Covid-19 Vaccine Effectiveness and the Test-Negative Design

Observational studies are emerging as fundamental sources of information about vaccine effectiveness outside the controlled environment of randomized trials, and they are being used to generate evidence of effectiveness against outcomes that are underpowered in trials, such as hospitalization or intensive care unit (ICU) admission, or for narrow subgroups. 1 These studies can monitor the waning of vaccine effectiveness or measure the performance of vaccines against novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants when large randomized, controlled trials are not feasible. 2 Thompson et al. 3 now describe in the Journal the application of a retrospective test-negative design to estimate coronavirus . . .