In a study of roughly 4,000 people at high risk for COVID-19, including health professionals, police officers, and firefighters, the US Centers for Disease Control and Prevention (CDC) found that the Pfizer-BioNTech and Moderna vaccines are 80% effective at preventing SARS-Cov-2 infections after the first dose and 90% after the second (Morbidity and Mortality Weekly Report, April 2). This is one of the first studies to estimate the effectiveness of the vaccines at preventing infection, and not just at protecting against the development of symptomatic COVID-19, as was tested in clinical trials. When discussing the results with The Washington Post, CDC director Rochelle Walensky said the study shows that vaccination in the USA is working. In a trial of 2,260 participants not yet reviewed by other experts, the Pfizer-BioNTech vaccine also proved effective at preventing COVID-19 in children aged 12 to 15. No serious side effects were registered. There were 18 cases of symptomatic infection in the placebo group and none in the group that received the vaccine (New York Times, March 31).
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