Medicine
Vaccination
Booster dose
Booster (rocketry)
Retrospective cohort study
Population
Pandemic
Confounding
Cohort
Observational study
Cohort study
Coronavirus disease 2019 (COVID-19)
Demography
Internal medicine
Immunology
Immunization
Environmental health
Disease
Infectious disease (medical specialty)
Physics
Astronomy
Antigen
Sociology