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Has global HIV incidence peaked?
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Commentary
The Lancet March 30, 2006
James D Shelton a, Daniel T Halperin b and David Wilson c
Rajesh Kumar and colleagues, in today's Lancet, document a declining prevalence of HIV-1 in young adults in South India.1 This evidence, along with other recent positive findings, provides impetus to re-examine the HIV pandemic. Many readers may be surprised that available evidence indicates that HIV incidence (rate of new infections) has peaked overall in Africa-indeed, it did so some years ago. And even the lagging indicator of HIV prevalence (rate of existing infections) has also declined in an increasingly impressive number of African settings and elsewhere.
One reason that this decline in incidence is poorly appreciated is because attention often focuses on prevalence. To understand the interplay of HIV incidence and prevalence, Kenya is illustrative (figure 1). According to modelling by the US Census Bureau, new infections peaked around 1992-93.2 However, because of the many years of latency between HIV infection and mortality, prevalence continued to rise even as incidence fell-and peaked around 1997 when mortality finally rose to match incidence. Clearly, incidence is key to assessing efforts to prevent the estimated 4¥9 million new infections annually.3 But incidence is hard to measure. It relies on modelling derived from prevalence data and limited cohort studies. Alternatively, prevalence in young people can provide a surrogate for incidence by generally reflecting newer infections.
The generalised hyperepidemics of southern and, to a lesser extent, some of eastern Africa form the epicentre of the global AIDS pandemic. Their genesis probably reflects a "perfect storm" of conducive factors, including limited male circumcision and significant networks of concurrent sexual partnerships involving both men and women, which fuelled a chain reaction of rapid transmission from one highly infectious, newly-infected person to another.4 Other African countries, especially in west Africa, where circumcision is widespread, display markedly lower and more stable epidemics.5
Modelled incidence patterns for some key African countries are shown in figure 2.2 Incidence in Uganda and Zambia peaked around 1987, in Botswana and Lesotho around 1994, and in South Africa around 1997. Only Mozambique shows rising incidence. Nigeria is rather representative of the low-level countries, displaying continued low and gradually declining incidence. It is wise to be cautious about modelling data, but the natural history of HIV is understood well enough that changing assumptions would be unlikely to change the overall pattern. And importantly, even prevalence declines in generalised epidemics are now accumulating. There are noteworthy declines in HIV prevalence reported in Uganda,6 Kenya,7 and Zimbabwe,8 along with urban Rwanda,9 Malawi,10 Ethiopia,11 and Haiti.12
What caused these incidence declines? Most important surely are purely epidemiological phenomena-those most susceptible become infected first (because of sexual behaviour and networks) and the susceptible pool shrinks. Moreover, at some point the chain reaction derived from the infectiousness of newly-infected people subsides. There has also been important behavioural change, notably in Uganda,6 Kenya,7 and Zimbabwe.8 Some safer behaviour was probably adopted by people on their own. Knowledge of the causes and consequences of AIDS, albeit imperfect, is widely known. But some behavioural change was probably related to programmatic prevention efforts. The incidence curves for Uganda and Kenya show proportionately greater and more sustained declines, supporting the role of behavioural change. Conversely, Botswana and Lesotho show secondary plateaus (and perhaps even a later slow increase) at appallingly high levels, suggesting that epidemic dynamics were almost entirely responsible for the decline, with little or no behavioural change.
What lessons emerge? First, given the considerable time lag between incidence and prevalence, assessing prevention activities on the basis of prevalence is a perilous undertaking-literally "behind the curve". Rather, incidence is the gold standard for assessing prevention, based on more systematic modelling and better measurement techniques. Second, the "perfect storm" conditions that spawned broad heterosexual hyperepidemics in Africa in the 1980-90s are probably uncommon elsewhere. Rather, we face lower-level but often tenacious epidemics. Lastly, we gain confidence in prevention by behavioural change. Over a decade ago, a strategy was developed to contain concentrated epidemics, with success in Thailand13 and Cambodia.6 We are now also seeing apparent success in some generalised epidemics in Africa. Our understanding of what drives widespread HIV transmission, including the pivotal role of concurrent partnerships, is increasing, and our experience in promoting reduction in the number of partners and effective use of condoms continues to accumulate.14
What of the overall global epidemic? Sub-Saharan Africa and India together harbour some three-fourths of HIV-infected people.3 Kumar and colleagues' finding of declining prevalence in young women and male attendees at sexually transmitted disease clinics, and stable prevalence in older women in south India, along with fairly stable prevalence in the less-affected north, provide encouraging evidence that India's HIV incidence has also peaked. Also, recently released numbers from China present relatively modest and probably fairly stable levels of new infections.15 Thus in all likelihood, new HIV infections have peaked globally.
While celebrating this progress, we must remain ever vigilant. Incidence is still unacceptably high (especially in southern Africa) and can reverse course if gains from behavioural changes slacken. We have both gloom and hope. Too many are infected and too many will be. But the overall global decline in incidence, along with growing numbers of countries with declining HIV prevalence, confer confidence that prevention efforts can work to turn the tide against AIDS at last.
The findings, interpretations, and conclusions expressed in this Comment are entirely those of the authors, and do not necessarily represent the view of USAID, the World Bank, its Executive Directors, or the countries they represent. We declare that we have no conflict of interest.
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