Aim: To estimate measles vaccine efficacy in the field in New Zealand, during the 1991 measles epidemic.
Methods: Notifications in the Wellington Area Health Board region from the first 21 weeks of the 1991 measles epidemic (1444 cases) were analysed to estimate vaccine efficacy. Estimates of 70% and 80% immunisation coverage were used in this analysis for the age groups of 1-4 years, 5-9 years, 10-14 years, and 15-19 years.
Results: A trend was demonstrated, suggesting a reduction in vaccine efficacy (VE) for the two older age groups at 70% coverage (1-4 years 69% VE, 5-9y 68%, 10-14y 46%, 15-19y 47%) and 80% coverage (1-4y 82%, 5-9y 81%, 10-14y 69%, 15-19y 69%). Potential sources of bias are highlighted and their impact on the vaccine efficacy estimates discussed.
Conclusions: There are multiple sources of bias that exist. If New Zealand is to improve and monitor the quality of its vaccination programmes, good quality information is required, which may include the need for immunisation registers, or ongoing case control studies in certain circumstances.