In clinical trials, preexposure prophylaxis (PrEP) has been shown to be highly effective in preventing HIV. There have been few studies, however, of its effectiveness when used in real-life situations by people who have diverse profiles. A matched, nested case-control study was undertaken to assess PrEP effectiveness in real-world situations.

Using data from a national health care system, the researchers identified 46,706 men at high risk for sexually acquired HIV infection, of whom 28,352 were PrEP users and 18,354 were not. A total of 256 men who were newly diagnosed with HIV infection were matched with 1,213 controls who weren't infected with HIV.

PrEP users accounted for 29% of cases of men with a newly diagnosed HIV infection compared with 49% of controls. Among PrEP users, cases were more likely than controls to have low PrEP consumption and to have discontinued its use. Overall PrEP effectiveness was 60%; effectiveness increased with PrEP consumption, ranging from 18% for low consumption to 93% for high consumption. If periods after PrEP discontinuation were excluded, PrEP effectiveness reached 86%. PrEP effectiveness was significantly lower in men younger than 30 years of age and in those who were socioeconomically deprived. Both groups had low PrEP consumption and high rates of discontinuation.

The authors note that the database they used didn't include information about sexual behaviors, blood test results, or the amount of exposure to HIV risk.