PrEP, sometimes known by the brand name Truvada, can stop new HIV transmissions. (Getty)

A gay man has been diagnosed with HIV despite taking preventative drug pre-exposure prophylaxis (PrEP) daily.

The 34-year-old – who has not been named – started taking PrEP in December 2018 and was diagnosed with HIV in July of this year.

The majority of cases where HIV is contracted while on PrEP are thought to be due to poor adherence, where a person misses doses or does not take the medication regularly enough.

However, this man was considered to be highly knowledgeable about PrEP and kept a diary recording when he took the medication, leading researchers to believe that it could be a genuine failure of the medication.

However, researchers cannot completely rule out the possibility that the man contracted HIV due to poor adherence to the drug.

PrEP failure is thought to be extremely rare.

Details of his case were presented last week at the 17th European AIDS Conference.

“There are a number of factors which indicate that this could be a case of PrEP failing to work,” said Matthew Hodson of NAM/AIDSmap.

“It should be noted though that the only reason this story was presented at an HIV conference is that the circumstances we see here are so very rare.

“There have been only a handful of cases where it’s suspected that someone acquired HIV while actively using PrEP. Acquiring HIV while using PrEP is extremely unusual, acquiring HIV while using condoms happens far more frequently.”

There have been only a handful of cases where it’s suspected that someone acquired HIV while actively using PrEP.

Hans Benhamin Hampel of Zurich University Hospital told the conference that it is almost impossible to prove if a person was fully adherent to PrEP when they contract HIV.

“The community would like to believe that this patient was just non-adherent, because that’s a simple reason for HIV infection,” he told AIDSmap.

“But there actually may be multiple factors that influence the efficacy of PrEP, and these may come together to create cases of PrEP failure like this.”

The man is on antiretroviral therapy and his viral load is now undetectable.

“PrEP, taken as directed, is still acknowledged by the CDC and other authorities as being more than 99% effective at preventing HIV, making it the most powerful prevention method that people without HIV can use when having sex with people who may have unsuppressed HIV.”

The man is now on antiretroviral therapy and his viral load is now undetectable – meaning he cannot pass the virus on to others.

There have been just a handful of recorded cases worldwide where people have contracted HIV while taking the preventative medication.

Last March, 27-year-old Steve Spencer from Sydney, Australia, revealed that he had contracted HIV while on the preventative medication.

He had been taking the drug for six years when he contracted the virus.

Research suggests that PrEP is up to 99 per cent effective at preventing HIV transmission when the daily pill is taken.