By Alan Mozes
WEDNESDAY, Feb. 25, 2015 (HealthDay News) -- There's new evidence that gay men not infected with HIV can stay that way if they take a pill called Truvada in the days before and after a sexual encounter with an infected partner.
The strategy is known as "pre-exposure prophylaxis," or PrEP. Prior studies had suggested that chronic, daily use of Truvada (a combo pill of tenofovir plus emtricitabine) could slash rates of HIV transmission in partners where one person was already infected and the other was not.
Now, two new studies -- one from Britain and one jointly conducted in France and Canada -- appear to bolster those results. They were presented Tuesday at the annual Conference on Retroviruses and Opportunistic Infections in Seattle.
The British investigation, called PROUD, involved 500 gay men and seems to reconfirm the effectiveness of standard, once-a-day PrEP. That study found that this dosing strategy lowered HIV infection risk by 86 percent among gay men.
The French/Canadian investigation, called IPERGAY, looked at PrEP taken on a different schedule -- only around the days surrounding a sexual encounter.
It found the same degree of protection -- an 86 percent lower risk for new infection -- among men who took two Truvada pills anytime between 2 and 24 hours prior to having sex with another man, followed by another two pills 24 and 48 hours after sex.
The IPERGAY study, involving 400 participants, was launched in 2012, but was halted prematurely in 2014 in light of strong indications that the approach is highly effective.
However, in a statement released by the U.S. Centers for Disease Control and Prevention, Dr. Jonathan Mermin said it is too soon to abandon the once-a-day regimen.
"CDC welcomes the findings presented today by the IPERGAY and PROUD teams, which provide additional evidence of the power of PrEP to reduce the risk of HIV infection," said Mermin, who is director of the CDC's National Center for HIV/AIDS, Viral Hepatitis, STD & TB Prevention.
But Mermin said it is not clear if the approach of only taking PrEP in the days before and after sex will work in all situations.
He noted that the strategy was only tested in men who were having frequent sex (an average of 10 times per month, and eight partners every two months).
So, in reality, that meant that some participants were taking as many as eight pills per week -- actually exceeding the older, once-per-day dosage.
Mermin said that, at this point in time, researchers simply don't know if the before/after sex PrEP method will work for gay men "who have sex less frequently and would therefore be taking PrEP less often."
He also pointed out that Truvada may need to build up in the body for a few days prior to a sexual encounter, in order to be optimally effective.
"There are also no data on how effective this regimen would be for heterosexual men and women and injection drug users," Mermin added, "or on adherence to this relatively complex PrEP regimen outside a trial setting."
So, at this point in time, the CDC "continues to recommend only daily use of PrEP, as approved by the FDA," he said. Still, the "IPERGAY findings, combined with other recent research, suggests that even with less than perfect daily adherence, PrEP may still offer substantial protection if taken consistently," Mermin added.
Experts note that studies presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.