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.
Poland
G A
GA
Department of Internal Medicine, Mayo Vaccine Research Group, Mayo Clinic and Foundation, Rochester, MN.
Jacobson
R M
RM
D002170:Canada / Q000453:epidemiology; D004196:Disease Outbreaks / Q000517:prevention & control*; D006801:Humans; D017589:Immunization Programs; D008457:Measles / Q000453:epidemiology* / Q000517:prevention & control*; D008458:Measles Vaccine / Q000008:administration & dosage; D017211:Treatment Failure; D014481:United States / Q000453:epidemiology
D008458:Measles Vaccine