HIV infections among PrEP users are extremely rare 

On 22 May 2022, the British website Aidsmap published an international research review of HIV infections diagnosed while on PrEP. This article summarizes the key research results presented in the review.  

Quick facts

  • PrEP (pre-exposure prophylaxis) is a medication that prevents HIV among HIV negative individuals who are at high risk of getting infected. 
  • When the PrEP is used according to the instructions, it gives almost 100% protection against HIV infection. 
  • There are approximately one million PrEP users worldwide. 
  • Only isolated cases of HIV transmissions have been reported among individuals who use PrEP as prescribed. 
  • The greatest risk of HIV transmission while using PrEP is associated with irregular use of the medication. 
  • HIV infections among individuals who have used PrEP as prescribed have usually been caused by exposure to a rare subtype of HIV that is resistant to the drugs used in PrEP medication. 

PrEP medication is effective against HIV infection 

PrEP medication prevents almost 100% of HIV infections when used correctly and the drug levels in the body remain high enough. 

In most cases where a person has been infected with HIV despite the use of PrEP, the medication has not been used according to the instructions and the drug levels in the body were insufficient to give protection against HIV. 

It is extremely rare for a PrEP user with a sufficiently high drugs levels in their body to be infected with HIV. Fewer than 20 such cases have been reported worldwide throughout the existence of PrEP medication, although it is estimated that there are around one million PrEP users. 

What is the cause of HIV infection among individuals who use PrEP as prescribed? 

Almost all HIV infections among individuals who use PrEP as prescribed are believed to have acquired in situations where the virus is resistant to drugs used in PrEP medication.  

In three cases where the PrEP user was infected with HIV, they also had rectal LGV which is a type of chlamydia. Although there is no direct evidence, doctors assume that inflammation of rectal tissue caused by LGV has contributed to the transmission of HIV. 

PrEP medication should always be used as prescribed 

Drug resistant HIV may also develop in situations where a person is infected with HIV after taking PrEP against the instructions and continues to use it without knowing that they have been infected with HIV. For this reason, the use of PrEP according to the instructions and regular HIV testing is important. 

Resistent virus mutations can also develop when a person is not taking PrEP as prescribed, acquires HIV and continues to take PrEP. After the infection, HIV can develop resistance because the drugs in PrEP do not fully suppress the virus.

PrEP medication does not work as treatment for HIV, although it contains the same drugs that can be used as part of the HIV treatment. 

How common are HIV strains resistant to PrEP drugs? 

Exposure to virus, which is resistant to drugs used in PEP treatment, is very unlikely. HIV is also not transmitted from a person who is on effective HIV medication, even if this person has a strain of HIV in their system that is resistant to the drugs used in PrEP treatment.  

There are several combinations of medicines to choose from in the treatment of HIV, and it can be carried out even in cases where the virus is resistant to one or more HIV medicines. 


PrEP medication has been used worldwide for ten years and is a very effective in  preventing HIV among people who are at high risk of getting infected.  

By far the most significant risk factor for getting HIV while taking PrEP is taking the medication against instructions. If PrEP is used as prescribed, the drug levels in the body remain adequate and PrEP prevents HIV infection by almost 100%. 

Previous articles about PrEP:

21.12.2021 Did you know that PrEP treatment includes regular monitoring?
1.7.2021 PrEP is now free in Finland!