Incremental effectiveness of 2 doses of measles-containing vaccine compared with 1 dose among high school students during an outbreak

A measles outbreak occurred among a highly vaccinated population in Alaska during 1998, providing an opportunity to determine the incremental efficacy of greater than or equal to2 doses of measles-containing vaccine (MCV) compared with 1 dose. Of 33 confirmed case patients identified, 31 had been vaccinated with 1 dose of MCV, 1 had received 2 doses, and vaccination status was unknown in 1 case. Seventy percent of cases were school-associated; 58% of cases occurred in 2 high schools. Of 3679 students attending the 2 schools, 50.4% and 45.5% had received greater than or equal to2 doses of MCV before measles introduction at the schools. The relative risk of developing measles among persons vaccinated with greater than or equal to2 doses of MCV compared with 1 dose was 0.06 (95% confidence interval, 0.01 - 0.44; P < .001), yielding an estimated incremental vaccine efficacy of 94.1% (95% confidence interval, 55.9% - 99.2%; P < .001). Rapid implementation of a mandatory second-dose MCV requirement probably limited the extent of this outbreak.