Failure to reach the goal of measles elimination: Apparent paradox of measles infections in immunized persons

Background: Measles is the most transmissible disease known to man. During the 1980s, the number of measles cases in the United States rose dramatically. Surprisingly, 20% to 40% of these cases occurred in persons who had been appropriately immunized against measles. In response, the United States adopted a two-dose universal measles immunization program. We critically examine the effect of vaccine failure in measles occurring in immunized persons. Methods: We performed a computerized bibliographic literature search (National Library of Medicine) for all English-language articles dealing with measles outbreaks. We limited our search to reports of US and Canadian school-based outbreaks of measles, and we spoke with experts to get estimates of vaccine failure rates. In addition, we devised a hypothetical model of a school where measles immunization rates could be varied, vaccine failure rates could be calculated, and the percentage of measles cases occurring in immunized students could be determined. Results: We found 18 reports of measles outbreaks in very highly immunized school populations where 71% to 99.8% of students were immunized against measles. Despite these high rates of immunization, 30% to 100% (mean, 77%) of all measles cases in these outbreaks occurred in previously immunized students. In our hypothetical school model, after more than 95% of schoolchildren are immunized against measles, the majority of measles cases occur in appropriately immunized children. Conclusions: The apparent paradox is that as measles immunization rates rise to high levels in a population, measles becomes a disease of immunized persons. Because of the failure rate of the vaccine and the unique transmissibility of the measles virus, the currently available measles vaccine, used in a single-dose strategy, is unlikely to completely eliminate measles. The long-term success of a two-dose strategy to eliminate measles remains to be determined.