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King County Public Health reports that a local man taking Truvada, the pre-exposure prophylaxis (also known as PrEP) that prevents the contraction of HIV, has tested positive for the disease.

Contracting HIV while correctly taking PrEP is extremely rare, according to Public Health, which says that there have been just four cases of PrEP failure noted in the medical literature out of the hundreds of thousands of people taking the drug, including over 6,500 people in King County.

"PrEP is very effective," says Matthew Golden, director of Public Health’s STD/HIV program in a blog post. "If taken consistently, it reduces the risk of acquiring HIV through sex by over 90%, and perhaps by over 95%. It is also more than 70% effective in preventing HIV infections transmitted through sharing injection equipment. So while it is very effective, like many drugs, it is not 100% effective."

PrEP works by blocking the HIV virus, and unlike antibiotics, resistance isn't a public health concern, says Jared Baeten, a professor and researcher at the University of Washington who has led studies on PrEP.

"If someone doesn't have HIV and they're having PrEP, they don't develop any resistance because there is no virus in them," Baeten says. "It's really different than antibiotic resistance."

In King County, the case of a man who contracted HIV while on PrEP was reported to Public Health by a medical provider. Public Health and the patient have both contacted known sexual partners who may have been exposed to HIV. PrEP does not protect people from contracting other STIs like syphilis, gonorrhea, herpes, chlamydia, and HPV, and rates of gonorrhea and syphilis are rapidly climbing in King County and around the country. Still, when it comes to preventing the spread of HIV, "PrEP works extraordinarily well," says Baeten. "The most important thing is to take it consistently and get tested."