We evaluated effectiveness of personal protective measures against severe acute respiratory disease coronavirus 2 (SARS-CoV-2) infection . Our case-control study included 211 cases of coronavirus disease (COVID-19) and 839 controls in Thailand . Cases were defined as asymptomatic contacts of COVID-19 patients who later tested positive for SARS-CoV-2; controls were asymptomatic contacts who never tested positive . Wearing masks all the time during contact was independently associated with lower risk for SARS-CoV-2 infection compared with not wearing masks; wearing a mask sometimes during contact did not lower infection risk . We found the type of mask worn was not independently associated with infection and that contacts who always wore masks were more likely to practice social distancing . Maintaining> 1 m distance from a person with COVID-19, having close contact for <15 minutes, and frequent handwashing were independently associated with lower risk for infection . Our findings support consistent wearing of masks, handwashing, and social distancing to protect against COVID-19.