Is Truvada, the Pill to Prevent H.I.V., 99 Percent Effective? Don’t Be So Sure

Truvada, the once-a-day pill to help keep people from contracting H.I.V., is on the cover of this week’s New York magazine, and Tim Murphy’s cover story focuses on how the pill is changing sex by drastically reducing gay men’s fear of infection.

It’s not hard to see why: Mr. Murphy writes, “When taken every day, it’s been shown in a major study to be up to 99 percent effective.” This is a claim I hear thrown around a lot among gay men in New York. And it’s wrong. The 99 percent figure isn’t a study finding; it’s a statistical estimate, based on a number of assumptions that are reasonable, but debatable.

Here’s how the estimate was reached: A major study of men who have sex with men, called iPrEx, found that H.I.V.-negative men who were prescribed daily Truvada were 44 percent less likely to contract the virus than those who were given a placebo. But a great many of the subjects did not take their prescribed medication regularly, or at all. Of 48 iPrEx subjects who were assigned to take Truvada and contracted H.I.V. anyway, just four had any detectable level of the drug in their system when they were diagnosed, indicating a 92 percent red... Read full article here

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