Comparison of protection afforded by single measles vaccination and late revaccination schedules.

An intervention study compared the protection afforded vaccinees by single measles vaccination and late revaccination schedules in 1990-94. During the intervention a single revaccination (after initial vaccination at nine months of age) was applied to children at any point in time between 12 and 23 months of age. Cases of measles aged 10-23 months were identified through an active surveillance system and in this period 5, 11 and 11 cases of revaccinated, single vaccination and unvaccinated children were identified. Measles incidence rates were 392.2-415.6 (mean = 405.6, SD = 6.7), 75.4-112.1 (median = 91.7, SD = 13.3) and 12.8-15.2 (mean = 13.9, DS = 0.99) per 100,000 population in children who were unvaccinated, with single vaccination and revaccinated respectively. Relative risk of contracting measles in children who were unvaccinated or with single vaccination was 26.5-32.5 (mean = 29.4, SD = 2.3) and 5.8-8.8 (mean = 6.8, SD = 1.2) respectively compared with revaccinated children. Vaccine efficacies that were determined were 73-81% (mean = 77.2, SD = 2.9) and 96.2-96.9% (mean = 96.6, SD = 0.27) for single vaccination and late revaccination schedules respectively. It was concluded that late revaccination affords vaccinees better protection than single vaccination through improvement in vaccine.