A second man taking the daily HIV-prevention pill Truvada has been infected by a rare drug-resistant strain of the virus.

The results were presented by Cleveland Clinic HIV specialist Howard Grossman, who unveiled the finding at the 2016 HIV Research for Prevention conference in Chicago on Tuesday.

The patient, who is not releasing his name or age to protect his privacy, is a gay man who had been taking the drug daily since January 2016. Hair and blood tests confirmed that he had the appropriate blood levels of the pill's two drugs — tenofovir and emtricitabine — to offer the nearly 99% protection it offers from the virus when taken daily.

The patient's long-term partner is HIV-positive but on treatment, with an undetectable viral load in his body. On two occasions and with two separate people, however, the couple had condomless sex with a third person.

Grossman found that the patient's strain of HIV was resistant to both of the drugs in Truvada, as well as all of the other drugs in its class, which work by blocking one of the enzymes needed by the virus to reproduce. The new strain did not match his partner's, meaning that the multi-drug-resistant virus must have been transmitted during one of the two sexual encounters.

The man, who is now HIV-positive, is on a mixture of antiretroviral drugs and has his infection under control, Grossman said.

The first documented case of a person on daily Truvada being infected with the virus was announced in February. For now, experts suggest that the cases should not be cause for alarm.

"We know PrEP is not 100% effective, and that’s something we need to be saying loudly and clearly," Mitchell Warren, executive director of AVAC, a global HIV-prevention advocacy group in New York City, told BuzzFeed News. "No prevention method — other than abstinence — is."

But while the risk exists, it needs to be weighed against the pill's well-demonstrated benefits, Warren said. "We have now seen hundreds of thousands of people on oral PrEP, and we’ve only seen two cases of so-called breakthrough infections."

Instead, Warren urged that we need better surveillance to track and identify the prevalence of drug-resistant strains of the virus. "This is a virus that’s been around for 35 years — it's so variable and mutates so rapidly — and that's one of the biggest challenges for developing any prevention."

Grossman agreed that the new case should not cause those taking or considering taking PrEP to reconsider using the drug out of fear.

"Truthfully, PrEP is the best intervention we’ve ever had to prevent HIV," he said. "I don’t think we’re going to see some huge explosion of failures. I think it’s going to be a sort of trickle, if anything."