MEASLES HERD-IMMUNITY - THE ASSOCIATION OF ATTACK RATES WITH IMMUNIZATION RATES IN PRESCHOOL-CHILDREN

Objective. - To examine the association between incidence of measles and immunization coverage among preschool-age children. Design. - An ecological study in which measles incidence was compared with immunization coverage among census tracts. The independent effects of race and population density were controlled for. Setting. - A recent measles outbreak in Milwaukee, Wis. immunization coverage data were estimated from a retrospective, school-based survey of Milwaukee grade school students. Patients. - One thousand eleven persons (less-than-or-equal-to 17 years) who had confirmed measles from September 1989 through June 1990. Main Outcome Measures. - Confirmed measles cases grouped by census tract, corresponding census tract preoutbreak immunization coverage, racial breakdown, and population density. Results. - Census tracts stratified into four levels, with mean immunization rates of 50.4%, 60.2%, 69.9%, and 81.0%, had respective median attack rates of 11.6, 5.0, 1.7, and 0.0 cases per 1000 persons (P < .01). The association between immunization coverage and measles attack rate remained significant even after controlling for race and population density. Conclusions. - Modest improvements in low levels of immunization coverage among 2-year-olds confer substantial protection against measles outbreaks. Coverage of 80% or less may be sufficient to prevent sustained measles outbreaks in an urban community.