Measles vaccine effectiveness and duration of vaccine-induced immunity in the absence of boosting from exposure to measles virus

Background. It is unknown whether vaccine-induced immunity is lifelong in the absence of periodic exposure to measles virus. After 27 years of no known exposure to measles, an outbreak in Palau in 1993 offered the opportunity to study this issue and the measles vaccine effectiveness. Methods. Household contacts of a sample of confirmed cases were interviewed for exposure, symptoms and vaccination status verified by records. Serum from symptomatic contacts was tested for measles antibodies. Results. Among 78 contacts 4 of 5 (80%) unvaccinated, 4 of 35 (11%) 1-dose vaccine recipients and none of 38 (0%) >1-dose recipients developed measles. Effectiveness of 1-dose vaccine was 86% (95% confidence interval, 60 to 95%). An additional dose significantly reduced the risk of measles (P = 0.048). Time since vaccination was not a significant risk factor for developing measles (relative risk, 1.6; 95% confidence interval, 0.3 to 9.4; persons vaccinated >15 years ago vs. <5 years ago). Conclusions. Similar to the estimates previously obtained in the area, measles vaccine effectiveness in Palau was lower than the estimates obtained in the US. A second dose of vaccine further reduced the risk for developing measles. We found no evidence that waning immunity was an important problem in this limited population with no known previous exposure to measles virus. The small number of vaccinated contacts precludes a definitive assessment.