A new meta-analysis (Johnson) of the efficacy of condoms in preventing HIV transmission via anal sex between gay men has found a considerably higher estimate of their efficacy than two previous analyses.
The new estimate by the US Centers for Disease Control and Prevention (CDC) finds that condoms, used 100% of the time, stop more than nine out of ten HIV infections. Two previous analyses, one published back in 1989 (Detels) and the other in 2015 (Smith) found that they only stopped seven out of ten infections.
This new estimate for condom efficacy in sex between men is much closer to the estimated efficacy for sex between men and women; a synthesis of studies finds an efficacy in the region of 80-85%.
Why is this new estimate of efficacy (91.6%, in the case where the HIV-negative partner is the receptive one) so much higher than previous ones (72.3% for the same risk in the 2015 analysis)?
There are several possible reasons, the CDC researchers think. One is that the new analysis examines condom efficacy in four different studies, whereas the previous ones only looked at a single study (Detels) or two (Smith).
However, they think that the crucial difference is that they look at condom efficacy per number of partners instead of per sex act.
Counting the number of partners may be a more reliable guide to risk than counting sex acts. This is because in cases where there are multiple sex acts between one couple, the risk tends to go down with time: at least one early study showed an extremely high risk (about a 25% chance of infection) during the first year of exposure but then a considerable falling-off of risk; transmissions became infrequent after a few years. This may be due to variation in viral load: partners with high viral loads transmit in the first year, while those with low ones may never do.
Because there is less risk of infection as time goes on, the risk of not using condoms also diminishes over time – and so, therefore, does their apparent efficacy.
If on the other hand, someone continues having sex with multiple partners, their infection risk does not diminish over time because their chances of encountering someone with a high viral load stays constant – and therefore so does the efficacy of condoms.