Increased protections during a measles outbreak of children previously vaccinated with a second dose of measles-mumps-rubella vaccine

Background. In 1989 a second dose of measles vaccine was recommended for US children to prevent school outbreaks of measles, Coverage of school age ch ildren with a second dose remains incomplete, and data on the effectiveness of this recommendation are limited. We investigated a measles outbreak in Mesa County, CO, in December, 1994, and evaluated the efficacy of preout-br eak immunizations at an elementary school (School A) where many students ha d received two doses. Methods, All reported suspected cases of measles were investigated; cases t hat met a clinical case definition were tested by a measles IgM antibody as say. A confirmed case required laboratory confirmation or had to meet the c linical case definition and be epidemiologically linked to a confirmed case . Vaccination records of students at School A were reviewed. The effectiven ess of one and two doses of measles vaccine was estimated using logistic re gression. Results. Sixty-two confirmed cases were reported, including 17 at School A. At School A the attack rate in unvaccinated children (7 of 16, 44%) was hi gher than in those with 1 dose (10 of 320, 3%) or 2 doses (0 of 289, 0%). E stimated vaccine effectiveness was 92% for 1 dose and 100% for 2 doses. Two doses were better than one dose in decreasing the likelihood of acquiring measles (P = 0.003). Conclusions. The lower attack, rate among two dose recipients provides evid ence that a two dose strategy can help prevent measles in schools. Administ ering the second dose at elementary school entry can help prevent the persi stence of susceptible cohorts of children and is likely to be important in sustaining elimination of indigenous transmission of measles in the United States.