Surgical face masks to prevent respiratory symptoms.

Competing Interests: Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The authors declare the following other interests: JAL is supported by grants from the Agency for Healthcare Research and Quality (R01HS029328), National Institute on Aging (R21AG081895, R24AG064025, U19AG065188, R01AG070054, P30AG024968-20S1, R01AG074245, R01AG069762, P30AG059988), National Heart, Lung, and Blood Institute (R01HL167023), and National Institute of Neurological Disorders and Stroke (U01NS105562).