[Assessing the effectiveness of vaccination programs]

Vaccines have contributed enormously to reducing the incidence of many communicable diseases. The protective efficacy of a vaccine refers to the health effects of the vaccine applied in optimal, ideal conditions, whereas the effectiveness of a vaccination program refers to the health effects of vaccination in the vaccinated individuals in clinical practice or within public health programs, which may differ widely from optimal conditions. Vaccine efficacy is estimated by randomized clinical trials. In contrast, effectiveness can be measured by various types of epidemiological studies: randomized community trials, in which the target vaccine is randomly assigned to a group and disease incidence in this group is compared with that of an unvaccinated group; cohort studies, which are observational epidemiological studies in which the vaccination status is known in healthy vaccinated (vaccinated cohort) and unvaccinated (unvaccinated cohort) people and the occurrence of the disease in the two groups is studied; and observational case-control studies, in which two groups are selected; one with the disease under investigation (cases) and the other without (controls), and vaccination histories are investigated in the two groups. Vaccine effectiveness may also be estimated by comparing attack rates in epidemic outbreaks or secondary attack rates in the home, or by screening.