I've come across a few resources like this one. In this case they claim:

unprotected vaginal intercourse with an HIV-positive man who is chronically infected and not on antiretroviral treatment (ART) carries an estimated 0.08% risk of infection for the woman per sexual act.

source: http://www.thelancet.com/journals/laninf/article/PIIS1473-3099%2809%2970021-0/abstract

The most widely cited study of per-act anal-transmission risk was published in 1999. It found that amongst men in high-income countries, unprotected anal intercourse with an HIV-positive insertive partner carried an estimated 0.82% risk of infection for the receptive partner per sexual act,

source: http://www.ncbi.nlm.nih.gov/pubmed/20139750

Problem is I can find no mention in either of these two studies that they are referring to people who contracted HIV from their partner who they knew already had HIV. Is it really that hard to contract HIV from someone who is already infected? Or are these AIDS resources just mistaken in their claims?

  • The male-to-female estimate from the first quote is a duplicate of skeptics.stackexchange.com/questions/34584/…
    – Oddthinking
    Oct 10, 2016 at 0:31
  • 1
    @Oddthinking Yes but that answer does not mention anything about sex per act with an infected partner. Oct 10, 2016 at 1:48
  • Note that this is figuring no STIs in the recipient. If they already have some other STI the odds go up considerably. Oct 10, 2016 at 19:12

1 Answer 1


The first statement is supported by the very comprehensive reference that you cite.

However, the second statement about anal intercourse is an underestimate according to the same reference, at least in the heterosexual situation.

The reference says:

Only two publications [89,92] reported M-to-F estimates for receptive anal intercourse (RAI)(pooled=0·017, CI:0·003-0·089)

In other words, the best estimate is 1.7%.

The US Center for Disease Control (CDC) gives an online table Estimated Per-Act Probability of Acquiring HIV from an Infected Source, by Exposure Act based upon the 2014 article "Estimating per-act HIV transmission risk: a systematic review" :

Receptive Penile-Vaginal Intercourse 8 per 10,000 (0.08%)

Receptive Anal Intercourse 138 per 10,000 (1.38%)


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .