Even though the Dutch man who contracted HIV while on PrEP had many sex partners, researchers say his case should be a reason to investigate unique patterns of transmission.

A Dutch man has become the third person to have contracted HIV while practicing PrEP, a daily regimen of Truvada that is supposed to keep an HIV-negative person from contracting the virus.

According to findings from Dr. Elske Hoornenborg, an infectious disease specialist at Public Health Service Amsterdam, presented at the Conference on Retroviruses and Opportunistic Infections in Seattle, the man was 50-years old and kept a diary of his sexual activity on a phone app.

Hoornenborg tried to speculate what might have gone wrong, and judging by the number of sex partners the man had (over 50 a month), she theorizes that while Truvada kept HIV exposure relatively in check, a small level of the virus may have stayed in his rectum without his knowledge before it had a chance to spread throughout the body.

Because of this, even a small dip of Truvada concentration in his rectum that may have occurred without another corresponding drop in blood levels may have given HIV a small opportunity to spread. The two drugs in Truvada (tenofovir disproxil fumarate/emtricitabine) may not have been enough to fight against such a unique circumstance.

“This underscores the importance of regular HIV testing in PrEP users and being aware of potential patterns of seroconversion,” Hoornenborg and her co-researchers said in their findings.

The man who contracted the virus started taking PrEP as part of AMPrEP, a PrEP study in Amsterdam led by Hoornenborg. When he first started, he tested negative for HIV. In his diary, he reported to have have sex with 75 partners the first month, 56 the second month, 56 the third month, 50 the fourth, 38 the fifth, 49 the sixth, and 66 the seventh, with 12 anal sex partners in total.

According to questionnaires, the man had barrier free anal sex with 90 partners during the first 12 weeks. During the duration of the study, while he was on PrEP, he was diagnosed twice with rectal gonorrhea and once with rectal Chlamydia.

This case marks the third known case someone has contracted HIV while on PrEP. The first one was a rare exception as well: a 43-year old Canadian man whose strain of HIV was found to be resistant to multiple drugs, including emtricitabine and tenofovir, the two main ingredients in Truvada.