PrEP Efficacy for Oral Sex and HIV Infection Risks

Q: PrEP Efficacy for Oral Sex and HIV Infection Risks

Hello： Please consult an infectious disease specialist, thank you.

PrEP questions?

Hello, according to the data from the Ministry of Health and Welfare, there is still a chance of HIV infection through unprotected oral sex, even though the risk is low.
If I use the 211 method to take PrEP, can I ensure that I won't get infected? Assuming I take two tablets of Truvada at 10:30, have unprotected oral sex at 13:00, and then take one tablet each on the following day and the third day at 10:30, what would be the risk of HIV infection in this scenario?

HH, 20~29 year old female. Ask Date: 2022/12/02

Dr. Du Shixi reply Urology

Hello: Please consult an infectious disease specialist, thank you.

Reply Date: 2022/12/02

More Info

PrEP, or pre-exposure prophylaxis, is a highly effective method for preventing HIV infection in individuals at high risk. It involves taking antiretroviral medication consistently to reduce the likelihood of contracting HIV. The regimen you mentioned, often referred to as the "211" method, involves taking two pills of PrEP (such as Truvada or Descovy) on the first day, followed by one pill daily for the next two days before engaging in high-risk sexual activities.
To address your question regarding the efficacy of PrEP when engaging in unprotected oral sex, it is important to understand the dynamics of HIV transmission. While it is true that unprotected oral sex carries a lower risk of HIV transmission compared to anal or vaginal intercourse, the risk is not zero. Factors that can increase the risk during oral sex include the presence of cuts or sores in the mouth, bleeding gums, or the HIV status of the partner.

In your scenario, if you took two pills at 10:30 AM and then engaged in unprotected oral sex at 1:00 PM, you would be within the window of time where PrEP is not yet fully effective. The full protective effect of PrEP typically takes about 7 days of consistent daily use to reach optimal levels in the bloodstream for anal or vaginal sex, and while some protection may be conferred earlier, it is not guaranteed. Therefore, the risk of HIV transmission during that specific encounter would still exist, albeit at a reduced level due to the initiation of PrEP.

Regarding the specific question of how much the risk is reduced, studies suggest that PrEP can reduce the risk of HIV transmission by up to 99% when taken as prescribed for anal sex. However, for oral sex, the risk is inherently lower, and the exact percentage reduction is less well-defined. Some estimates suggest that the risk of HIV transmission through oral sex is significantly lower than through anal sex, but it can still occur, particularly if there are other risk factors present.

In conclusion, while using PrEP significantly lowers the risk of HIV infection, it does not provide a 100% guarantee, especially if the regimen is not fully adhered to prior to the sexual encounter. If you are concerned about potential exposure, it is advisable to consult with a healthcare provider for further guidance, including the possibility of post-exposure prophylaxis (PEP) if the exposure occurred within 72 hours. Regular HIV testing and open communication with sexual partners about HIV status and prevention methods are also crucial components of a comprehensive strategy to reduce the risk of HIV transmission.

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