Untested Individuals May Account for 70% of Sexually Transmitted HIV Infections

By Liz Highleyman 

June 27 was National HIV Testing Day in the United States, an opportunity for public health officials to encourage people who do not currently know their HIV serostatus to get tested. As reported in the June 26, 2006 issue of AIDS, HIV positive individuals who have never been tested and are unaware of their serostatus may be responsible for 50% to 70% of new sexually transmitted HIV infections in the U.S., according to a new estimate from the Centers for Disease Control and Prevention (CDC).

Gary Marks and colleagues used a mathematical model to calculate the relative contribution of known HIV positive individuals and those who are unaware of their status to new sexually transmitted HIV infections.

In making their calculations, the authors relied upon the following assumptions:

About 25% of the estimated 1 million HIV positive individuals in the United States do not know they are infected. One-third of patients who know they are HIV positive are at low risk for transmitting the virus because they have low or undetectable HIV viral load. Aware patients are more likely to have low viral load because they are more likely to be receiving appropriate treatment. Individuals who do not know they have HIV are, on average, 57% more likely to have unprotected anal or vaginal intercourse with HIV negative partners, compared with individuals aware of their HIV positive status. A CDC meta-analysis of 11 studies published last year by the same authors found that people who know they are HIV positive were 53% to 68% less likely to engage in high-risk sexual activity compared with untested HIV positive individuals.

The model did not take into account:

The contribution of concurrent sexually transmitted infections, which can increase the risk of transmitting and contracting HIV. Findings that the risk of HIV transmission is considerably greater within the first several months following seroconversion, when viral load tends to be high.

Results

The researchers estimated that HIV positive individuals who are unaware of their serostatus account for 54% to 70% of new HIV infections, depending on the number of uninfected partners with whom they have unprotected sex. The rate was 54% if aware and unaware HIV positive people had sex with the same average number of uninfected partners, and 70% if unaware individuals had unprotected sex with twice as many uninfected partners.

Using the lower bound, the transmission rate from unaware individuals was 3.5 times higher than the transmission rate from people who know their HIV status.

If all HIV positive individuals got tested in a timely manner and reduced their high-risk sexual activity with HIV negative partners by a little more than half, the rate of new sexually transmitted HIV infections could be reduced by about 30% per year.

Conclusion

The authors concluded that the unaware HIV positive group “contributes disproportionately” to new sexually transmitted HIV infections relative to their percentage of the HIV positive population, and that “the HIV/AIDS epidemic can be lessened substantially by increasing the number of HIV-positive persons who are aware of their status.”

They suggested that targeted HIV counseling and testing campaigns, as well as more widespread routine HIV screening, could have a significant impact on future HIV incidence and prevalence; targeted testing could potentially have a particularly strong effect among gay and bisexual men of color.

07/7/06