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OCR for page 447
Monitoring the Spread of
HIV Infection
Charles F. Turner and Robert E. Fay
The current picture of the AIDS epidemic is clouded. In 1986, the
U.S. Public Health Service (PHS, 1986) estimated that approxi-
mately 1.0 to 1.5 million Americans are infected by the HIV virus.
These infected persons, often unaware of their illness, can transmit
the disease to others. (Incleec3, high rates of transmission have been
observed tFischT et al., 1987] even when one partner had been diag-
nosec3 with AIDS and both partners had been counseled about the
dangers of unprotected sexual intercourse.) Current studies suggest
that a large proportion of this infected population may eventually
clevelop AIDS. Thus, in terms of both the spread of the epidemic
and its ultimate cost, the estimate that there are 1.0 to 1.5 million
infected persons presents an even grimmer picture than the actual
number who have developed AIDS (72,645 reported cases in the
This paper was prepared during the spring of 1987 in reaction to proposals for routine
(or mandatory) testing of all hospital patients, marriage license applicants, and others,
in order to obtain "better" epidemiological data on the spread of HIV. The paper was
widely circulated in manuscript form during the summer and fall of 1987. This back-
ground paper is the same manuscript that was originally circulated (save for updating
of numbers and copyediting); thus it does not review recent experiences of the National
Center for Health Statistics and the Research Triangle Institute in their efforts to im-
plement a survey program of the type described in this manuscript. This manuscript is
included as a background paper to the committee's report because it describes the de-
sign problems that must be solved in order to use sample surveys to monitor HIV preva-
lence (see Chapter 1~. Where appropriate, editorial notes have been appended to de-
scribe significant events that have occurred since the paper was prepared in 1987. ED.
At the time the original manuscript was prepared, Charles Turner was a Scholar in Res-
idence and Robert Pay a consultant at the National Research Council.
447
OCR for page 448
448 ~ BACKGROUND PAPERS
United States as of September 5, 198S, according to the Centers for
Disease Control tCDC, 19884~.
Regrettably, there appears to be far less certainty about na-
tional estimates of the number of infected individuals than about
estimates of the number of AIDS cases. At present there exists no
comprehensive system for monitoring the prevalence of human im-
munodeficiency virus (HIV) in the United States or any other nation.
In this paper, we briefly review the uncertainties inherent in
current estimates of the prevalence of HIV infection in the United
States. Subsequently, we identify some of the statistical require-
ments of a reliable system for monitoring the spread of HTV infection
in the U.S. population and the pitfalls involved in reliance upon data
clerivecl from mandatory (or routine) screening of convenient popula-
tions such as applicants for marriage licenses, hospital patients, and
so on. We will then outline some considerations involved in mounting
a practical survey program to provide the raw data required for such
a monitoring system.
While this discussion draws upon the situation that prevailed in
the United States in early 1987, the issues ant] methods we discuss
continue to be of relevance in this country (see Chapter 1), and may
have applications in other nations.
CURRENT ESTIMATES
Various attempts have been made to estimate the total number of
persons infected with HIV. Table 1 presents three such estimates for
the United States, in addition to the PHS estimates. It will be noted
that these estimates vary widely; the largest estimate (2.49 million)
is more than three times larger than the smallest (0.75 million).
We will briefly review some of the sources of uncertainty in these
estimates. We commence by observing that the estimates are highly
discrepant, anal, therefore, insufficient to provide reliable monitoring
of the spread of HIV infection in the population. Furthermore, it
appears that the uncertainties in these estimates follow from the
procedures (and data) that have been used to generate the estimates.
Proportional Mocle]
A key element in the calculations of Curran et al. (1985) and Sivak
and Wormser (1985), for example, was the fraction
NAIDS
NHIV
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MONITORING THE SPREAD OF HIV ~ 449
TABLE 1 Estimates of Number of Persons Infected with HIV in the
United States
Source Population As of Estimate
Curran et al. (1985) Total population May 1985 750 thousands
PHS (1986) IV drug users and June 1986 1.25 millions
homosexual men
PHS (1987) IV drug users and Nov. 1987 1.17 millions
homosexual men
Sivak and Wormser (1985) Total population July 1985 1.76 million
Bees (1987) Total population Dec. 1984 2.49 million
aEstimated as the interval from 0.5 million to I.0 million. The midpoint of the interval is listed
in the table.
bEstimated as the interval from 1.0 million to 1.5 million. The midpoint of the interval is listed
in the table.
CEstimated as the interval from 945,000 to 1.41 million. The midpoint of the interval is listed
in the table.
where NAIDS is the total number of reported AIDS cases and NHIV
is the total number of persons infected with the AIDS virus (HIV).
This fraction can change substantially as an epidemic progresses.
For infectious diseases with Tong latency periods such as AIDS, the
fraction will be zero for a long time after the infection enters a
new population. This occurs because the denominator (number of
infections) can increase rapidly while the numerator (number of di-
agnosed cases) remains zero. Similarly, in a static population that
has been saturated with infection, the denominator of the fraction
may remain constant since all vulnerable members of the popula-
tion have been infected, however, the numerator (cliagnosed cases)
will continue to grow. The instability of this fraction makes its use
problematic, particularly when estimates from one subpopulation are
applied to another subpopulation in which the infection may have
been established for a different length of time.
Multiplicative Mode}
A different procedure was used to derive the most widely quoted
estimate of HIV prevalence in the Unitecl States (presented in the
Public Health Service's [1986] Coolfont report). That report con-
cluded that "by extrapolating all available data, we estimate that
there are between 1 and 1.5 million infected persons in those groups
tIV drug users and homosexual men] at present." Although explicit
calculations are not shown, the Coolfont report indicates that its
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450 ~ BACKGROUND PAPERS
authors estimated that 2.5 million American men between the ages
of 16 and 55 are exclusively homosexual throughout their lives and
5 to 10 million more have some homosexual contact. Similarly, they
estimated (without explicit reference to a source) that 750,000 Amer-
icans inject heroin or other intravenous (IV) drugs at least once a
week and that similar numbers inject drugs less frequently. These
estimates of population size were then multiplied by estimates of the
prevalence of HIV infections among these groups in order to gen-
erate the widely quoted estimate that there are 1.0 to 1.5 million
infected persons in these two groups. (The prevalence rates used in
these calculations were not published but the report states that HIV
prevalence estimates range from 20 to 50 percent for homosexual
men and from 10 to 50 percent for users of {V drugs.)
This procedure (and a hybrid! employed in the calculations of
Sivak and Wormser) is vulnerable to errors of unknown magnitude
in both multiplicands. Note, for example, that the Coolfont estimate
used data collected by Kinsey and coworkers (1948) in the 1940s
in order to estimate the current number of male homosexuals in
the United States. Even 30 years ago, Kinsey's data were widely
regarded as unreliable for use in making such estimates because
the research did not employ probability sampling and because the
respondents were disproportionately drawn from the Midwest and
from the college-eclucated segment of the population (see Terman,
1948; Wallis, 1948; Cochran et al., 1953~. Today, a further leap
of faith is required since the relative size of the (self-reported) ho-
mosexual population must be assumed to have remained unchanged
since the 1940s. Furthermore, we note that estimates of the preva-
lence of HIV infection among homosexual men were not derived from
representative samples.
These factors introduce substantial uncertainty into the resul-
tant estimates of HIV prevalence. Indeed, the multiplication used in
this estimation introduces some unique problems in that errors in the
two multiplicands are related, although the form of the relationship
is not known with any precision. Consider, for example, the defini-
tion of what has loosely been termed the "homosexual population."
If the definition of this population is restricted to persons whose
sexual contacts have been exclusively (or predominantly) homosex-
ual, the population will be smaller than if men who have hacI any
homosexual contacts since the onset of the epidemic are included.
Clearly, however, the estimates of HIV prevalence will also change
(by unknown amounts) depending upon which definition is used.
(May and Anderson t1987] provide useful models representing the
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MONITORING THE SPREAD OF HIV ~ 451
underlying processes.) Parallel uncertainties afflict estimates of the
number of persons who use IV cirugs and the rates of infection in this
subpopulation.
Temporal Distribution Mode}
The most recent estimate -(as of June 1987) of the number of persons
infected with HIV in the United States was produced by Rees (1987)
using a different approach. Rees fit a normal distribution to the
frequency of AIDS cases as a function of time elapsed between the
initial HIV infection and the diagnosis of AIDS. The parameters of
this distribution fib = 15, ~ = 5) were selected on the basis of a
relatively small sample (n = 144) of persons who developed AIDS
from blood transfusions.
The resultant estimate that 2.49 million Americans are infected
with HIV is the largest of the four estimates in Table 1. Indeed,
this estimate is actually more extreme than it appears since it refers
to an earlier date than the other estimates (December 1984) and it
includes only "HIV infections that will result in AIDS over the next
30 years or so" (Rees, 1987:345~.
Although the procedures proposed by Rees open another avenue
for modeling, his estimates must be treater! with caution. Note
that Rees's selection of the normal distribution and its parameters
(p = 15, ~ = 5) was based upon the relative timing of AIDS onset
for those who have developed AIDS from transfusions. The usual
appropriate denominators (i.e., the number of persons infected via
transfusions or the total number of persons infected via transfusion
who will eventually develop AIDS) were not available.
An important implication of Rees's choice of parameters is that
only 5.3 percent of those eventually (1eveloping AIDS (under his
model) would be expected to do so in the first seven years after
infection. This rate appears quite low. Such a low presumed rate of
AIDS onset in the early years after infection implies large multipliers
for each AIDS case observed during this period. It will be several
years before the validity of Rees's assumptions can be fully tested;)
however, the fit of Rees's moclel is particularly poor in the first
year. Rees's fitted values for the first year of infection add up to
20.98 AIDS cases through 1983, compared to S.5 observed cases.
(Fractional observed cases result from Rees's splitting of ambiguous
cases between adjacent years.)
iFor present purposes, the committee concluded that a mean of 8 may be more appro-
priate (see Chapter 2, and Lui et al., 1988~. ED.
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452 ~ BACKGROUND PAPERS
Because of such uncertainties in estimation procedures, it is,
perhaps, not surprising that the estimates shown in Table 1 vary by
a factor of 3. Nor is it surprising that similar uncertainties exist in
other nations. (Rees's model, for example, estimates that 109,000
U.K. residents were infected by mid-1985. Rees observes, however,
that "there seems to~be a general opinion that there were about
30,000 infections in the micIdle of 1986, between a quarter and a
third of my estimate for a year earlier" (Rees, 1987:345~.
PROPOSAL
Inaccurate estimates of the size of the population infected with HTV
are potentially dangerous because (among other reasons) such es-
timates may generate a false sense of security or a false sense of
alarm among those charged with formulating policies to cope with
the epidemic. The 1986 report of the Institute of Medicine/National
Academy of Sciences (TOM/NAS, 1986) concluded that better in-
formation is needed to quantify the number of persons infected with
HIV. This report recommended undertaking "extensive and repeated
surveys of seropositivity to determine the incidence and prevalence of
infection by age, race/ethnicity, geographic area, and sex" (p. 200~.
Epidemiological precedence for this approach is somewhat limited,
but several factors specific to the AIDS epidemic argue for the use of
national sample surveys to obtain these data. Specifically,
. For many infectious diseases there is only a short period
between infection and manifestation of symptoms. For
HIV infection, however, the latency period is several
years; thus, the size of the currently infected population
will remain unknown if one relies solely upon systems
that monitor the number of AIDS cases.
. Sample surveys are typically ineffective at measuring
very rare characteristics unless the specific subpopula-
tion can be identified in advance. Regrettably, it appears
that HIV infection is no longer extremely rare. Sample
survey methods can now give effective estimates of the
size of this population, without the need to explicitly
identify high-risk individuals, such as homosexual males
or intravenous drug users.
Controlling the spread of the AIDS epidemic will depend
critically upon public education programs and other so-
cial interventions. Periodic sample surveys of the preva-
lence of HIV infection in the population will provide
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MONITORING THE SPREAD OF HIV ~ 453
one important indicator of the overall impact of these
programs.
A clearer picture of the magnitude of the epidemic in terms of the
prevalence and incidence of infection and its distribution by age, sex,
geographic area, ethnic group, and other distinguishing characteris-
tics is a prerequisite for designing and targeting intervention efforts
in a rational and cost-effective manner. Without such information,
risk reduction efforts will always be targeted less than optimally. Ad-
ditionally, there will be no way of monitoring on a broad scale how
effective our interventions are in combatting the spread of the epi-
demic. Information on trends in HIV prevalence will also be essential
in planning for the provision of health care.
REQUIREMENTS
Given the range of estimates shown in Table 1, few thoughtful ob-
servers would question the need for more reliable evidence on the
current prevalence of HIV infection in the American population.
With a minimal amount of reflection, most observers would also con-
clude that our need for up-to-date information on HIV prevalence
will continue into the future at least until such time as AIDS ceases
to be a medical threat. Thus, reliable estimates of HIV prevalence
must be obtained at regular intervals in order to track the spread of
the epidemic.
In addition to the requirement that HIV prevalence estimates be
generalizable to the population and regularly updated, it is also desir-
able that the estimates permit disaggregation sufficient for prevalence
(and changes in prevalence over time) to be monitored in particular
demographic (and other) subgroups of the population. Thus, preva-
lence estimates that allow us to separately track the spread of the
infection among unmarried youth, for example, may be as critical as
estimates of prevalence in the entire population.
Even this minimal set of desiderata constrains our data-gathering
activity in several important ways. It requires that our estimates be
.
representative of the entire population that is at risk,
. based on data collected at regular intervals, and
. capable of providing estimates for subpopulations of in-
terest.
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454 ~ BACKGROUND PAPERS
LIMITATIONS OF SAMPLES OF CONVENIENCE
The requirement that prevalence for the American population be
characterized demands that probability samples be drawn from that
population. Samples of convenience even when they are extremely
large are not sufficient to satisfy the preceding list of statistical
needs. Prevalence ciata derived from blood donors, armed forces re-
cruits, persons voluntarily seeking testing, and other special popula-
tions are constitutionally incapable of providing reliable information
on the key question: What is the prevalence of HIV infection in the
general population?
The assumption that prevalence per se or trencis in prevalence in
these special populations will be mirrored in the general population
requires a leap of faith. Given the seriousness of the AIDS epidemic,
such leaps should be discouraged to the extent scientifically possible.
What is needed are better data. With better data such assumptions
can be avoidecl.2
It follows from such considerations that proposals to initiate
mandatory or routine HIV-antibody testing of hospital patients, ap-
plicants for marriage licenses, and so forth will not provide reliable
evidence on the prevalence of HIV infection in the population.3
These convenient populations provide pieces of information from
a larger puzzle, but they provide this information in a manner that
does not readily permit us to reassemble that puzzle. We know,
for example, that hospital patients are a population that is both
substantially older and sicker than the general population. We also
know that some patients may be admitted to hospitals several times
during a brief period and thus may be double- or triple-counted with
such an approach. Healthy aclults, on the other hand, are unlikely to
be hospitalized.
2This is not to say that HIV prevalence data should no longer be collected from these
convenient sources. However, careful studies are needed that document the nature of the
interrelationships (if any) between trends in HIV prevalence in the general population
(estimated from probability samples) and trends in HIV prevalence in convenient (but
self-selected) samples such as blood donors and military recruits. Over time, such studies
might provide an empirical basis for broadening the interpretation of trend data derived
from such special populations.
3Mandatory (or routine) testing of hospital patients has been proposed for monitoring
the course of the epidemic as well as for other purposes, such as "protecting health
care workers." The deficiency of such proposals for epidemiological purposes does not,
of course, imply that such testing might not serve these other purposes. It should be
noted, however, that the recent report of the Institute of Medicine/National Academy
of Sciences (1986:124) recommended that "the question of whether to undergo tHIV]
testing should be a personal health care decision to be made by an individual, ideally
following counselling by health care professionals."
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MONITORING THE SPREAD OF HIV ~ 455
The fact that the hospital population is considerably older than
the general population will doubtlessly introduce a downward bias
in prevalence estimates for any sexually transmitted disease that has
been introduced recently into the population. On the other hand,
persons with AIDS are more likely to be hospitalized than members
of the general population. Furthermore, persons with HIV infection
(but not diagnoses! as having AIDS) may also be more likely to be
hospitalized.4 Such factors would introduce an upward bias in preva-
lence estimates derived from hospital populations (versus estimates
derived from the general population). Finally, one cannot rule out
the possibility that a program of routine screening as a condition of
hospital admission would discourage persons from seeking hospital
care if they believe they may be infected. Besides the terrible im-
pact this would have on the individuals involved, it would introduce
another source of bias into hospital-basec! prevalence measures.
Adjustments of hospital prevalence data might be attempted.
For example, projections to the national population might adjust
the hospital data to match the age distribution of the national pop-
ulation. Even the most ingenious adjustments, however, could not
escape the essential uncertainties of using HIV prevalence rates for
persons who pass through hospitals to estimate rates for persons
who are not hospitalized. Age (or other) adjustments merely restrict
the domain of our assumptions. Thus an age-adjustment procedure
might have us assume that the prevalence of HIV infection among
hospitalized 20- to 29-year-olds, for example, is equivalent to that
for 20- to 29-year-olds in general. The validity of such assumptions
would remain unknown.
Doubtlessly, other restrictions could be introduced. For exam-
ple, (liagnosed AIDS patients might be exclu~led from the hospital
estimates (relying on the CDC case-reporting system for an estimate
of the number of persons who are both HIV-antibody positive and
cliagnosed with AIDS). We might also derive estimates of HIV preva-
lence using only patients admitted for reasons that appear medically
unrelated to HIV infection (e.g., accidental injuries).
Even such seemingly attractive strategies have the potential for
introducing bias. We know, for example, that the propensity of
inclividuals to seek meclical care (given similar symptoms) is corre-
lated with a number of factors including socioeconomic status. The
4Subsequent to the preparation of this draft (in early 1987) Des Jarlais and coworkers
(1988) reported a sharp rise in non-AIDS deaths from pneumonia (not pneumocpstis
carinii), tuberculosis, and endocarditis among HIV-infected IV drug users. ED.
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456 ~ BACKGROUND PAPERS
prevalence of HIV infection is unlikely to be the same in all so-
cial classes, and prevalence estimates derived from hospital samples
necessarily reflect the joint operation of socioeconomic variations in
hospital admission rates and differences across social classes in the
rates of HIV infection. Furthermore, although using "accidental" in-
jury cases may seem to introduce an element of randomness into the
sampling, the probability of accidental injury may not be indepen-
dent of HIV antibody status. For example, {V drug users may be at
greater risk of accidental injury due to impaired cognitive function-
ing (and, therefore, more likely- to appear in a sample of hospitaTizecl
accident victims). Since IV drug users are at risk of HIV infection
due to needle-sharing practices, prevalence rates could be biased by
the joint operation of these two factors.
Inventive readers will be able- to imagine a myriad of other po-
tential biases, methods that might reduce them, further biases that
remain or are newly introduced, and so forth ad infinitum. Such
exercises are limited only by the fertility of one's imagination. The
lesson taught by such exercises is that data from hospital and other
seemingly "convenient" populations can never provide completely
trustworthy evidence of the prevalence of HIV infection in the gen-
eral population.
A concrete example of the limitations of convenience samples is
provided by recent reports in the popular merlin that the prevalence
of infection cletectecT among military recruits in the United States diet
not increase cluring the first 15 months of the military's testing pro-
gram (see, for example, Washington Post, May 15, 1987~.5 Although
this result may appear encouraging, it is actually quite (lifficult to
interpret. We cannot, for example, rule out the possibility that po-
tential military recruits who had engaged in high-risk behaviors were
discouraged from volunteering for military service by publicity about
the mandatory testing of recruits.
Rather than expanding the quixotic enterprise of mandatory
testing to include other "convenient" populations, we believe a pro-
gram of regularly conducted surveys that obtain blood for testing
from probability samples of the national population should be con-
sidered. Such a research program might, if properly (resigned and
executecl, provide a simpler, more reliable, less controversial, and
less costly way of monitoring the spread of HIV infection in the pop-
ulation. In the following pages we outline some of the factors that
should be consiclered and tested in planning such an endeavor.
MAIDS Rate Remains Stable Among U.S. Military Recruits Since Testing Started in
1985; Statistics Puzzle Experts," Washington Post, May 15, 1987:A1.
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MONITORING THE SPREAD OF HIV ~ 457
DESIGN CONSIDERATIONS
Overview
An Oversimple summary of such a survey program might in-
volve the following steps. The population to be surveyed would be
identified. An appropriate probability sampling procedure would be
designedly, and survey procedures would be pretested. During this
preliminary stage, a decision might be made to exclude certain seg-
ments of the total population from the survey (e.g., the elderly and
young children). If the survey used a household sampling frame as
one component of its sample design, sampled households would sub-
sequently receive a letter which would (1) advise them that an inter-
viewer would be calling at their household and (2) inform them of the
purpose of the survey and the safeguards ensuring their anonymity.
Subsequently, a survey interviewer (together with a phIebotomist)
would visit the household. The interviewer would ascertain how
many eligible respondents lived in the household (excluding persons
outside the target population), and a respondent would be selected
at random from among the eligible respondents (if any). The survey
interviewer would~ ask the designated respondent a short series of de-
mographic questions and subsequently request that the respondent
provide a blood sample.
Results of HIV testing of these blood samples would provide a ba-
sis for estimating HIV prevalence in the total population and selected
demographic subgroups.6 Repeated annually (or at another suitable
interval), such surveys might provide a reliable way of monitoring
the spread of HIV infections in the population.
With this oversimplified summary of the procedure in mind let
~ - ~
us review in greater detail some of the important considerations in
the design and execution of such a survey.
Sample Size and Efficiency
For purposes of initial discussion, a sample of approximately 10,000
persons might be considered for HIV screening. We might, in ad-
dition, restrict the sample to persons age 18 to 54. This restriction
would avoid problems of consent below age 18 and reflect the prac-
tical judgment that those 55 and over are likely to contribute little
Gin addition to providing statistical data on prevalence, the stored sera from this program
could provide a valuable resource for future biomedical and epidemiological researchers.
It is important, therefore, that plans be made for long-term storage of the sera collected
in these surveys.
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460 ~ BACKGROUND PAPERS
that the prevalence rate is high. Thus stratum 3, which has a lower
(presumed) prevalence rate, would be allocated 55 percent of the
sample even though it represents 75 percent of the total population.
(The final survey estimates, of course, would employ an appropriate
reweighting to derive national estimates that take account of the
disproportional allocation of the sample to these geographic strata.)
A complex sample design employing only these three geographical
strata could reduce the sampling variance by as much as 24 percent
(compared to a design that allocated the sample in proportion to the
total population of these areas). Further refinements of the sample
design (e.g., taking into account variations in AIDS prevalence by
age, marital status, or race and ethnicity) might reduce the sampling
error even further, perhaps by as much as 40 to 50 percent in total.8
Interview Data
Such a survey should collect, at a minimum, information on age,
race and ethnicity, gender, and marital status from the respondents.
Respondents are almost always willing to supply such information,
so these questions should not threaten respondent cooperation. The
resulting information about the spread of HIV infection in different
geographic areas and among various demographic subgroups could be
of great practical and scientific value. Questions on sexual practices
and {V drug use, on the other hand, would also be highly informative
but could reduce the level of cooperation. Hence, we suggest that
such sensitive questions should be omitted from the national prob-
ability sample proposed here, unless pretesting indicated that these
data could be obtained without decreasing the response rate for the
blood test.
Cost
An HIV survey of the sort outlined is likely to cost less than $400
per sample case (this includes development and processing costs).
Thus, the cost of an annual survey of 10,000 respondents should not
scare must be exercised to ensure that decisions to decrease the sampling error of the
overall estimate do not compromise the ability to obtain adequately reliable estimates
of HIV prevalence in important subpopulations. So, for example, the design should not
allocate sample disproportionately to male respondents, even though men make up the
preponderance of current AIDS cases. (Changes over time in HIV prevalence among
women can provide an important indicator of the extent of heterosexual transmission
in the population. Thus, estimates must be reliable enough to track changes in the
prevalence rate for women.)
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MONITORING THE SPREAD OF HIV ~ 461
exceed $4 million. (Because some of the costs of the survey are fixed,
increasing the sample size by a factor of four should not produce a
fourfold increase in survey costs.)
With the annual federal budget for AIDS projected to exceed $1
billion in the near future and with even larger costs being borne by
individual AIDS victims, state and local governments, insurers, and
the health care system),-the proposed expenditure does not seem
prohibitively expensive. When compared to the costs of mandatory
testing programs that have recently been discussed, the costs of
even a vastly expan(le(1 survey program would be small. The cost
of HIV testing performed at alternative sites with appropriate pre-
and posttest counseling averages approximately $40 per individual
(IOM/NAS, 1986:17~. Even if the cost of blood tests and counseling
for persons applying for marriage licenses were only one quarter of
this amount (i.e., $10 per inclividual), the total annual cost would
be only approximately $50 million in the United States. Mandatory
screening of all patients admitted to hospitals would be much more
expensive (in 1982 there were 39 million such admissions).
Auspices
In most cases, government statistical agencies obtain higher levels of
cooperation from the public than other survey organizations (see, for
example, National Research Council, 1978:42, Table 1~. We think
therefore that an effort by one or more federal statistical agencies
might be a reasonable approach. The possibility of using a private
survey research firm, with a well-established record for quality, to
carry out this work should not be excluded. Incleec3, there are cir-
cumstances (see below) in which such a strategy would be mandatory.
The possibility of adding HIV testing to an ongoing survey should
also not be excluded. Survey designers, however, would need to be
sensitive to possible adverse reaction to such "piggybacking." Sur-
veys are frequently perceived as involving an implicit social contract
between the survey taker ant! the respondent. Respondents con-
tacted to take part in an immunological health survey may more
readily appreciate the importance of blood testing than respondents
in another survey who might view an adcled request for a blood
sample as irrelevant to their original commitment. Furthermore, re-
searchers gathering survey data for other purposes might justifiably
worry about the effects that such piggybacking might have on their
own data collection.
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Pretesting
Most surveys require a period of "pretesting" to evaluate the effect
of the questionnaire and other survey procedures on the quality of
the data to be collected. Such testing would be especially critical in
developing an effective strategy for this survey, given the emotionally
charged atmosphere surrounding public discussions of AIDS. Thus,
greeting respondents at the door with "GoocT afternoon. May T please
have a sample of your blood?" is unlikely to yield the desired re-
sults. A carefully structured interview, conclucted by a well-trained
interviewer and followed by blood sampling by a quaTifiecl health pro-
fessional, may succeed. A letter stating the purposes an importance
of the survey and detailing privacy ant! confidentiality provisions may
help to set the stage for the interview, if mailect to the respondents
in advance of the initial contact. Supplemental reading material (or
a videotaped presentation) on the importance of the survey may also
help to inform respondents of their expecter! roles. We would antic-
ipate that many details of the survey strategy would be refined and
tested in a pilot phase (prior to the formal data-gathering phase of
the survey). We will not attempt to anticipate all of those details
here. However, three issues are sufficiently important to merit fur-
ther discussion: (1) assuring the completeness of survey coverage,
(2) guaranteeing the confidentiality of survey data, and (3) providing
blood test results to interested respondents. We briefly discuss each
of these issues below.
Aclequacy of Survey Coverage
Some readers may wonder whether it is feasible to achieve high
levels of respondent cooperation in surveys that ask randomly se-
lectec! respondents to provide blood samples. Although it may seem
unusual to request that survey respondents provide blood samples,
precedent suggests that such surveys are possible and do produce
relatively high levels of cooperation. The National Health and Nutri-
tion Examination Survey (NHANES), for example, performs physical
examinations and administers a range of tests (including blood tests)
to large probability samples of the U.S. population (approximately
21,000 examinations were performed in the most recent surveys).
The recent experience of the NHANES program has been that ap-
proximately 91 percent of the public consent to a lengthy household
health interview, and approximately 73 percent subsequently report
to the survey's medical facility for examinations. (It should be noted
that although NHANES could provide HIV prevalence estimates, its
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MONITORING THE SPREAD OF HIV ~ 463
sampling schedule would not yield timely estimates. According to
NCHS personnel, national estimates from the next NHANES survey
will not be available until 1991.)
Nonresponse to the NHANES medical examination occurs for
various reasons. Empirical studies suggest that nonresponse rates do
not vary substantially by sex or race. Examination rates were 71.8
percent for females, 74.4 percent for mates, 72.7 percent for whites,
75.6 percent for blacks, and 74.3 percent for other racial groups.
There is, however, a monotonic decrease in examination rates with in-
creasing age. Medical examinations were completed with 81 percent
of designated respondents aged 6 months to 17 years, 74 percent of
18- to 34-year-olds, 69 percent of 35- to 54-year-olds, and 64 percent
of persons aged 55 or oilier. The NCHS (1982:Appendix 1) reports,
nonetheless, that various comparative studies indicate that respon-
dents who do not undergo medical examinations in the NHANES pro-
gram have self-reported health characteristics similar to respondents
who do undergo those examinations. Similarly, Forthofer (1983:507)
finds that after standard NCHS nonresponse and posts/ratification
adjustments, "there is excellent agreement in the marginal distribu-
tion of variables between NHANES-~] for examined persons and the
1976 National Health Interview Survey (NHIS)" (which achieved a
96 percent response rate).
The response rates from the NHANES demonstrate that most
respondents are willing to provide blood samples in a national health
survey. Three features of the proposed HIV survey might encourage
even higher response rates than those obtained in the NHANES.
First, the survey focuses on a health problem of pressing national
importance. Second, restricting the survey interview to a few ques-
tions will minimize the burclen on respondents. Finally, by cirawing
blood samples in the respondents' homes, the survey eliminates the
need for respondents to travel to a central site for testing.
Although these factors encourage the belief that blood samples
can be obtained from a substantial proportion of eligible respondents,
it must be recognized that HIV blood testing is an emotionally and
politically charged issue. As noted above, careful pretesting (and
other measures) will be required to explore the feasibility and refine
the details of any such measurement program. It is vital that sufficient
time and resources be devoted to these preliminary research activities
because they will have a major impact on the quality and usefulness
of the resultant data.
Several potential impediments to adequate survey coverage can
be identified at the outset. First, household-based sampling frames
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464 ~ BACKGROUND PAPERS
will not obtain data from persons who are not attached to a house-
hold. Such sampling frames will miss persons who are homeless
or who live in prisons, hospitals, and other institutions. An HTV
prevalence survey using a household sampling frame will, therefore,
underrepresent some important subgroups (e.g., current or past IV
drug users) if those subgroups are disproportionately homeless or
institutionalized. In designing a survey, separate estimates must be
obtained for important subpopulations that will be missed or un-
derrepresented in household surveys (e.g., by drawing samples from
prisoners or persons in drug treatment centers).
More troubling than the Toss of identifiable segments of the popu-
lation from the sampling frame is the loss of an unidentifiahie fraction
of the population because some respondents choose not to cooperate
with the survey. As this fraction increases, the claim that the survey
"represents" the general population is weakened. For our purposes,
this threat is particularly serious because we are seeking to assess a
relatively uncommon characteristic (HIV seropositivity) in a situa-
tion where one might expect noncooperation to be correlates! with
the respondent's known or suspected seropositivity. That is to say,
persons who know or suspect that they are infected may be more
likely to refuse to participate in the survey.9
Minimizing such refusals will be major challenge. Indeed, it must
be recognized at the outset that it is both impossible to eliminate
refusals entirely and likely, even under the best of circumstances, that
these refusals will introduce some bias. As a consequence, estimation
of HIV prevalence will require some imputation of missing data.~°
Furthermore, it is important to recognize that the survey effort
could fad! completely because of substantial noncooperation by mem-
bers of the public. Hopefully, this failure would be detected during
pretesting, so that the cost of failing wouicT be minimized.
Guaranteeing Anonymity
A first step in reducing noncooperation is to ensure that both in fact
and in the perceptions of potential respondents, the survey poses no
actual or potential threat. Given the level of public concern about
9Subsequent to the drafting of this paper, Hull and coworkers (1988) Reported results
demonstrating this phenomenon in a situation in which confidential testing was offered
to clients of an STD clinic ED.
1OTo assist in this process, the suggestion has been made that a recent sample of re-
spondents from another survey (e.g., the Health Interview Survey) might profitably be
used. Data from the previous interview would assist in the analysis of the biases caused
by nonresponse to the HIV survey.
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MONITORING THE SPREAD OF HIV ~ 465
AIDS and the particular vuInerabilities of the groups presently ex-
periencing the bulk of AIDS cases, we believe it is essential that
bloocT test data be collected in a way that ensures the anonymity
(not merely the confidentiality) of these data. Even if persons fa-
miliar with the excellent record of the federal statistical system in
preserving confidentiality are reassured by past history, it is unlikely
that their trust will be shared by all members of the public. This
may be particularly true for respondents who feel vulnerable to dis-
criminatory actions as a result of being identified as carriers of the
AIDS virus.
Obviously, as a practical matter one must know the addresses
of respondents in order to draw a sample. Thus, total anonymity
of the respondents is impossible. We would propose, however, that
the blood test results be obtained in a manner that guarantees those
results will be anonymous. Procedures to achieve this end might
include the following:
.
To the fullest extent possible, no linkage should be allowed at
any stage of the fieldwork or data processing between addresses
and the identification numbers assigned to respondents.
As soon as practical after completion of the relevant portion
of fieldwork, all materials that might allow the identification of
actresses should be destroyed. If practicable, this should be
done on a continual basis at the local sites.
No information allowing linkages between addresses and iclenti-
fication numbers should be transmitted from the fieldwork sites
.
to the central site.
All blood samples should be identified only by encrypted i(lenti-
fication numbers.
All sampling records should be destroyed as soon as practical
after completion of the relevant portion of the fieldwork.
Blood samples should be tested by one central laboratory, and its
testing procedures shouIcI be subject to strict security and quality
control measures. Tests should not be performed until sampling,
fieldwork, and other survey records have been destroyed. The
results of the blood testing should be transmitted only to the
central site (not to the fieldwork sites).
All interview ciata obtained from respondents (e.g., age, sex,
race) when entered into a data file and combined with all remain-
ing geographic information, must conform to present stanciards
for public use tapes released by the Bureau of the Census.
.
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466 ~ BACKGROUND PAPERS
The foregoing are initial suggestions for the types of precautions
that will be required to guarantee anc Amity of the data records.
The ultimate test will be whether or not the most knowledgeable
members of the sampling, fieldwork, and statistical staffs of the
survey agency can state with certainty that they would be unable to
identify the ciata derived from any individual respondent (using all
records that remained available from the survey, together with the
data file). This litmus test should be satisfied prior to the merging of
the encrypted HIV test results with the information obtained in the
· , ~
survey interview.
Perception of Anonymity
In addition to guaranteeing the anonymity of the survey data records,
it is crucial that the public at large, all respondents to the survey,
and key social groups (especially those presently bearing the brunt
of the AIDS epidemic) be convinced that the design of the survey
preclucles any threat to the respondents in the survey. Moreover,
each of these audiences must be convinced that the resultant data
will play an important role in the U.S. attempt to cope with the
AIDS epidemic.
We suggest that representatives of these different audiences
shouIcl be appointed to review the design and execution of the sur-
vey. This group shouicl be provided with access to survey sites and
appropriate staff to allow them to undertake an independent review
to ensure that the anonymity of survey respondents has been fully
protected. Their certification of this fact should be made a prereq-
uisite for the merging of the encrypted blood test results with the
other survey data.
To further ensure that the anonymity of the data gathering
would not be compromised (anal to further reassure the respondents
on this point), it may be clesirable for the survey to be performed in
cooperation with the Bureau of the Census so that the protections
of Title 13 of the U.S. Code would apply (Title 13 provides crimi-
nal penalties under federal law for violations of the confidentiality
of census data). Careful pretesting shout be undertaken, however,
to ascertain whether the "reassurance" provided to respondents by
such legal protections will outweigh any reluctance of respondents
to provide sensitive information directly to employees of a federal
statistical agency. If a substantial fraction of the population (partic-
ularly groups with atypical HIV prevalence rates) feels threatened by
the request to provide blood samples to interviewers from a federal
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MONITORING THE SPREAD OF HIV ~ 467
statistical agency, it may be advisable for data gathering to be done
by a nongovernmental survey research organization.
Informing Respondents of Test Results
There is an inevitable tension between the desire to provide fail-safe
protection of a respondent's anonymity and the desire to provide
test results to individuals who want this information. The design
we have outlined precludes direct notification of indiviclual survey
respondents (because names are never obtained and acldress records
would be destroyed). Nonetheless, there are ways in which test re-
sults might be provided without directly recontacting respondents.
We believe, however, that it would be inadvisable to provicle such
information without appropriate counseling and support services.
Counseling, of course, requires personal contacts that would jeopar-
dize anonymity.
Alternative procedures should be considered that would protect
the anonymity of the survey data while permitting respondents to
use the blood samples drawn in the survey to obtain information
about their HIV status (together with appropriate counseling). Any
proposed procedure should have the following characteristics:
. It should eliminate the need to ask respondents for their
names.
· It should eliminate the need to retain survey records
that might permit the personal identification of blood
samples for any respondent. It thereby ensures that all
such records can be destroyed prior to testing of the
blood samples.
It should ensure that respondents who wish to know
their HIV status can use the sample drawn during the
survey to find that out, and it should do so without
requiring that respondents reveal to the interviewer their
interest in learning the test results.
. It should not force blood test results upon respondents
who may not wish to know their status.
. It should guarantee that appropriate counseling will be
provicled along with blood test results.
.
This list of requirements might be user! as a preliminary screening
device for vetting proposed procedures. Survey development and
pretesting are likely to reveal other requirements that should be met.
One approach that satisfies these preliminary requirements would
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468 ~ BACKGROUND PAPERS
provide respondents with an encrypted identification number to be
presenter! at a local medical facility if they wished to learn their blood
test results. (They wouIc3, of course, be counseled at that time.) The
encrypted number could be given to respondents in a seated envelope
so that it would not be seen by the interviewer. Obviously, this
number shout be different from the serial number on the survey
interview form or the encrypted! identification number placed on the
blood sample. This number might also be printed on a distinctive
form so that it could not be copied, and the respondent might be
instructed to place some personal information (e.g., height, eye color,
sex) on the form so that it could not be used by anyone else to obtain
the respondent's blood test results.
An alternative strategy might involve dividing blood samples
in half. One half of the sample wouIcT be taken for testing by the
survey organization. The other half of the blood sample would be left
with respondents, who would be given the address of a local medical
facility that would provide free testing of the blood sample together
with appropriate counseling. Respondents conic! then choose for
themselves whether they wished to know their HIV test results. (HIV
tests presently under development may make this alternative quite
practical because future tests may require only droplets of blood
lanced from a finger and blotted on filter paper.)
The foregoing suggestions are only two of a large number of
possible strategies. There is no need to commit to any particular
strategy prior to beginning the design and pretesting of the survey.
However, the five attributes listed above are a reasonable starting
point for vetting any strategies proposed for informing respondents
of their test results. Further consideration might also be given to
the option of not providing test result data in order to provide more
assured protection of the anonymity of the blood test results.
CONCI`USION
Public and scientific awareness of the impending magnitude of the
AIDS epidemic has increased substantially in recent years. Yet an as-
pect of unreality remains. Thus, many individuals and whole nations
must face critical decisions without trustworthy information on the
prevalence of the AIDS infection in the population. If the infection
is as pervasive as the present fragmentary evidence suggests, a se-
ries of annual estimates based upon representative national samples
can serve to remind us all of how real and threatening the AIDS
epidemic will become in the near future. Such estimates will permit
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MONITORING THE SPREAD OF HIV ~ 469
more reliable tracking of the spread of the infection, and they will
facilitate evaluation of the overall impact of educational and other
interventions clesignecl to retard the spread of the AIDS epidemic.
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Representative terms from entire chapter:
hiv prevalence
