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Gay Men Better at Predicting When They Won't Have Sex than When They Will

A study in which a group of HIV negative gay and bisexual men from New York City were asked to predict each day whether they would have sex the following day, and then compared their prediction with what actually happened, found that men generally overestimated the likelihood they would have sex. This study, published in the April 1 Journal of Acquired Immune Deficiency Syndromes, suggests possible pre-exposure prophylaxis (PrEP) dosing strategies.

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The only time men accurately predicted whether they would have sex the next day was in the days they predicted it was very unlikely: if men reported that there was only a zero to 5% chance of sex the following day then, sure enough, only about 4% actually ended up having sex, and there was no case in which someone had sex after having predicted that there was zero chance of having sex the following day. The sexual optimists, on the other hand, were often disappointed: only 45% of men who predicted a 90% chance of sex the following day actually ended up having it.

The Study

In this study, 92 gay men and other men who have sex with men (MSM) from New York over 18 years of age were asked to keep online sex diaries every day for 30 days. The mean completion rate was 78%. The men were asked to say whether they had had sex -- where sex meant "anal intercourse with a casual partner" -- and also rate the likelihood, from zero to 100%, that they would have anal sex with a casual partner the following day. 

This was a very mixed group of MSM. Their mean age was 33, but a quarter were under 22 and a quarter over 44; 54% were white, 14% were black, and 14% were Latino; 16% defined as bisexual rather than gay. Roughly equal proportions worked full-time, worked part-time work, and were unemployed or a student; two-thirds had been to a college or university. Only 1 in 6 had a current steady relationship. 59% reported they had sex on 3 or more days per week.

Although there was a correlation between the prediction of sex and its occurrence, the men were in general more likely to overestimate rather than underestimate their chances of casual sex the following day. If men said they thought there was a 100% chance of sex the following day, only 58% actually had it; if a 50% chance, only 20% had it; and if a 20% chance, only 10% had it. Only if men rated their chances of sex as very low were their forecasts accurate: for instance, where men rated their chances as 1%-10%, about 5% had sex, and if they thought there was no chance, they did not end up having sex.

Implications for PrEP Regimens

The researchers comment that these findings have implications for studies of intermittent PrEP. While so far intermittent PrEP studies have tended to instruct participants to take PrEP if they think they are likely to have sex the next day, it might be safer, and in certain situations more cost-effective, they comment, to tell participants to "take PrEP daily unless you are definitely not going to have sex tomorrow; then you can miss a dose."

This strategy would have to be quite strict, however. Although participants generally overestimated their chances of having sex, when the researchers worked backwards, i.e., looked at occasions of sex and then looked at whether participants had predicted it, they found a number of "false negatives" -- cases where people had ended up having sex even though they had rated the probability as low.

This means that, if a group of gay men similar to those in this study were told to take PrEP if they thought they would have sex, there was a substantial number of "false positives," to the extent that only 20% of PrEP doses taken would actually being needed. However 3.8% of occasions of sex would be "false negatives," i.e., not covered by PrEP. There were only no "false negatives" if participants were absolutely certain they would not have sex the following day.

Given that, while "false positives" only waste money, but a "false negative" may result in an HIV infection, the researchers recommend that the safest advice would be to advise PrEP takers that they can skip a dose only there is "no chance" they will have sex the following day.

While most studies of PrEP have involved taking a daily dose of antiretroviral drugs to prevent HIV, some studies have looked at -- and are looking at --intermittent dosing of one sort or another. Intermittent dosing may lower the possibility of side-effects, but the main motive for investigating it is cost. With each Truvada (tenofovir/emtricitabine) pill costing about $25 in the U.S., the researchers estimated it could save $4700 per person per year if people took PrEP only every other day, or only 50% of the time.

Recently the French Ipergay study, which randomized gay men to take two Truvada or placebo pills 2 to 12 hours before they anticipated sex, and then to take 2 more in the 2 days after sex if it actually happened, stopped its randomized phase because of higher-than-expected effectiveness. Data on adherence showed that the amount of drug used in Ipergay was about 50% of what would be used with full adherence to daily dosing. The first data from Ipergay were presented at the 2015 Conference on Retroviruses and Opportunistic Infections.

An ongoing international study, HPTN067 (ADAPT), randomizes participants to 3 different regimens: daily PrEP; PreP twice per week regardless of the chance of sex, with an extra post-exposure dose if sex occurs (time-driven); or 1 dose of PrEP 24 hours before anticipated sex, followed by 1 post-exposure dose 2 hours after sex (event-driven). This study was closed to follow-up in December and results are expected soon.

Looking at various sexual patterns and comparing them with the ADAPT and Ipergay intermittent regimens, clearly if men had no sex one week, money would be saved if PrEP was taken according to either of the event-driven regimens, compared to the time-driven regimen. But if they did have sex, the Ipergay regimen would end up with the highest pill usage because it requires 4 pills. If there was 1 occasion of sex in the week, or 2 on consecutive days, then the ADAPT event-driven regimen would be the most economical with only 2 pills used. But if there were 2 occasions of sex separated by a day or more, then the ADAPT time-driven regimen would be as economical as the event-driven regimen.

These results are based on 100% prediction reliability, i.e., that in all event-driven regimens, the participants do end up having sex. However, the researchers comment that due to the chance of "false negatives" the Ipergay protocol may actually be the safest, for 2 reasons. If men end up having sex more than twice per week, their PrEP coverage in Ipergay would be at least 4 doses per week, which is the level that the iPrEx study predicts would be effective. Also, Ipergay allows them to take Truvada on the day in which they anticipate sex, rather than the day before. They thus recommend the ADAPT fixed-dose regimen, with a dose only omitted if there is no chance of sex, for men who have sex no more than once a week on 1 day or 2 consecutive days, but the Ipergay regimen for men who have sex on more than 2 days a week or on days that are not consecutive.

One major limitation of this study was that it only looked at casual sex, even in the case of the 16% of men who had a steady partner. Men’s ability to predict sex -- or to postpone it if PrEP has not been taken -- may be greater in situations of sex with a regular partner, and more research is needed on the predictability of sex within regular relationships.

2/23/15

Reference

JT Parsons, HJ Rendina, C Grov, et al. Accuracy of Highly Sexually Active Gay and Bisexual Men's Predictions of their Daily Likelihood of Anal Sex and its Relevance for Intermittent Event-driven HIV Pre-Exposure Prophylaxis. Journal of Acquired Immune Deficiency Syndromes68(4):449-455. April 1, 2015.