Commentary: decline of HIV-2 prevalence in West Africa: good news or bad news?

The HIV pandemic is a continuing global health emergency. At the end of 2005, WHO and UNAIDS estimated that ~40 million people are infected with HIV, with .60% of infections affecting people living in sub-Saharan Africa. The pandemic is very much dominated by infections with HIV type 1 (HIV-1), which was discovered in 1983. In 1987, a second type of virus was identified: HIV type 2 (HIV-2). It rapidly became clear that the epidemic of HIV-2 is limited to relatively few areas, mainly in the Portuguese-speaking part of Africa. The epicentre is located in West Africa, in and around Guinea Bissau. This second HIV epidemic never developed into a pandemic, and an increasing body of evidence indicates that the epidemic is now on the decline: in the countries that were affected most in the beginning, the prevalence of HIV-2 is decreasing, and this is happening even where HIV-1 is on the increase. 1 The article by Schim van der Loeff et al. 2 in this issue of the journal adds to this body of evidence by documenting the decline in HIV-2 in The Gambia, a country in the vicinity of Guinea Bissau. During an observation period of 16 years in the sexually transmitted infections (STI) research clinic of the Medical Research Council (MRC) Laboratories near the capital Banjul, the prevalence of HIV-2 infections decreased from 7 to 4% among clinic attendees, while the prevalence of HIV-1 increased from 4 to 18% and that of dual infections remained stable at ~1%. Importantly, these discordant trends do not appear to be explained by changes over time in the population attending the clinic. The study is a good example of the use of prevalence data in high-risk populations to examine trends over time when the prevalence of the infection is low in the general population. In The Gambia, during the period Schim van der Loeff et al. 2 studied these patients, sentinel surveys among pregnant women showed only a very slight decrease in the prevalence of HIV-2, from 1 to 0.8% and a slight increase in the prevalence of HIV-1 from 0.7 to 1%. The STI clinic of the MRC in The Gambia has, thus, played the role of a magnifying glass. A continued decline in HIV-2 would be both bad and good news for Africa. Bad news, because it is preferable to be infected with HIV-2 rather than HIV-1, and, compared with HIV-1, the HIV-2 virus is both less transmissible through the sexual route, less transmissible vertically from mother to child, and less pathogenic, with a slower progression to AIDS. 1