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 effectiven ess. Methods. Household contacts of a sample of confirmed cases were i nterviewed for exposure, symptoms and vaccination status verified by r ecords. Serum from symptomatic contacts was tested for measles antibod ies. 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 de veloped measles. Effectiveness of 1-dose vaccine was 86% (95% confiden ce interval, 60 to 95%). An additional dose significantly reduced the risk of measles (P = 0.048). Time since vaccination was not a signific ant risk factor for developing measles (relative risk, 1.6; 95% confid ence interval, 0.3 to 9.4; persons vaccinated >15 years ago vs. <5 yea rs ago). Conclusions. Similar to the estimates previously obtained in the area, measles vaccine effectiveness in Palau was lower than the es timates obtained in the US. A second dose of vaccine further reduced t he risk for developing measles. We found no evidence that waning immun ity was an important problem in this limited population with no known previous exposure to measles virus. The small number of vaccinated con tacts precludes a definitive assessment.