I had unprotected anal sex by penetrating (top) a guy for the first time and it lasted for a few minutes. We performed oral sex on each other and he also swallowed my semen. He claims to be negative since his last testing a month ago but I am not sure if he could have acquired it after the test and before having sex with me. I felt a light burning sensation on my penis and went to get PEP (two tablets/once a day) on the third day after my potential exposure. I was tested negative when I went to get my PEP prescription.

Based on my situation, what is the effectiveness of PEP to prevent me from acquiring HIV?

Thank you!
Hello and thank you for contacting us with your question.

We understand that you would like to know about the effectiveness of PEP to prevent you from acquiring HIV after you had unprotected anal sex with someone whose HIV status is unknown.

Unprotected anal sex is classified as a High Risk activity for transmission of HIV.

Post-exposure prophylaxis, or PEP, is a way to help prevent the transmission of HIV in an HIV-negative person who may have been recently exposed to HIV. It involves taking HIV medications as soon as possible after a potential exposure to HIV. PEP is very effective but will not prevent 100% of HIV transmissions from occurring. It must be started within 72 hours of exposure to HIV. For PEP to be effective, a person must have high adherence to the full course of PEP drugs and should have no further exposures to HIV while taking PEP. (1)

An HIV-negative person who thinks they may have been exposed to HIV within the last 72 hours should consider taking PEP. When a person presents for PEP at a clinic or emergency room, a risk assessment will determine whether PEP should be started based on their risk for HIV infection. Assessment of HIV risk is based on the type of exposure and the likelihood that the contact person was HIV positive. (1)

Based on guidelines here in British Columbia, the first step when a person such as yourself requests PEP is to perform a baseline HIV test. If the test is positive, PEP is not indicated, and you should be referred for follow-up treatment.(2)

The next step is typically an assessment of your exposure including the following details:

-presence of an STI in either yourself or the source person

-circumcision status for insertive male partners

-condom use

-degree of physical injury associated with the sexual act

In a case like yours, where your baseline test was negative, and the source is known but their status is unknown, the next step is to contact the source person for an interview and HIV testing. If they test negative, PEP is not indicated.

As you can see, the decision to start a person on PEP is not taken lightly, at least here in British Columbia. Although patients are encouraged to seek PEP in situations where they are at risk, it is not intended to be used as a primary source of protection against HIV. There is potential for adverse effects with any drug and cost is always a consideration.

PEP medications need to be taken consistently and correctly—every day for four weeks—or the risk of HIV infection will increase. A person taking PEP should take extra precautions to avoid exposure to HIV while taking PEP. The use of PEP is only intended to reduce the risk of infection associated with one exposure. If a person continues to engage in behaviours that can transmit HIV, such as sharing needles or having unprotected sex, while taking PEP, their risk of getting HIV increases. (1)

Now to your question about the effectiveness of PEP in your situation. You say you started pep on the third day after your exposure, we will assume this was within 72 hours. You don't mention if you adhered to the entire 28 day course of treatment. Assuming that you did, studies show that when antiretroviral therapy is offered in a timely manner (within 72 hours) after an HIV exposure, the risk of acquiring HIV decreases by approximately 80%. (2) The estimated risk of a single exposure event for the insertive partner in anal intercourse is 0.11%. (2)

Recommendation: HIV testing is always strongly recommended for high risk exposures. For people like yourself who have received PEP, BC guidelines recommend the following follow up:

* 2 and 4 weeks after exposure (while on PEP): CBC and differential, creatinine, eGFR, if any abnormalities were present on baseline testing.

* 3 and 6 weeks after end of PEP: HIV Ag/Ab test

* 3 months after end of PEP: HIV Ag/Ab test

Consider using a condom for anal intercourse in the future to reduce your risk of acquiring HIV. If this event becomes a common occurrence for you, you might also consider going on PrEP.

AIDS Vancouver Helpline Volunteer,

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