Figure 1. Development of Cohorts in a Study of SARS-CoV-2 Infections After Vaccination With and Without Prior Infection.

PCR indicates polymerase chain reaction.

^aIndividuals were exact matched based on infection status with no replacement allowed on a 1:3 ratio by sex, 5-year age group, nationality, and calendar week of first vaccine dose. Matching did not exactly reach 1:3 ratio because of low frequency in older age categories and for specific nationalities.

^bSevere disease included hospitalization that did not require intensive care; critical disease included patients admitted to the intensive care unit.

^cDeaths were not related to COVID-19.