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Demonstrating that taking a daily pill to prevent H.I.V. infection can work in the real world, San Francisco’s largest private health insurer announced Wednesday that not one of its 657 clients receiving the drug had become infected over a period of more than two years. That outcome contradicted some critics’ predictions that so-called pre-exposure prophylaxis, or PrEP, would lead to less condom use and more H.I.V. infections. A study published in Clinical Infectious Diseases found that the San Franciscans on PrEP, almost all of whom were gay men, did use fewer condoms — and contracted several other venereal diseases as a result. But none got H.I.V. (McNeil Jr., 9/2)

Lifting a ban on spending city money on needle exchanges for intravenous drug users prevented 120 new cases of HIV in two years in Washington, according to a new study that researchers hope can help other communities deal with a surge in addicts shooting up. (Ungar, 9/3)