Printed in Great Britain The Measurement and Interpretation of Age-Specific Vaccine Efficacy

The age-specific efficacy of a vaccine depends upon the mechanism of vaccine action and the force of infection in the unvaccinated subpopulation. Consequently, inferences concerning the persistence of vaccine-induced immunity cannot in general be made directly from age-specific vaccine efficacy estimates. The definition, estimation and interpretation of age-specific vaccine efficacy measures are discussed using simple models of vaccine action. The methods are applied to data on measles and pertussis vaccines. The persistence of vaccine-induced immunity is of in-terest in assessing the effectiveness of a vaccination programme and in determining policy on the use of boosters. For some vaccines, such as diphtheria vaccine, this may be studied directly by measuring antibody persistence in circumstances where there is no natural boosting. For others, such as pertussis vaccine, a suitable laboratory test is not available. Information on the persistence of vaccine-induced immunity must then be derived from age-specific attack rate