Evaluation of an early two-dose measles vaccination schedule

Abstract

Vaccination at 6 months of age followed by routine revaccination is recommended when exposure of infants to measles is likely. Dade County, Florida, began this early two-dose schedule during a large epidemic in 1986-1987 (i.e., 22% of cases occurred in infants aged 6-11 months). This schedule was continued routinely in high-risk areas. The effect of an early two-dose schedule on measles prevention in the county was examined by comparing measles vaccination coverage and epidemiology before (1985-1987) and after (1988-1996) the schedule became routine. To assess serologic response, seroprevalence of measles antibody among children aged 4-6 years in 1995 was examined. To evaluate vaccine effectiveness, a case-control study was conducted among preschool-aged children. Among those aged 2 years, vaccination coverage with ≥1 dose increased from 75% to 94% in 1996. The number of annual cases declined, and endemic measles transmission reportedly ended after 1993. Seroprevalence of plaque reduction neutralization antibody (titer > 1:120) among those receiving vaccination according to an early two-dose schedule and a single dose at age ≥12 months was 94% (95% confidence interval: 89, 98) and 98% (95% confidence interval: 95, 100). In these groups, vaccine effectiveness was comparably high. Early two-dose measles vaccination is associated with improved coverage and a comparably high level of humoral immunity and clinical protection as a single dose at age ≥12 months. This strategy can be useful in areas at high risk for measles among infants.