Click for next page ( 65


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 64
Biological Challenges to Post-Eradication OVERVIEW All of the viruses currently under consideration for eradication rely on highly effective vaccines and well-defined immunization programs to inter- rupt transmission. Major biological challenges after eradication include: . knowing how and when to stop immunization; improving vaccine technology and production should a vaccine ever be needed even after the cessation of immunization; safely containing viruses in the lab in the post-eradication era; and continuing and improving surveillance for the detection of vaccine- associated cases, recrudescence of infection, new zoonotic transmissions, and the emergence of recombinant viral strains. Overcoming these challenges will require a better understanding of pathogen transmission and viral biology. For example, vaccine-preventable viruses (e.g., polio and measles) are characterized by boom-and-bust epidemic cycles which exhibit extraordi- nary non-linear dynamics due to the complex population-level interactions that influence transmission. Mathematical modeling that takes these inter- actions into account provides a robust scientific framework for predicting the impact of mass vaccination and exploring immunization cessation strat- egies. It can help us answer questions such as: How extensive must vaccina- tion be to interrupt transmission in a clefined population? What age class should mass vaccination target? Are catch-up campaigns effective? 64

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 6s Even after eradication and the cessation of immunization, it may not be desirable to completely eliminate all traces of the infectious agent because of its use in basic scientific and vaccine advancement research, as well as the need for an emergency stockpile in case of recrudescence. Thus, post-eradi- cation strategies must consider safe laboratory containment of the virus to minimize the risks of accidental or intentional re-introduction. In addition to recrudescence of wild-type virus, other potential post- eradication outbreaks could result from vaccine-associated infections, new recombinant strains of virus (e.g., between circulating HIV and newly in- troduced SIV), or new zoonotic transmissions (e.g., the existence of a pri- mate reservoir must be taken into account while planning future eradica- tion, and eventually post-eradication, strategies for HIV/AIDS). Most notably, vaccine-associated paralytic poliomyelitis (VAPP) demonstrates how vaccine-associated cases of disease can occur even when disease due to wild-type virus is eliminated. Post-eradication strategies will require con- tinual surveillance, more information about the duration of shedding and the persistence of the vaccine-derived virus in the environment, and con- tinuing vaccine coverage even in areas where wild-type virus has been eraclicated. Viruses have extraordinary evolutionary strategies about which we have very little unclerstanding. Continual surveillance and improved sampling methods are essential for tracking new genetic variants, particularly as more vaccines are introduced worldwide and rarer genotypes are selected for. The chance that new viruses could evolve underscores the need for continuer! development of improved vaccines and vaccine delivery systems. HERD IMMUNE Y AND TO DESIGN OF VACCINATION PROGRAMS Professor Roy M. Anderson, Ph.D., F.R.S. Department of Infectious Disease Epidemiology Imperial College School of Medicine University of London, London, U.K. The past four decades have witnessed remarkable success in the control of viral diseases by mass vaccination. The most notable of these is the eradication of smallpox in 1977 (Fenner et al., 1988), which resulted from an intensive worldwide immunization campaign. The success of the small- pox campaign has provided hope that other viral infections for which effective and safe vaccines are available- particularly polio and measles- can also be eradicated, given the will and financial resources. However, there are still many problems associated with pathogen transmission that

OCR for page 64
66 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION must be resolved before eradication can be achieved. These problems result from variation in vaccine uptake among countries, increased mixing of populations between cities and towns worldwide, and the high transmissi- bility of viruses within high-density populations. The development of a safe, effective, and cheap vaccine is only the first step albeit a vital onetoward community-based control of infectious disease. Population-leve! processes, such as the demography of the human host, human behavior, and the biological factors that influence transmis- sion all play critical roles in determining the impact of mass vaccination. The dynamics of the interaction between an infectious agent and its human host population are complex and often highly non-linear in form due to variation in the course of infection within the human host and the interac- tion of demography (e.g., net birth rate) and host behavior (e.g., patterns of mixing) (Anderson, 1994~. The resultant complex patterns are often sea- sonal; they are driven by both climatic influences on the likelihood of transmission and changes in behavior (e.g., school attendance and aggrega- tion of children). They can also be longer-term as a result of the dynamic interaction between the exhaustion (by infection) and renewal (by new births) of the supply of susceptible individuals. Longer-term cycles occurred in many developed countries prior to and after the initiation of routine mass immunization and are a well-known phenomenon. Once mass vaccination is initiated within a defined community, these complex interactions may be influenced in a manner that is not easily understood in the absence of a detailed template for analysis and interpre- tation. Mathematical models that combine the processes underlying the typical course of infection in the host with those that determine transmis- sion between hosts provide a robust scientific framework for the prediction of intervention impact and the formulation of cost-effective policies (Ander- son and May, 1990; Anderson et al., 1997~. This summary provides a review of recent progress in this type of mathematical modeling, with a particular focus on the factors that influence the persistence of infection and disease in communities with high rates of vaccine uptake. The child- hooct vaccine-preventable viral and bacterial infections, such as measles, mumps, rubella, polio, and pertussis, provide the empirical basis for much of the theory. Basic Principles Simple theory provides many insights into the likely impact of a defined immunization program targeted at a particular infectious agent. One of the central epidemiological concepts underlying this theory is the basic repro- ductive number, Ro' which is clefined as the average number of secondary cases of infection generated by one primary case in a susceptible commu-

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERaDICATION 67 nitv. The magnitude of Ro is determined by a blenci of parameters that influence the typical course of infection within the human host with param- eters that determine transmission between inclivicluals. For a clirectly trans- mittec3 viral or bacterial infection that exhibits little antigenic variability (i.e., one dominant serotype), the approximate value of Ro is given by the expression: Ro = [L - A]/[A - M], where L is human life expectancy, M is the average duration of protection from maternally derived antibodies, ancI A is the average age of infection in an unvaccinated community. The value of L can be replaced by an equiva- lent term representing the net birth rate of the community, since it is this parameter that generates the renewal of the supply of susceptibles. For example, for measles in the United States prior to wide-scale immunization, with L, A, and M values of 70, 5, anc! 0.5, respectively, each primary case of infection generated 14 to IS secondary cases in a totally susceptible community. In the case of endemic persistence within a community, where many have recovered from infection and are immune to re-infection, the effective reproductive number, R. is unity in value: each primary case generates, on average, exactly one secondary case. The effective reproductive number, R. is the average number of secondary cases generated by one primary case in a population that is not entirely susceptible to infection (i.e., the presence of those who are immune due to recovery or immunization). In cases where seasonal factors influence transmission and the transmission dynamics of the virus generates longer-term oscillations in incidence, the magnitude of R will fluctuate above anci below unity in value. The magnitude of Ro in an unvaccinated community can be determined from either cross-sectional or longituclinal serological surveys which clefine, by age, what percentage of the population is seropositive for specific anti- gens of an infectious agent. The rate of increase in seropositivity between two age classes provides a quantitative measure of the age-specific inci- clence of infection, which is sometimes referrec3 to as the "attack rate" or "force of infection." A serological approach to epiclemiological surveillance is much more accurate than case reports of infection, since the latter tend to vary in reliability depending on the prevailing incidence of infection. Under-report- . . . , . . . . lIlg IS common w ten an infection IS rare, anc over-reporting can arise cluring an epidemic phase in a recurrent epidemic situation. Serology works well for viral infections but is more problematic for bacterial disease, due to the decline over time in detectable antibodies to past infection. A diagrammatic illustration of a cross-sectional serological survey is

OCR for page 64
68 :~ 0.6 Q o a) In C o o 0.9 0.8 0.7 0.5 0.4 0.3 0.2 0.1 o CONSIDERATIONS FOR VIRAL DISEASE ERADICATION Age-specific serology l i.... ~ .... :~ 1~,~.: Used to calculate the average age at infection, A, and the average duration of maternal antibody protection, M. 2.5 5 7.5 10 Age in Years = Maternal Antibodies Antibodies from Infection 12.5 15 17.5 20 FIGURE 3-1 Diagrammatic example of a cross-sectional serological survey (L = 70 yrs, A = 5 yrs). It records the fraction of a sample of sera collected from a popula- tion that are seropositive to the antigens of a defined infectious agent, stratified by host age. documented in Figure 3-1. The pattern displayed provides considerable information relevant to the design of mass vaccination programs. For ex- ample, the trough in susceptibility, which occurs after the decay of mater- nally derived protection and before the rise resulting from infection, defines the optimum age for vaccination, given the poor efficacy of many vaccines if delivered when the titer of maternally derived antibody is high. Cross-sectional surveys can be repeated yearly and then combined to provide a longitudinal pattern of immunity and a precise picture of the "herd immunity" profile of a population over time. The specificity and sensitivity of saliva-based serological tests for many viral infections suggest that surveillance based on herd immunity profiles should be more widely adopted. Gaps or troughs in herd immunity profiles can provide policy

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 69 guidance for the introduction of "top up" age-targeted immunization pro- grams in situations where overall levels of vaccine uptake are moderate to high. If stratified by location and ethnic or other social group, the profiles can also be used to identify social groups or communities with low uptake levels. Finland is exemplary in the quality of serological data collected to monitor infectious disease incidence and the impact of particular mass vaccination programs. Few other countries use this approach to infectious disease surveillance. Mass Vaccination Theory also sheds light on the degree of mass immunization required to block transmission in a defined population. If the average age at immuniza- tion is V, and A and L are as defined previously, then the critical proportion of each yearly birth cohort that must be effectively immunized to block transmission, Pc' is given by the simple expression (Anderson and May, 19921: Pc = ILL - AJ/tA _ Ad. The critical fraction is minimized by keeping the value of V as low as possible. For infections, such as measles, that have a low average age at infection (A), cohort immunization must be very high (typically in excess of 90% to 95%) to block transmission within most urban populations. Theo- retically, in rural areas with lower densities and higher average ages at infection, the critical level of uptake to block transmission is somewhat lower. Practically, however, the values Of Pc derived for urban areas must be applied even to rural communities due to ever-increasing connectedness between urban and rural areas. The expression for Pc defined above oversimplifies the tasks required for eradication. For example, two important factors that affect the value of Pc are a decrease in vaccine efficacy in the presence of high titers of mater- nally derived antibody (these decline rapidly from birth, with a detection half life of roughly 6 months for most viral infections), and vaccines of less than perfect efficacy, even in the absence of maternal antibodies. Both of these factors yield a more complex expression for the value f Pc Once Pc is derived, a graph can be plotted for any given infection and vaccine of defined properties relating the average age at vaccination (V) and vaccine efficacy (e) to the critical fraction of a cohort that must be immu- nized (Pc) (see Figure 3-2; Anderson et al., 1997~. The efficacy of most current vaccines is far less than perfect: estimates range from 72% to 88% for mumps, 90/O to 95% for measles, and 96% tO 99% for rubella (Plotkin and Orenstein, 1999~.

OCR for page 64
70 0.99 - -' 0.98 - g 0.97- a ._ 0.96- 0.95 - CONSIDERATIONS FOR VIRAL DISEASE ERADICATION Fraction that must be vaccinated to block transmission (A = 5 years) lo\ Q-0.97 \ Q-0.98 Q = 1.0 1 . 1 1 1 1 1 1 1 1 1 - - - / Maximum vaccine efficacy, Q 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Average age at vaccination, V (years) FIGURE 3-2 The impact of age at vaccination and vaccine efficacy on the critical level of cohort vaccination required to block transmission. NOTE: Vaccine efficacy changes with age due to presence of maternal antibodies. In the case of highly transmissible infections (high Ro values), such as measles, a lower than 100% vaccine efficacy strongly hinders the task of blocking transmission. As illustrated in Figure 3-2, the fraction of the co- hort that must be immunized in order to block transmission is greater than one, implying that more than one round of immunization of a given cohort is required for effective blockage (i.e., two-stage immunization policies) (Bottiger et al., 1987~. Even two-stage immunization may not be sufficient if either the average age at infection is very low, children who are not protected by the first immunization can never be protected due to nutri- tional or genetic factors, or if those not immunized in the first round of immunization are also not immunized in the second round at a later age. An example of the consequences of low average age at infection is the situation in Lagos, Nigeria, a large city in a developing country with a high birth rate. An average age at infection of around one to two years prior to mass vaccination requires that immunization be effectively administered near birth in order to block transmission. However, if delivered too soon after birth, the presence of maternally derived antibodies significantly reduces vaccine efficacy. In short, the combination of high transmissibility (low average age at infection), imperfect vaccine efficacy, and behavioral or

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 71 social predisposition to remaining unimmunized suggests that eradication in some parts of the world may be very difficult. Mass immunization influences the epidemiology of infectious agents in several ways. First, it lowers transmission success (i.e., from infected to still susceptible individuals), thereby increasing the average age at infection. If the likelihood of serious disease resulting from infection increases with age, low to moderate vaccine coverage may increase net morbidity, a particu- larly worrisome situation for rubella vaccination campaigns in developing countries. Every effort should be made to achieve high uptake. Second, immunization tends to lengthen the inter-epidemic period. Third, a trough of susceptibility moves across the herd immunity profile in older age classes, due to decreased transmission rates and exposure following the mass im- munization (see Figure 3-3~. All of these epidemiological phenomena, which have been both predicted by theory and observed in practice, need to be considered when monitoring the impact of mass vaccination. How to Vaccinate and at Which Age The design of immunization programs involves many different factors, such as cost and sustainability. Developed countries usually use cohort immunization at one or two different ages for any given vaccine or combi- nation of vaccines (e.g., measles-mumps-rubelIa [MMR]~. Practicalities dic- tate that ease of access to infants and children via health clinics or schools is critical in determining at what age vaccination is offered. It is essential that as high a fraction of children as possible are immunized at as young an age as possible, while taking into account the complexities induced by maternal antibodies. For example, even though many countries offer MMR vaccination at around two years of age, the observed distribution of ages at immunization is not always clustered around this age as it should be. An alternative or addition to cohort immunization is a pulse or "catch- up" immunization strategy involving particular days (or weeks) designated as "immunization days" and publicized by the press. On immunization days, health care services offer vaccination to all children of a particular age range. Immunization days must occur at regular intervals, perhaps every one to two years in the early stages of the program and less frequently as overall coverage rises and infection incidence falls. This approach has been used with considerable success in South American countries (de Quadros et al., 1996~. Analyses based on mathematical models of viral transmission confirm that catch-up campaigns can effectively disrupt spread, particularly when infections exhibit seasonal oscillatory trends in incidence or inter-epidemic periods lasting a few years (Agur et al., 19931. The optimum time for a vaccination day or week is during a trough in incidence between epidemic

OCR for page 64
72 0.8 g :~ An o ~ 0.6 a) 00 o ._ ~ 0.4 IL 0.2 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION Susceptibility trough 0<1m 6-m 2- <1 m 6-m 2 - 1-m 1- 4 1-m 1- 4- r ~ H~;5 1981 1979 6- 10- 14- 20- 30- >60 8- 12- 16~ 25- 40- Age I so:' 1 987 1 985 1991 FIGURE 3-3 Herd immunity profile for rubella in Finland, recording the trough in susceptibility moving across the surface following the initiation of mass immuniza- tion in 1981. The y-axis records the fraction of a sample of sera collected from a population that are seropositive to the antigens of the rubella virus, stratified by host age and year of collection (Ukkonen, 1996~. cycles. Catch-up campaigns are especially valuable in developed countries, where they serve to mop up susceptible pockets within the population. However, it is important to recognize that any cessation or decline in effectiveness of either cohort or pulse programs will rapidly lead to a build- up of susceptibles, particularly in high birth rate communities. Increased

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 73 susceptibility makes a population vulnerable to the reintroduction of infec- tion from other countries or areas with lower vaccine coverage. Molecular epidemiological studies have revealed how travelers carry infections, such as measles and rubella, across continents, thereby creating short chains of transmission within susceptible pockets in highly vaccinated populations (Bellini and Rota, 1998~. Persistence or Eradication Chains of transmission often persist even within highly vaccinated com- munities in developed countries. A number of factors create difficulties cluring the final push for the elimination of indigenous transmission. First, successful programs tend to increase the average age of infection. Conse- quently, cases of infection are often observed in clusters in older age classes (i.e., older than the age class for which immunization is first offered>. Second, incidence increases in the younger age classes (i.e., younger than the age class for which immunization is first offered). Third, the synchrony of epidemics among different spatial locations decreases. Prior to wide- spread immunization, the epidemic cycles of most childhood viral and bac- terial infections are highly correlated in different spatial settings within countries. However, synchrony decreases significantly as vaccine coverage rises, incidence falls, and inter-epidemic periods lengthen (Bolker and Grenfell, 19963. Controlling these minor epidemics may require that catch- up campaigns be timed differently. Eradication is especially difficult when there is variation in vaccine uptake among regions, areas, or spatial locations. In developed countries, vaccine uptake in poor inner-city communities is often low. Pockets of low immunity, particularly if linked with overcrowding, poor public health care facilities, and high birth rates, provide reservoirs of infection for the sustenance of transmis- sion. The ever-growing connectedness of urban centers across the world via air, road, anal rail (even within Africa) suggests that continued immunization is necessary in all regions until vaccine uptake is uniformly high across the globe. Increased vaccine uptake reduces effective community size, which re- sults in greater fade-out (i.e., a greater number of weeks during which there are no reported cases of infection) (see Figure 3-4~. Cost-Effectiveness of Mass Immunization The main obstacles to eradication of measles and polio are often per- ceived to be financial. In today's world of rising health care costs, where many different interventions are possible in both developed and developing countries, cost-effectiveness is a major factor to consider when deciding which intervention to use. An increasing number of vaccines are available

OCR for page 64
74 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION o 8 o .= o ~ 6 Q o a) IL 4 2 o ~ ~ ~^ am. rot ~ ~~ ~ US/Can. cities 1 921 -40 Various islands, 1 950-65 British cities, 1 960-64 _ ~~,.~*.i7.~.~,=~.<'~.~.~.~ ~.~.~Y~ 103 104 05 1 o6 107 5x107 Population size FIGURE 3-4 Critical community size for measles, defined as the population size at which fade-out (proportion of weeks in a year when no cases reported) of cases rises rapidly to approach unity. for both viral (e.g., varicella) and bacterial infections (e.g., pneumococcal infections). Pharmaceutical companies and government health agencies of- ten use cost-benefit analyses to determine which vaccine to use. However, these analyses tend to grossly underestimate the benefit of vaccination pro- grams, because the current health economic methods of analysis typically only take into account the direct effects of immunization on the vaccinated individual. In practice, immunization has important indirect effects as well because it decreases transmission among those still unvaccinated. The mag- nitude of these indirect benefits increases rapidly as overall vaccine cover- age increases and, as illustrated in Figure 3-5, may comprise a significant fraction of the overall benefit. The magnitude of the indirect benefit is calculated in a way that takes into account the impact of immunization on transmission success as a function of vaccination coverage. The time frame over which benefit is calculated (i.e., the number of years) is critical for an accurate assessment of cost versus benefit. Morbidity Induces! by Immunization All vaccines carry a small risk of adverse effects in the immunized patient (Peltola and Heinonen, 1986~. When the disease is common, risk of serious morbidity from infection is many orders of magnitude greater than risk associated with immunization. The values of the two risks converge when vaccine coverage approaches the level required to block local trans-

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 87 Known or believed to represent natural infections, and five of them have been identified in chimpanzees from west central Africa (P. t. troglodytes). A sixth strain was isolated from a wild-caught chimpanzee of unknown geographic origin which was classified as a P. t. schweinfurthii on the basis of mtDNA analyses (Gao et al., 1999; Peeters et al., 1992~. All three groups of HIV-1 are significantly more closely related to the five SIVcpz(P.t.t.) isolates than to the one SIVcpz(P.t.s.) strain, indicating that the cross- species transmissions that gave rise to all three groups of HIV-1 (M, N. and O) occurred in west central Africa (Gao et al., 1999~. HIV-1 groups N and O viruses are essentially restricted to west central Africa (Mauclere et al., 1997; Simon et al., 1998), and chimpanzee and group N human viruses from Cameroon form a unique subcluster in phylogenetic trees, implicating this country as the likely site of origin for HIV-1 group N (Corbel et al., 2000; Simon et al., 19981. Although HIV-1 group M is spreading globally, the greatest diversity of group M viruses has been found in the western parts of the Democratic Republic of Congo (i.e., Kinshasa), which is consis- tent with this being the region of the initial group M expansion (Vidal et al., 2000~. Kinshasa is outside the range of chimpanzees, but it is close to the natural range of P. t. troglodytes and is by far the largest city in the region. Together, these findings provide compelling evidence that HIV-1 arose as a consequence of three independent SIVcpz transmissions from naturally in- fected chimpanzees in west central Africa. Although the routes and circumstances of cross-species transmissions are unknown, it is believed that human infection with SIVcpz and SIVsm resulted from exposure to infected blood during the hunting and field dress- ing of animals, the preparation of primate meat for consumption, and bites and scratches from infected pets or wounded animals (Hahn et al., 2000~. Given that humans throughout sub-Saharan Africa are in frequent contact with primate species other than chimpanzees and sooty mangabeys, the possibility of additional zoonotic transfers of primate lentiviruses must be considered. Indeed, a recent survey of bushmeat markets in Cameroon revealed that up to one-third of all primates offered for sale were SIV- infected (Peelers et al., 20013. Peeters and colleagues found that over 130 of 400 wild-caught monkeys from 13 different species had serum antibodies that cross-reacted with HIV-1 andlor HIV-2 antigens. PCR amplification of viral sequences confirmed SIV infection in a subset of these animals and revealed the existence of four new SIV lineages not previously known to infect primates in the wild. This study thus provided conclusive evidence that humans are routinely exposed to a wide variety of primate lentiviruses through the hunting and handling of SIV-infected primates. Commercial logging represents an important economic activity in many west central African countries; it has led to road construction into remote forest areas, human migration, and the development of social and economic

OCR for page 64
88 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION networks which support this industry (Auze! and Hardin, 2000; Geist, 1988; Wilkie et al., 2000~. Hunters are now penetrating previously inacces- sible forest areas and using modern weapons and a newly developed infra- structure to capture and transport bushmeat, including many primates, from remote areas to major city markets. These socioeconomic changes, combined with current data on SIV prevalence and genetic complexity in mild living primates, strongly suggest that the magnitude of human expo- sure to SIV has increased dramatically, as have the social and environmen- tal conditions that support the emergence of new zoonotic infections. It remains unknown whether SIVs other than SIVcpz and SIVsm have the ability to infect humans. Molecular evidence for such cross-species transmissions does not exist; however, such infections might have gone unrecognized. An example is the recent identification of a Cameroonian man who had an indeterminant HIV serology but reacted strongly with an SIVmnd V3 loop peptide (Souquiere et al., 20011. Although SIV infection was not confirmed in this individual, the finding suggests that at least some naturally occurring SIVs (other than SIVcpz and SIVsm) have the potential to infect humans. In fact, several recently reported SIV isolates (Georges- Courbet et al., 1998; Souquiere et al., 2001) replicate well in primary human lymphocytes in vitro, as do SIVcpz (Gao et al., 1999; Peeters et al., 1992) and SIVsm (Peelers et al., 1994~. Thus, to determine whether addi- tional zoonotic transmissions of SIVs have already occurred, screening as- says that can reliably recognize and distinguish the wide variety of SIVs now known to infect wild-living primates will have to be developed. It is also important to distinguish between the initial transmission of a new SIV and the many additional factors that promote secondary transmis- sions and, ultimately, epidemic spread in the human population. The fac- tors that trigger epidemic outbreaks of newly introduced SIVs are unknown but could possibly involve situations where the recipient of a cross-species transmission event is already infected by an existing HIV. In these cases, superinfection and recombination could generate mosaic viruses of consid- erable genetic and biological complexity. Evidence that such events have taken place in primates has come from studies of SIVs infecting sabaeus monkeys (SIVagmSab), red-capped mangabeys (SIVrcm), and mandrills (SIVmnd2) (Georges-Courbet et al., 1998; fin et al., 1994b; Souquiere et al., 20011. In each case, mosaic viruses comprised of different SIV lineages are widely distributed in their respective host species and thus represent cases where cross-species transmission and recombination have led to suc- cessful virus adaptation and dissemination, perhaps even outcompeting the previous incumbent SIVs. As the prevalence rates of HIV-1 group M viruses are rising in west central Africa, recombination of newly introduced SIVs with circulating HIVs has become a more probable scenario.

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION TABLE 3-1 SIV Reservoirs and Human Zoonotic Risk: Future Studies 89 Determine the full spectrum of SIV-infected non-human primates in equatorial Africa: . Identify all SIV-infected primate species. Assess the prevalence, geographic distribution, and natural history of SIV infection in wild primate populations. Determine the frequencies, routes, and circumstances of primate-to-primate . . . cross-species transmissions. Characterize all major SIV lineages at the biological and molecular level: . Determine the spectrum of SIV genetic and biological diversity. Molecularly clone and sequence representatives of all major SIV lineages. . . . . lentlvlruses. Characterize the origins and evolutionary history of the entire group of primate Determine to what extent humans are exposed to SIV and whether such exposure has led to additional zoonotic transmissions: Develop novel screening and confirmatory assays that can detect and distinguish the wide variety of SIV infections now known to exist in wild primate populations. Establish effective surveillance mechanisms for humans at risk for zoonotic . , . ntectlons. Elucidate the viral, host, and environmental factors that facilitate zoonotic transmission and promote subsequent epidemic spread. Monitor the emergence of HIV/SIV recombinants. In summary, the current HIV-1 group M pandemic provides compel- ling evidence for the rapidity, stealth, and, ultimately, the extraordinary clinical impact that can result from even a single zoonotic transmission event. It is now clear that humans are routinely exposed to a plethora of primate lentiviruses through the hunting of primates, and that the magni- tude and breadth of this exposure has previously been underestimated. In light of these data, a complete and accurate assessment of all SIV- infected non-human primate species in geographic areas where these are abundant seems necessary (Table 3-1~. Since most SIV-infected primates, especially the great apes, are endangered, strategies that avoid a further increase in hunting will need to be employed. Such strategies would rely on non-invasive methods, such as the use of urine and fecal samples to detect SIV-specific antibodies and viral nucleic acids (Santiago et al., 2001~. Stud- ies are also needed to determine whether transmission of simian lentiviruses other than SIVcpz and SIVsm to humans have already occurred. This will require the screening of human sera with diagnostic tests which can detect and distinguish a wide range of primate lentiviral infections.

OCR for page 64
i: 9o CONSIDERATIONS FOR VIRAL DISEASE ERADICATION Finally, the potential of recombination between currently circulating HIVs and newly introduced SIVs must be considered, and surveillance mechanisms must be established to detect their possible emergence. Such recombinants could evade susceptibility to vaccines that are based on only one virus group or subtype. Because experimental HIV vaccines will even- tually be tested in countries worldwide, the occurrence of new zoonotic SIV infections and their possible impact on immunization efforts will need to be examined. The existence of a primate reservoir must be taken into account while planning future eradication strategies for HIV/AIDS. VACClNE-ASSOCIAl~D CASES DUE TO IMMUNIZATION WITH LIVE VIRUS VACCINES Jeffrey I. Cohen, M.D. Head, Medical Virology Section, Laboratory of Clinical Investigation National Institutes of Health, Bethesda, MD Live virus vaccines, including those for smallpox, measles, and poliovi- rus, have dramatically reduced or in some cases eliminated disease caused by these viruses. As disease due to wild-type virus is eliminated, however, vaccine-associated cases become of increasing concern. Vaccinia, which has been used for 200 years to prevent smallpox, can cause postvaccinal encephalitis, progressive vaccinia, eczema vaccinatum, and generalized vaccinia. In a national survey in 1968, about 300 of 14 million vaccinees suffered severe side effects, and 9 fatalities were reported. All but one of the fatalities were due to postvaccinal encephalitis or pro- gressive vaccinia. Fatal cases of eczema vaccinatum have been reported in contacts of vaccinees. A case of severe generalized vaccinia occurred in a vaccinated asymptomatic HIV military recruit. The smallpox viral genome contains 150 genes that are very similar to vaccinia and 37 genes that are smallpox-specific or divergent from those in vaccinia. These latter genes frequently encode host-interactive proteins. The vaccine strain of measles virus rarely causes disease. Vaccine virus has been detected in lung, liver, bone marrow, or brain tissues of only three patients who had severe disease after vaccination. One patient had HIV, one had severe combined immunodeficiency, and one had no known immu- nocleficiency. The latter two patients died from measles vaccination. The measles vaccine and wild-type virus share more than 95.5/O of the same nucleotide sequences. The changes that are responsible for attenuation of the measles virus are unknown.

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION Vaccine-Associated Paralytic Poliomyelitis 91 The first reports of vaccine-associated paralytic poliomyelitis (VAPP) occurred shortly after the introduction of the oral polio vaccine (OPV). Since 1973, the number of VAPP cases has exceeded the number of cases of wild-type polio in the United States. From 1980-1989, VAPP was associ- ated with 1 out of every 2.5 million doses of OPV in the United States. VAPP occurs primarily in unvaccinated or inadequately vaccinated persons, and more commonly in infants. In the United States from 1980 to 1995, about 40% of VAPP cases occurred in OPV recipients, 30% in close con- tacts, 25% in immunodeficient persons, and 5/O were community acquired. The latter persons had not been recently vaccinated and were not known to be in direct contact with vaccine recipients. The percentage of patients in each risk group has remained fairly stable over time. Immunodeficient patients have a 3,000- to 6,000-fold greater risk of developing VAPP. In one study (Sutter and Prevots, 1994J, 96% of VAPP cases were due to B cell deficiency; the other 4/O were due to long-term corticosteroid use. So far, there have been only two cases of VAPP associ- ated with HIV. In Romania, but not in the United States, VAPP has been associates} with increasing numbers of intramuscular injections given nine to thirty days before OPV. Nucleotide sequencing indicates that less than 1% of OPV bases (polio is an RNA virus and does not have base pairs) differ from those of its neurovirulent parent. Only two or three base changes are needed for OPV type 2 (OPV2) or OPV3 to revert to neurovirulence, while several base changes are needed for OPV1. This coincides with the fact that OPV2 and OPV3 are isolated more frequently than OPV1 from patients with VAPP. VAPP may be due to neurovirulent revertant viruses that develop dur- ing replication in the gastrointestinal tract, recombination between differ- ent strains of OPV, or recombination between OPV and wild-type strains. In one study, within 2 days of receiving OPV3, one of the attenuating mutations in the virus reverted to the wilcl-type sequence, and the shed virus was more neurovirulent (Evans et al., 19853. When OPV was the preferred vaccine, there were eight to nine VAPP cases per year in the United States. After the initiation of a sequential regimen of inactivated poliovirus vaccine (IPV)-OPV, the number of cases declined to two to five per year. With an all IPV regimen, VAPP should be virtually eliminated. However, there is concern about the continued release of neurovirulent revertants of live OPV into the environment even after vaccination is terminated. Thus, VAPP may continue to occur for some finite period of time. Recently, two antibody-deficient patients with VAPP shed virus in their stool for over five years after their last vaccination. Comparison of the sequence of these viruses with that of OPV suggests that

OCR for page 64
92 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION the viruses had been replicating in the patients for about ten years. How- ever, most antibody-deficient patients probably shed virus for less than six months. In contrast, immunocompetent persons usually shed virus for less than three months. Several studies have suggested that OPV has a limited circulation in the environment. In Cuba, for example, where OPV is administered for two months of the year, the virus has been detected for only 2 or 3 months after vaccinations (Ochoa and Lago, 19871. Similarly, VAPP cases in Romania have been closely associated with specific vaccination campaigns (Strebel et al., 1994~. In most cases, sequencing of VAPP isolates shows greater than 99/0 similarity to OPV, indicating that the VAPP isolates have circulated for a very short period of time. However, other studies show that neurovirulent forms of OPV can circulate at length. For example, analysis of the nucleotide sequence of OPV2 isolated from sewage in Israel suggested that the virus had been circulating for six years (Shulman et al., 2000~. A similar study from Japan found neurovirulent virus in sewage and river water three months after OPV vaccination (Yoshida et al., 2000~. These neurovirulent strains of OPV were not associated with VAPP in either of these two studies. In Poland, from March to December 1968, there was an outbreak of poliovirus type 3 four months after vaccination with a live attenuated OPV3, USOL virus. There were 464 cases of paralytic disease. Nucleotide sequencing of isolates from seven epidemic cases, four healthy vaccinees, and one healthy contact all showed USOL-like viruses. The seven isolates exhibited a change in sequence associated with neurovirulence; none of the healthy vaccinees or contacts exhibited such a change. A large number of cases of polio occurred in China from 1991 to 1993. Sequencing of isolates from 34 patients indicated that the virus was a recombinant derivative of wild-type polio type 1 and OPV1. Analysis of the sequenced viruses suggested that all of the recombinants were derived from a mixed infection of a single person with wild-type and OPV type 1. The recombinant virus spread rapidly over 2,200 kilometers in 3 years. Recent outbreaks of paralytic polio have occurred clue to circulating vaccine-derived poliovirus (cVDPV) in several areas, suggesting that neurovirulent revertants of OPV can persist. For example, from July to November 2000, 20 cases of cVDPV due to OPV1 occurred in the Domini- can Republic and Haiti. About 85% of the patients were under six years of age, and all of the patients were either unvaccinated or inadequately vacci- nated. The viral nucieotide sequence showed a 97/O genetic similarity to OPV, suggesting that the virus had either circulated for two years in the area or had undergone prolonged replication in an immunodeficient per- son. The epidemic was rapidly terminated after intensive vaccination with OPV. A similar epidemic of cVDPV due to OPV2 occurred in 32 persons in

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 93 Egypt from 1988-1993. Analysis of the viral sequence suggested that the virus had circulated for 11 years. Like the Caribbean epidemic, vaccination coverage was low in Egypt cluring this time, anct circulation of OPV-derivect virus stopped when vaccine coverage increased. From March to fuly 2001, 3 cases of cVDPV occurred in the Philippines. These cases were clue to virus cleriveci from OPV1. Conclusion These VAPP cases emphasize (a) the need for continuing polio vaccina- tion in polio-free areas until global eradication is achieved, (b) the necessity of continued surveillance for poliovirus and flaccid paralysis, (c) the need for aciclitional information about duration of shecicling and persistence of virus in the environment, and (cl) the importance of global eradication of . . pot .lovlrus. Remaining questions, ancI possible answers, inclucle the following: 1. Will immunocompromised carriers of OPV continue to shed the virus into the environment? Yes, but only for a limited time- most often weeks to months, but in some cases for up to as long as 10 years. 2. What proportion of immunocompromised persons (including those with HIV) shed OPV for prolonged periods of time? Probably less than 10% of antibocly-cleficient patients sheet virus for long periods of time; the percentage is probably lower among HIV-infectec! patients since they often retain the ability to produce antibodies. 3. How long will neurovirulent revertants of OPV be shed into the environment? Basec3 on results from sewage studies in Japan and Israel, OPV can be cletecteci for up to nearly five years after vaccination. 4. What is the threshold rate of vaccine coverage needed to suppress circulation of OPV, and upon what does the rate depend? The rate is probably similar to that required to prevent circulation of wilcl-type polio, and it probably clepencis on the strain of OPV, population density, level of hygiene, and climate. 5. How long can OPV circulate in populations, anal how transmis- sible is it? OPV can circulate for 11 years according to the Egyptian out- break of polio associated with OPV, and 2 years according to the Carib- bean outbreak. 6. Should IPV be given for a period of time after OPV is discontinued to allow clearing of virus from shedders? Yes, if possible, especially since infants have the highest risk for VAPP and will not be immune when . . . VaCClIlatlOI1 IS stoppec . 7. How long should intensive surveillance be continued" after IPV is

OCR for page 64
94 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION stopped Probably at least 10 years, in view of the long shedding period and the recent occurrence of polio due to circulating vaccine-related virus. 8. What is the best way to detect circulating OPV and respond to outbreaks? Intensive surveillance for cases of acute flaccid paralysis and poliovirus is required, and further research is needed. REFERENCES Agur Z. Cojocaru L, Mazor G. Anderson RM, and Danon YL. 1993. Pulse mass measles vaccination across age cohorts. Proceedings of the National Academy of Sciences 90:11698-11702. Anderson RM. 1994. Populations, infectious disease and immunity: A very nonlinear world. Philosophical Transactions of the Royal Society of London, Series B 346:457-505. Anderson RM and May RM. 1990. Immunization and herd immunity. Lancet 335:641-645. Anderson RM and May RM. 1992. Infectious Diseases of Humans: Dynamics and Control. Oxford: University of Oxford Press. Anderson RM, Donnelly CA, and Gupta S. 1997. Vaccine design, evaluation, and commu- nity-based use for antigenically variable infectious agents. Lancer 350:1466-1470. Auzel P and Hardin R. 2000. Colonial history, concessionary politics, and collaborative management of Equatorial African rain forests. Pp. 21-38 in Hunting and Bushmeat Utilization in the African Rain Forest, M Bakarr, G Da Fonseca, W Konstant, R Mittermeier, and K Painemilla, eds. Washington, D.C.: Advances in Applied Biodiversity Science Series, Conservation International. Bellini WJ and Rota PA. 1998. Genetic diversity of wild-type measles viruses: Implications for global measles elimination programs. Emerging Infectious Diseases 4:29-35. Bibollet-Ruche F. Galat-Luong A, Cuny G. Sarni-Manchado P. Galat G. Durand JP, Pourrut X, and yeas F. 1996. Simian immunodeficiency virus infection in a pates monkey (Erythrocebus patas): Evidence for cross-species transmission from African green mon- keys (Cercopithecus aethiops sabaeus) in the wild. The Journal of General Virology 77:773-781. Bolker BM and Grenfell BT. 1996. Impact of vaccination on the spatial correlation and persistence of measles dynamics. Proceedings of the National Academy of Sciences 93:12648-12653. Bottiger M, Christenson B. Romanus V, Taranger J. and Strandell A. 1987. Swedish experi- ence of two-dose vaccination program aiming at eliminating measles, mumps, and ru- bella. British Medical Journal 295:1264-1267. Chen Z. Telfer P. Gettie A, Reed P. Zhang L, Ho DD, and Marx PA.1996. Genetic character- ization of new West African simian immunodeficiency virus SIVsm: geographic cluster- ing of household-derived SIV strains with human immunodeficiency virus type 2 sub- types and genetically diverse viruses from a single feral sooty mangabey troop. Journal of Virology 70:3617-3627. Chen Z. Luckay A, Sodora DL, Telfer P. Reed P. Gettie A, Kanu JM, Sadek RF, Yee J. Ho DD, Zhang L, and Marx PA. 1997. Human immunodeficiency virus type 2 (HIV-2) seroprevalence and characterization of a distinct HIV-2 genetic subtype from the natural range of simian immunodeficiency virus-infected sooty mangabeys. Journal of Virology 71 :3953-3960.

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 95 Corbet S. Muller-Trutwin MC, Versmisse P. Delarue S. Ayouba A, Lewis J. Brunak S. Martin P. Brun-Vezinet F. Simon F. Barre-Sinoussi F. and Mauclere P. 2000. Env sequences of simian immunodeficiency viruses from chimpanzees in Cameroon are strongly related to those of human immunodeficiency virus group N from the same geographic area. Jour- nal of Virology 74:529-534. Courgnaud V, Pourrut X, Bibollet-Ruche F. Mpoudi-Ngole E, Bourgeois A, Delaporte E, and Peeters M. 2001. Characterization of a novel simian imm~'nodeficiency virus from Guereza Colobus (Colobus guereza) in Cameroon: A new lineage in the nonhuman primate lentivirus family. Journal of Virology 75:857-866. de Quadros CA, Olive ~M, Hersh BS, Strassburg MA, Henderson DA, Brandling-Bennett D, and Alleyne GA. 1996. Measles elimination in the Americas: Evolving strategies. Journal of the American Medical Association 275:224-229. Evans DM, Dunn G. Minor PD, Schild GC, Cann AJ, Stanway G. Almond JW, Currey K, and Maizel JV Jr. 1985. Increased neurovirulence associated with a single nucleotide change in a noncoding region of the Sabin type 3 poliovaccine genome. Nature 314(6011):548- 550. Fenner F. Henderson DA, Arita I, Jezek Z. and Ladnyi ID. 1988. Smallpax and Its Eradica- tion. Geneva: World Health Organization. Gao F. Yue L, White AT, Pappas PG, Barchue J. Hanson AP, Greene BM, Sharp PM, Shaw GM, and Hahn BH. 1992. Human infection by genetically diverse SIVsm-related HIV-2 in west Africa. Nature 358:495~99. Gao F. Yue L, Robertson DL, Hill SC, Hui H. Biggar RJ, Neequaye AK, Whelan TM, Ho DD, Shaw GM, Sharp PM, and Hahn BH. 1994. Genetic diversity of human immunodefi- ciency virus type 2: Evidence for distinct sequence subtypes with differences in virus biology. Journal of Virology 68:7433-7447. Gao F. Bailes E, Robertson DL, Chen Y. Rodenburg CM, Michael SF, Cummins LB, Arthur LO, Peeters M, Shaw GM, Sharp PM, and Hahn BH. 1999. Origin of HIV-1 in the chimpanzee Pan troglodytes troglodytes. Nature 397:436~41. Geist V. 1988. How markets for wildlife meat and parts, and the sale of hunting privileges, jeopardize wildlife conservation. Conservation Biology 2:15-26. Georges-Courbot MC, Lu CY, Makuwa M, l~elfer P. Onanga R. Dubreuil G. Chen Z. Smith SM, Georges A, Gao F. Hahn BH, and Marx PA. 1998. Natural infection of a household pet red-capped mangabey (Cercocebus torquatus torquatus) with a new simian immuno- deficiency virus. Journal of Virology 72:600~08. Hahn BH, Shaw GM, De Cock KM, and Sharp PM. 2000. AIDS as a zoonosis: Scientific and public health implications. Science 287:607014. Jin MJ, Rogers J. Phillips-Conroy JE, Allan JS, Desrosiers RC, Shaw GM, Sharp PM, and Hahn BH. 1994a. Infection of a yellow baboon with simian immunodeficiency virus from African green monkeys: Evidence for cross-species transmission in the wild. Jour- nal of Virology 68:8454-8460. Jin MJ, Hui H. Robertson DL, Muller MC, Barre-Sinoussi F. Hirsch VM, Allan JS, Shaw GM, Sharp PM, and Hahn BH. 1994b. Mosaic genome structure of simian immunodefi- ciency virus from west African green monkeys. The European Molecular Biology Orga- nization Journal 13:2935-2947. Kaye JA, de Mar Melero-Montes M, and Jick H. 2001. Mumps, measles, and rubella vaccine and the incidence of autism recorded by general practitioners: A time trend analysis. British Medical Journal 322:460~63. Kuiken C, Foley B. Hahn BH, Marx P. McCutchan F. Mellors J. Mullins }, Wolinsky S. and Korber B. 1999. Human Retroviruses and AIDS: A Compilation and Ana,lysis of Nucleic Acid and Amino Acid Sequences. Los Alamos, NM: Los Alamos National Laboratory.

OCR for page 64
96 CONSIDERATIONS FOR VIRAL DISEASE ERADICATION Mauclere P. Loussert-Ajaka I, Damond F. Fagot P. Souquieres S. Monny Lobe M, Mbopi Keou FX, Barre-Sinoussi F. Saragosti S. Brun-Vezinet F. and Simon F. 1997. Serological and virological characterization of HIV-1 group O infection in Cameroon. AIDS 11:445- 453. Ochoa EG and Lago PM. 1987. Epidemiological surveillance and control of poliomyelitis in the Republic of Cuba. Journal of Hygiene, Epidemiology, Microbiology, and Immunol- ogy 31(4):381-389. Peeters M, Honore C, Huet T. Bedjabaga L, Ossari S. Bussi P. Cooper RW, and Delaporte E. 1989. Isolation and partial characterization of an HIV-related virus occurring naturally in chimpanzees in Gabon. AIDS 3:625-630. Peeters M, Fransen K, Delaporte E, Van den Haesevelde M, Gershy-Damet GM, Kestens L, van der Groen G. and Plot P. 1992. Isolation and characterization of a new chimpanzee lentivirus (simian immunodeficiency virus isolate cpz-ant) from a wild-captured chim- panzee. AIDS 6:447~51. Peeters M, Janssens W. Fransen K, Brandful J. Heyndrickx L, Koffi K, Delaporte E, Piot P. Gershy-Damet GM, and van der Groen G. 1994. Isolation of simian immunodeficiency viruses from two sooty mangabeys in Cote d'Ivoire: virological and genetic characteriza- tion and relationship to other HIV type 2 and SIVsm/mac strains. AIDS Research and Human Retrovi7uses 10:1289-1294. Peeters M, Pourrut X, Bibollet-Ruche F. Courgnaud V, Abela B. Mpoudi E, Hahn B. and Delaporte E. 2001. Ongoing exposure of humans to simian immunodeficiency viruses in West Central Africa poses a risk for additional zoonotic transmissions. Paper presented at 8th Conference on Retroviruses and Opportunistic Infections, Foundation for Retrovirology and Human Health, Chicago, February 4-8, 2001. Peltola H and Heinonen OP. 1986. Frequency of true adverse reactions to measles-mumps- rubella vaccine: A double-blind, placebo-controlled trial in twins. Lancet 1:939-942. Plotkin SA and Orenstein WA, eds. 1999. Vaccines, 3rd ed. Philadelphia: WB Saunders. Santiago M, Rodenburg C, Mamaeva O. Kilby J. Moldoveanu Z. Fahey B. Muller M, Ayouba A, Shaw G. McClure H. Heeney J. Nerrienet E, Boesch C, Wrangham R. Gao F. and Hahn B. 2001. AIDS as a zoonosis: characterizing the primate reservoir. Paper presented at 8th Conference on Retroviruses and Opportunistic Infections, Foundation for Retrovirology and Human Health, Chicago, February ~8, 2001. Sharp PM, Bailes E, Chaudhuri RR, Rodenburg CM, Santiago MO, and Hahn BH. 2001. The origins of AIDS viruses: where and when? Pl7ilosophical Transactio'~s of t1,e Royal Society of London, Series B 356:867-876. Shulman LM, Manor Y. Handsher R. Delpeyroux F. McDonough MJ, Halmut T. Silberstein I, Alfandari J. Quay J. Fisher T. Robinov J. Kew OM, Crainic R. and Mendelson E. 2000. Molecular and antigenic characterization of a highly evolved derivative of the type 2 oral poliovaccine strain isolated from sewage in Israel. Journal of Clinical Microbiol- ogy 38(10):3729-3734. Simon F. Mauclere P. Roques P. Loussert-Ajaka I, Muller-Trutwin MC, Saragosti S. Georges- Courbot MC, Barre-Sinoussi F. and Brun-Vezinet F. 1998. Identification of a new hu- man immunodeficiency virus type 1 distinct from group M and group O. Nature Medi- cine 4:1032-1037. Souquiere S. Bibollet-Ruche F. Robertson DL, Makuwa M, Apetrei C, Onanga R. Kornfeld C, Plantier JC, Gao F. Abernethy K, White LJ, Karesh W. Telfer P. Wickings EJ, Mauclere P. Marx PA, Barre-Sinoussi F. Hahn BH, Muller-Trutwin MC, and Simon F. 2001. Wild Mandrillus sphinx are carriers of two types of lentivirus. Journal of Virology 75:7086-7096.

OCR for page 64
BIOLOGICAL CHALLENGES TO POST-ERADICATION 97 Strebel PM, Aubert-Combiescu A, Ion-Nedelcu N. Biberi-Moroeanu S. Combiescu M, Sutter RW, Kew OM, Pallansch MA, Patriarca PA, and Cochi SL. 1994. Paralytic poliomyelitis in Romania, 1984-1992. Evidence for a high risk of vaccine-associated disease and reintroduction of wild-virus infection. American Journal of Epidemiology 140(12):1111- 1124. Sutter RW and Prevots R. 1994. Vaccine-associated paralytic poliomyelitis among immuno- deficient persons. Infections in Medicine 11(6):429038. Ukkonen P. 1996. Rubella immunity and morbidity: impact of different vaccination programs in Finland 1979-1992. Scandinavian Journal of Infectious Diseases 28(1):31-35. UNAIDS (Joint United Nations Programme on HIV/AIDS). 2000. AIDS Epidemic Update: December 2000. Online. Available at www.unaids.org. van der Loeff MFS and Aaby P. 1999. Towards a better understanding of the epidemiology of HIV-2. AIDS 13 Suppl A:S69-S84. van Rensburg EJ, Engelbrecht S. Mwenda J. Laten JD, Robson BA, Stander T. and Chege GK. 1998. Simian immunodeficiency viruses (SIVs) from eastern and southern Africa: Detec- tion of a SIVagm variant from a chacma baboon. Journal of General Virology 79:1809- 1814. Vidal N. Peeters M, Mulanga-Kabeya C, Nzilambi N. Robertson D, Ilunga ~7, Sema H. Tshimanga K, Bongo B. and Delaporte E. 2000. Unprecedented degree of human immu- nodeficiency virus type 1 (HIV-1 ) group M genetic diversity in the Democratic Republic of Congo suggests that the HIV-1 pandemic originated in central Africa. Journal of Virology 74:10498-10507. Wilkie D, Shaw E, Rotberg F. Morelli G. and Auzel P. 2000. Roads, development, and conservation in the Congo Basin. Conservation Biology 14:1614-1622. Yoshida H. Horie H. Matsuura K, and Miyamura T. 2000. Characterisation of vaccine- derived polioviruses isolated from sewage and river water in Japan. Lancet 356(9240):1461-1463.