Demographic Variation in the Epidemic
The proportion of HIV infections found in women is a function of both biological and behavioral factors and varies across Africa. In household-based surveys representing the adult population, ratios of infection in women compared with men range from below 1 to over 2, but in the majority of populations, women are between 50 and 100 percent more likely to be HIV-infected (Stover et al., 2006). This, in turn, leads to infants and children of HIV-positive women being a high-risk group. See Appendix A for a discussion of the impacts of mother-to-child transmission and prevention of mother-to-child transmission (PMTCT) prophylaxis on infants and children.
Biologically, two factors influence the relative risk of women compared with men: transmission probability and survival once infected. Evidence is inconsistent as to the relative probability of transmission from men to women compared with that from women to men. In a systematic review of transmission probabilities per sex act, the risk of transmission from men to women of 0.08 percent [95 percent confidence interval [CI] 0.06–0.11] was found to be twice that from women to men of 0.04 percent [95 percent CI 0.01–0.14] in high-income countries. In low-income countries, the relationship was reversed, with the risk of transmission from women to men being 0.38 percent [95 percent CI 0.13–1.10], compared with 0.30 percent from men to women [95 percent CI 0.14–0.63] (Boily et al., 2009). However, these figures represent the risk in observed couples and may not capture the risk in young women, which may be greater because the cervix is immature (Glynn et al., 2001; Moss et al., 1991). In addition, other factors, such as pregnancy and the impact of other sexually transmitted infections (STIs), may differentially impact the risk experienced by women in comparison with men: pregnancy was found to increase the risk of acquisition in women in some studies
(Gray et al., 2005), while STIs were found to have a greater impact on the risk of men acquiring infection (Rottingen et al., 2001). The question arises of whether these differences in transmission rates might be driven by the nature of the sampling rather than by behavior or biology. Further research in this area is needed to determine the demographic differences in transmission suggested by these data.
Differential survival once HIV-infected can also affect the relative prevalence of HIV infection in men and women. In African cohort studies, age-specific mortality does not differ between men and women, but because women are on average infected at a younger age, and duration from infection to death decreases with age, women on average survive longer than men (Todd et al., 2007; Zaba et al., 2007). Additionally, access to and uptake of treatment are better for women, in part because of antenatal clinic testing and in part because women are more likely to seek care (Cooke et al., 2010). The result is better survival as well as greater relative prevalence of HIV among women.
The distribution of risk behaviors among men and women combines with biological factors in determining the relative risk of women compared with men. Where HIV risk is concentrated in high-risk groups—men who have sex with men, injecting drug users, and commercial sex workers—men are more likely to be infected than women. In the latter case, there is a small very high-risk fraction of sex workers and a larger pool of clients who are at risk (Alary and Lowndes, 2004; Vuylsteke et al., 2003). It is when risk is widespread throughout the general population that HIV is found more often in women than in men (Barnighausen et al., 2008; Boerma et al., 2003). Early in the HIV/AIDS epidemic in Africa, men were more likely to be infected and die (Gregson and Garnett, 2000), but as the epidemic progressed, women began to predominate, and this pattern is likely to continue.
As noted, part of the reason more cases of HIV infection are found in women is that women are on average infected at a younger age than men (Stover et al., 2010), and the population distribution in Africa is a pyramid shape, with population growth resulting in a young population. The highest prevalences are found among 15- to 24-year-old women and 24- to 35-year old men (Stover et al., 2010). This pattern appears to be a function of the pattern of risk by age and the age difference between sexual partners, as well as the increased susceptibility to infection experienced by young women (Glynn et al., 2001; Gregson et al., 2002).
With the spread of HIV, those initially infected age, and the mean age of prevalence of infection should rise until a stable age distribution is achieved. With declining HIV incidence in the young, who engage in reduced risk behavior (International Group on Analysis of Trends in HIV Prevalence and Behaviours in Young People in Countries Most Affected by HIV, 2010), as well as increased survival of those on treatment (Jahn et al., 2008), the average age of those infected will rise further.
HIV/AIDS-associated mortality among young adults has led to an increase in numbers of orphans (Grassly and Timaeus, 2005; Gregson et al., 1994), with a
higher fraction of children losing both parents than would be expected if parental deaths were unlinked (Grassly and Timaeus, 2005). Over time, the disadvantage experienced by children losing their parents has translated into a higher risk of pregnancy, STIs, and HIV infection as the orphaned children have reached adolescence (Birdthistle et al., 2008; Floyd et al., 2007; Gregson et al., 2005; Robertson et al., 2010).
Alary, M., and C. M. Lowndes. 2004. The central role of clients of female sex workers in the dynamics of heterosexual HIV transmission in sub-Saharan Africa. AIDS 18(6):945-947.
Barnighausen, T., F. Tanser, Z. Gqwede, C. Mbizana, K. Herbst, and M. L. Newell. 2008. High HIV incidence in a community with high HIV prevalence in rural South Africa: Findings from a prospective population-based study. AIDS 22(1):139-144.
Birdthistle, I. J., S. Floyd, A. Machingura, N. Mudziwapasi, S. Gregson, and J. R. Glynn. 2008. From affected to infected? Orphanhood and HIV risk among female adolescents in urban Zimbabwe. AIDS 22(6):759-766.
Boerma, J. T., S. Gregson, C. Nyamukapa, and M. Urassa. 2003. Understanding the uneven spread of HIV within Africa: Comparative study of biologic, behavioral, and contextual factors in rural populations in Tanzania and Zimbabwe. Sexually Transmitted Diseases 30(10):779-787.
Boily, M. C., R. F. Baggaley, L. Wang, B. Masse, R. G. White, R. J. Hayes, and M. Alary. 2009. Heterosexual risk of HIV-1 infection per sexual act: Systematic review and meta-analysis of observational studies. The Lancet Infectious Diseases 9(2):118-129.
Cooke, G. S., F. C. Tanser, T. W. Bärnighausen, and M. L. Newell. 2010. Population uptake of antiretroviral treatment through primary care in rural South Africa. BMC Public Health 585.
Floyd, S., A. C. Crampin, J. R. Glynn, N. Madise, M. Mwenebabu, S. Mnkhondia, B. Ngwira, B. Zaba, and P. E. Fine. 2007. The social and economic impact of parental HIV on children in northern Malawi: Retrospective population-based cohort study. AIDS Care 19(6):781-790.
Glynn, J. R., M. Carael, B. Auvert, M. Kahindo, J. Chege, R. Musonda, F. Kaona, and A. Buvé. 2001. Why do young women have a much higher prevalence of HIV than young men? A study in Kisumu, Kenya and Ndola, Zambia. AIDS 15(Suppl. 4):S51-S60.
Grassly, N. C., and I. M. Timaeus. 2005. Methods to estimate the number of orphans as a result of AIDS and other causes in sub-Saharan Africa. Journal of Acquired Immune Deficiency Syndromes 39(3):365-375.
Gray, R. H., X. Li, G. Kigozi, D. Serwadda, H. Brahmbhatt, F. Wabwire-Mangen, F. Nalugoda, M. Kiddugavu, N. Sewankambo, T. C. Quinn, S. J. Reynolds, and M. J. Wawer. 2005. Increased risk of incident HIV during pregnancy in Rakai, Uganda: A prospective study. The Lancet 366(9492):1182-1188.
Gregson, S., G. P. Garnett, R. Shakespeare, G. Foster, and R. M. Anderson. 1994. Mathematical model simulations and empirical estimates of the impact of HIV-1 on orphanhood in sub-Sarahan Africa. Health Transition Review 4(Suppl. 1):S65-S92.
Gregson, S., and G. P. Garnett. 2000. Contrasting gender differentials in HIV-1 prevalence and associated mortality increase in eastern and southern Africa: Artefact of data or natural course of epidemics? AIDS 14(Suppl. 3):S85-S99.
Gregson, S., C. A. Nyamukapa, G. P. Garnett, P. R. Mason, T. Zhuwau, M. Carael, S. K. Chandiwana, and R. M. Anderson. 2002. Sexual mixing patterns and sex-differentials in teenage exposure to HIV infection in rural Zimbabwe. The Lancet 359(9321):1896-1903.
Gregson, S., C. A. Nyamukapa, G. P. Garnett, M. Wambe, J. J. Lewis, P. R. Mason, S. K. Chandiwana, and R. M. Anderson. 2005. HIV infection and reproductive health in teenage women orphaned and made vulnerable by AIDS in Zimbabwe. AIDS Care 17(7):785-794.
International Group on Analysis of Trends in HIV Prevalence and Behaviours in Young People in Countries Most Affected by HIV. 2010. Trends in HIV prevalence and sexual behaviour among young people aged 15-24 years in countries most affected by HIV. Sexually Transmitted Infections 86(Suppl 2):ii72-ii83.
Jahn, A., S. Floyd, A. C. Crampin, F. Mwaungulu, H. Mvula, F. Munthali, N. McGrath, J. Mwafilaso, V. Mwinuka, B. Mangongo, P. E. Fine, B. Zaba, and J. R. Glynn. 2008. Population-level effect of HIV on adult mortality and early evidence of reversal after introduction of antiretroviral therapy in Malawi. The Lancet 371(9624):1603-1611.
Moss, G. B., D. Clemetson, L. D’Costa, F. A. Plummer, J. O. Ndinya-Achola, M. Reilly, K. K. Holmes, P. Piot, G. M. Maitha, S. L. Hillier, N. C. Kiviat, C. W. Cameron, I. A. Wamola, and J. K. Kreiss. 1991. Association of cervical ectopy with heterosexual transmission of human immunodeficiency virus: Results of a study of couples in Nairobi, Kenya. The Journal of Infectious Diseases 164(3):588-591.
Robertson, L., S. Gregson, and G. P. Garnett. 2010. Sexual risk among orphaned adolescents: Is country-level HIV prevalence an important factor? AIDS Care 22(8):927-938.
Rottingen, J. A., D. W. Cameron, and G. P. Garnett. 2001. A systematic review of the epidemiologic interactions between classic sexually transmitted diseases and HIV: How much really is known? Sexually Transmitted Diseases 28(10):579-597.
Stover, J., N. Walker, N. C. Grassly, and M. Marston. 2006. Projecting the demographic impact of AIDS and the number of people in need of treatment: Updates to the Spectrum projection package. Sexually Transmitted Infections 82(Suppl. 3):iii45-iii50.
Stover, J., P. Johnson, T. Hallett, M. Marston, R. Becquet, and I. M. Timaeus. 2010. The Spectrum projection package: Improvements in estimating incidence by age and sex, mother-to-child transmission, HIV progression in children and double orphans. Sexually Transmitted Infections 86(Suppl 2):ii16-ii21.
Todd, J., J. R. Glynn, M. Marston, T. Lutalo, S. Biraro, W. Mwita, V. Suriyanon, R. Rangsin, K. E. Nelson, P. Sonnenberg, D. Fitzgerald, E. Karita, and B. Zaba. 2007. Time from HIV seroconversion to death: A collaborative analysis of eight studies in six low and middle-income countries before highly active antiretroviral therapy. AIDS 21(Suppl. 6):S55-S63.
Vuylsteke, B. L., P. D. Ghys, M. Traore, Y. Konan, G. Mah-Bi, C. Maurice, D. Soroh, J. N. Diarra, T. H. Roels, and M. Laga. 2003. HIV prevalence and risk behavior among clients of female sex workers in Abidjan, Côte d’Ivoire. AIDS 17(11):1691-1694.
Zaba, B., M. Marston, A. C. Crampin, R. Isingo, S. Biraro, T. Bärnighausen, B. Lopman, T. Lutalo, J. R. Glynn, and J. Todd. 2007. Age-specific mortality patterns in HIV-infected individuals: A comparative analysis of African community study data. AIDS 21(Suppl. 6):S87-S96.