Epidemic Measles in a Highly Vaccinated Population

Abstract

During November, 1975, to May, 1976, measles occurred at a rate of 20.3 cases per 1000 in a purported immunized population, of whom historical and serologic survey revealed that 9 per cent had no history of either measles illness or vaccination and 18 per cent did not have detectable measles antibody. Antibody was detectable in 92 per cent of those vaccinated at ≥13 months, 80 per cent at 12 months and 67 per cent of those vaccinated when less than one year old (P<0.001), but no significant differences existed with increasing years since vaccination (P>0.1). A second vaccination increased detectable antibody prevalence only in those originally vaccinated when less than nine months old (42 to 80 per cent, P<0.02). During a measles outbreak, more cases occurred in those receiving vaccine when less than 12 months old than in those vaccinated at ≥12 months (37 per cent vs. 9 per cent, P<0.001). A second vaccination protected those originally vaccinated at <12 months (35 per cent ill without a second vaccination vs. 2 per cent with, P<0.001). Thus, a single measles vaccination of children <12 months old does not protect; a second vaccination will protect this group. (N Engl J Med 296:585–589, 1977)