Consistent condom use in anal sex stops 70% of HIV infections, study finds, but intermittent use has no effect

One ongoing problem in assessing the effectiveness of
different HIV prevention methods is that anal sex is under-studied. We do not
have enough data on rectal viral loads and their effect on transmission, or on
whether HIV treatment reduces transmission via anal sex as well as it does for
vaginal sex.

We are also unclear about to what extent condoms actually
prevent HIV transmission in anal sex. This last fact may seem surprising, given
that condoms have been recommended since the mid-1980s as the only effective
HIV prevention method for gay men who have anal sex.

In fact there is only
one large study in gay men, dating from 1989. In this study of 2914
gay men, HIV incidence among those who said they used condoms 100% of the time
was 70% lower than in men who did not use them at all. There has been one small
study in the era of ART, which found an efficacy of approximately 75%.

These are somewhat lower than efficacies computed for
vaginal sex, which is in the order of 80-85%, and may reflect both that HIV is
at least ten times more easily transmitted via anal than vaginal sex, and also
that condoms may be more likely to fail during anal sex.

So a new study estimating condom efficacy in anal sex
is very overdue, and given the need to compare condom efficacy against newer,
biomedical prevention methods, very useful.