Epidemiology contributes to vaccine development and -delivery by estimating disease burdens, identifying target populations for immunization, designing clinical trials to show vaccine-induced benefits and risks, and for formulating immunization policy.
To estimate disease burden, a case definition is needed first. This may have several sets of criteria, depending on how definite the diagnosis is to be.
Epidemiological studies to determine time, place, and person are referred to as descriptive epidemiology. Epidemiological studies to determine causes and effects are referred to as analytical epidemiology.
Epidemiological studies may potentially be biased (systemic errors). Bias must be distinguished from confounding (interfering factors that are linked with both, exposure and outcome).
Vaccine efficacy studies assess causality between vaccination and disease reduction in a prospective cohort design. Participants are assigned to receive vaccine or placebo usually in a double-blind, randomized fashion and efficacy is calculated from incidence reduction during the study period.
Impact studies are observational studies and encompass e.g. describing disease reduction in a population over time after introduction of a vaccine, as well as cohort studies where vaccine effectiveness is calculated from cohorts in which vaccine administration is only observed.
Observational studies are time-saving and less costly compared with prospective cohort studies. However, they are prone to various types of bias, e.g. recall bias if exposure status and outcome status are based on the participant's memory.