Individuals who believe themselves to be HIV-negative cannot rely on serosorting as an HIV prevention strategy, according to researchers from the university of California, writing in the May 31st edition of AIDS. The investigators developed a mathematical model that took into account the risk of HIV transmission posed by individuals who believe they are HIV-negative, but actually have acute HIV infection. They found that HIV-negative men were at much higher risk if they chose to have unprotected sex with men they believed to be HIV-negative, but who were actually seroconverting, than if they had unprotected sex with HIV-positive men on successful treatment.
Infectiousness is highest during the first few months of HIV infection and serosorting behaviours by individuals with acute HIV infection may, the investigators conclude, actually be contributing to the spread of HIV.
A recent study found that approximately half of all HIV infections are due to transmission from recently infected undiagnosed individuals, and studies conducted in Australia and the US suggest that gay men who believe themselves to be HIV-negative are having unprotected sex with partners who have declared themselves to be similarly HIV-uninfected.
There is also anecdotal evidence from gay dating websites that increasing numbers of men are reporting they are “HIV and STD clean” and are insisting that their partners have the same perceived disease status. The results of the latest study suggest that rather than protecting their health, they may unwittingly be risking both their own and other men’s health.
Correct and consistent use of condoms is an effective method of HIV prevention. But several studies have revealed that some individuals are selecting their sexual partners on the basis of reported HIV status to either minimise their risks of HIV infection or, if they are HIV-positive, the risks of onward transmission of HIV.
If condoms are not used for penetrative sex, it is essential that HIV status is accurately reported. However, some individuals who say that they are HIV-negative (even if they have had a recent HIV test) may have been recently infected with HIV.
Because the effectiveness of serosorting depends on accurate disclosure, investigators modelled the risks of HIV transmission for individuals disclosing that they were ‘HIV-negative’ and ‘HIV-positive.’
The model for ‘HIV-negative’ individuals took into account the transmission risks involved at different stages of HIV infection. Unsurprisingly, the investigators calculated that the transmission risk per 10,000 for individuals who were actually HIV-negative was zero. For individuals who believed themselves to be HIV-negative, but who had chronic, asymptomatic HIV infection, the transmission risk was calculated as seven in 10,000. This risk increased for individuals thinking themselves HIV-negative, but with advanced HIV infection to 36 in 10,000. The highest transmission risk of all was for ‘HIV-negative’ individuals with acute HIV infection who had a transmission risk of 82 in 10,000.
The investigators then modelled the risk of HIV transmission from individuals who knew they were HIV-positive and disclosed this status. They calculated that asymptomatic individuals not taking antiretroviral treatment had a transmission risk of 7 per 10,000. This risk fell to 1 per 10,000 for individuals taking stable and effective antiretroviral therapy. However, for individuals with advanced HIV disease with risk was 36 per 10,000.
“During the period of recent HIV infection, individuals typically have a much higher viral burden than they do for most of the time they are infected”, write the investigators. They add, “as the proportion of recently infected potential sex partners in the population increases, the effectiveness of disclosure for preventing HIV transmission by serosorting decreases.”
Indeed, the investigators suggest that “as a result of differences in the proportion of potential partners who are recently infected, the risk of acquiring HIV from one sexual exposure with one randomly selected ‘HIV-negative’ discloser from a high-risk population may actually be greater than the risk associated with a randomly selected HIV-positive discloser.”
They note that few in their cohort of HIV-positive individuals were taking potent HIV therapy so they “conservatively” estimated transmission to be one-seventh of that from untreated individuals with asymptomatic infection.
“Our conservative calculations show that serosorting based on disclosure is not likely to be an effective prevention strategy when the prevalence of recently infected ‘HIV-negative’ disclosers comprises approximately 4% of the potential sex partner population. This is a realistic calculation based on current data.” They add that given the prevalence of undiagnosed acute HIV infection, “HIV-uninfected individuals who try to serosort may be more likely to become HIV infected than if they had not tried to serosort in the first place.”
They conclude “individuals at greatest risk of HIV infection predictably belong to those very groups having the greatest proportions of recently infected people. By ignoring the increased potential for HIV transmission by recently infected individuals, serosorting may paradoxically increase the number of new HIV infections in certain populations.”
Butler DM et al. Serosorting can potentially increase HIV transmissions. AIDS 21: 1218 – 1220, 2007.