Twice vaccinated recipients are better protected against epidemic measles than are single dose recipients of measles containing vaccine

Objective-To study measles risk after revaccination. Design-A population-based case-control study during an epidemic season. Main outcome measure-Relative serologically confirmed measles risk. Participants and methods-153 vaccinated cases, mostly from rural areas, were serologically confirmed as measles at the central laboratory in 1988-89. A randomly selected group of 453 controls from either municipalities of vaccinated cases or from areas where measles attack rate was >600/105, was identified via the population registry. Vaccination and measles histories of cases and controls were determined from official vaccination cards. Results-Once and twice vaccinated had crude relative risk 15.6 and 2.3 compared with thrice vaccinated. When cases who had received their first vaccination at less than 14 months of age were omitted from analysis, once vaccinated had 4.0 (95% CI 1.2, 16.6) times higher age adjusted measles risk compared with twice vaccinated. When, omission was extended to cases from one particular municipality where even revaccinees had high measles risk during an explosive outbreak the corresponding risk ratio was 17.8 (2.8, 67.8). Conclusions-Twice vaccinated have better protection against epidemic measles compared with single dose recipients.