Can Someone on PrEP Get HIV from Unprotected Sex with an Undetectable Partner?

The risk of a person on PrEP contracting HIV through unprotected sex with an HIV positive partner with an undetectable viral load is extremely low. In this scenario, the combination of an undetectable viral load and daily Truvada use as PrEP significantly reduces the chances of HIV transmission to nearly zero.

The Two Layers of Protection

Firstly, the HIV positive partner's undetectable viral load serves as a strong protective barrier. People living with HIV whose virus is durably suppressed by antiretroviral therapy (ART) do not sexually transmit the virus to HIV-negative partners. This success is due to consistent adherence to ART, allowing individuals to lead worry-free lives without the risk of infecting their partners.

The second layer of protection comes from the use of PrEP. HIV-negative individuals who take a daily Truvada pill as pre-exposure prophylaxis (PrEP) lower their risk of acquiring HIV by approximately 92%. With both of these factors in place, the risk of HIV transmission through unprotected sex is dramatically reduced to nearly zero.

Adherence and Real-World Considerations

Though the statistics show that perfect adherence to PrEP has a high success rate, humans are not always perfectly consistent. According to data, chemical prophylaxis, similar to condom use, fails at a rate of roughly 92-98% depending on the statistics cited. The use of female contraception as a backup further reduces adherence rates.

However, when combined with the undetectable viral load status of the HIV positive partner, the risk of HIV transmission drops to an extremely low level. An undetectable viral load means that there are only minimal amounts of the virus in the blood, semen, or other bodily fluids, making HIV less infectious and requiring a larger quantity to successfully infect a new host. For comparison, Ebola virus is highly infectious, requiring only one viral particle to cause infection.

Limitations and Real-World Scenarios

It's important to note that statistics are not guarantees, and there have been rare cases where people on PrEP did develop HIV. In most of these cases, the individuals were in the latent window period before starting PrEP. A small number of cases involved individuals contracting a PrEP-resistant strain of HIV, often from partners who were likely not receiving adequate treatment. These outcomes are statistically expected but can be emotionally devastating for those affected.

While the risk is extremely low, it is not completely eliminated. It is crucial for individuals to communicate openly with their healthcare providers and partners about the status of their viral load and adherence to treatment regimens.