Vaccine Effectiveness Studies in the Field

The original trials of vaccines against infection with severe acute respiratory disease coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (Covid-19), have clearly shown vaccine efficacy. However, questions crucial to vaccination policy remain, at best, only partially answered. These include the effect of new virus variants, the timing between vaccine doses, the effect of vaccines on asymptomatic infection in contrast to severe disease, the waning of vaccine immunity, and the potentially enhanced effectiveness of mix-and-match strategies that might be used with booster shots. Wherever possible, vaccine efficacy should be assessed by means of randomized trials. Valid comparison is . . .