Population immunity to measles in the United States, 1999.

To estimate population immunity, we examined measles immunity among residents of the United States in 1999 from serological and vaccine coverage surveys. For persons aged >or=20 years, serological data from the third National Health and Nutrition Examination Survey (1988-1994) were used. For persons <20 years of age, immunity was estimated from results of the National Immunization Survey (1994-1998), state surveys of school entrants (1990-2000), and vaccine coverage surveys of adolescents (1997). To estimate immunity from vaccine coverage data, 95% vaccine efficacy was used for recipients of a single dose at >or=12 years of age and 99% vaccine efficacy was used for those with failure of a first dose who were revaccinated. Overall, calculated population immunity was found to be 93%. Although there was not much variation in immunity by region and state, in some large urban centers immunity among preschool-aged children was as low as 86%. Overall, geographic- and age-specific estimates of a high population immunity support the epidemiological evidence that measles disease is no longer endemic in the United States.