Measles vaccine efficacy determined from secondary attack rates during a severe epidemic

In February, March, and April 1974, a severe epidemic of measles, with 71 cases and three deaths, occurred on the Cheyenne and Standing Rock Sioux Indian Reservations in South and North Dakota. The attack rate was 9.0 cases per 1,000 persons, and associated with the illness were 24 cases of pneumonia and ten cases of otitis media. Age-specific attack rates were highest in those under one year of age. Using secondary attack rates in persons under nine years of age who were vaccinated and unvaccinated family contacts of cases, vaccine efficacy was measured as 97.3% (95% confidence interval 80.1 to 99.9%).