Most of the MSM enrolled in this study were exposed to possible HIV infection only intermittently, and about two-thirds of MSM had a window of opportunity to take a pre-exposure dose of chemoprophylaxis prior to sexual activity. This information implies that in terms of frequency of exposure, iPrEP may be a more appropriate regimen than daily PrEP. Unfortunately, those lacking the opportunity to take a timely pre-exposure dose were also at high behavioural risk for HIV infection.
Factors in favour of iPrEP over daily PrEP include reduced costs, decreased pill burden, and possibly, reduced toxicity and drug side effects. Cost-effectiveness analysis has shown that implementation of daily PrEP would be extremely challenging, even in the developed world [14
]. In the developing world, where most of the infections occur, daily PrEP would generally be not affordable.
With respect to pill burden, daily intake of drugs for considerable time in the absence of overt disease has proven to be difficult for many people. Toxicity, in particular renal toxicity, and dysfunction have been reported in association with TDF treatment [16
]. Fewer doses of drugs may reduce mild side effects, such as cramping, diarrhea and flatulence, and may therefore positively affect adherence.
As expected, sex was significantly more often reported on the weekend, but differences with other days were small. For those who almost exclusively have sex on the weekend, this may provide an opportunity to develop pill-taking routines, which may help to improve adherence.
With regard to sex planning, there were no significant differences between the different days of the week. With 60% to 65% of the first sex on a day being planned, the majority of men would principally be able to take pre-exposure doses, but still, about one-third of men would not. For these men, a regimen consisting of a number of weekly standing doses followed by post-exposure doses may be more appropriate in terms of opportunities for adherence. We need to keep in mind, however, that in these analyses, sex planning is used as a proxy for pill-taking behaviour, and that we do not know whether those who plan their sex will also take their pre-exposure doses as indicated.
Factors in our assessment that were independently and significantly associated with more frequent sex were use of alcohol, erectile dysfunction drugs, group sex, sex with a foreigner, buying and selling sex, and a history of HIV testing. Use of erectile dysfunction drugs, group sex, sex with a foreigner, buying and selling sex, and a history of HIV testing may be indicators of a more active sex life, while alcohol use may be associated with decreased control over one's sexual behavior. Together, these factors may help to identify those who should be targeted for daily PrEP.
Being at a younger age, not identifying as homosexual, having receptive anal intercourse, and not engaging in group sex were independently and significantly associated with unplanned sex. Those who are younger and those not engaging in group sex may be less likely or able to plan their sex due to lack of experience or decreased control over their sexual desire. Lack of control over partner selection or the necessity to respond to sexual opportunities may explain the association between not identifying as homosexual, receptive anal intercourse, and unplanned sex.
The association between younger age, receptive anal intercourse and unplanned sex is problematic, since the former two are also known as risk factors for HIV infection [17
]. Alternative PrEP regimens, such as two standing doses per week plus a post-exposure dose, if proven efficacious, or other regimens or prevention methods, may therefore be necessary for those who have unplanned sex.
The results presented in this paper are subject to a number of limitations. First of all, our data were collected in a sample of men at high risk for HIV infection who may not be representative of all MSM. Men at lower risk may have different sex frequency and sex planning profiles. In addition, younger and less educated men were more likely to have been lost to follow up in the current study. Since both younger age and lower education were found associated with unplanned sex, absence of these men may have biased our results regarding sex planning upwards.
Finally, our data are cross sectional and do not provide information about changes in sex frequency and sex planning profiles over time. For the same reason, our risk factor analysis does not allow causal inference; for example, it is unknown whether the risk factors occurred before or after sexual frequency and sex planning profiles were established.