Early two-dose measles vaccination schedule in Guinea-Bissau: good protection and coverage in infancy

Previous studies from Africa have suggested that there is little benefit to be gained from early two-dose measles vaccination schedules. Two-dose schedules have been associated with no improvement in coverage due to immunization of the same individuals on both occasions, low return rate, high refusal rate, low vaccine efficacy, and fear of blunting of the antibody response. Because of the poor results achieved previously with two-dose measles vaccination schedules, we studied patterns of participation, reasons for non-participation, vaccination coverage and relative efficacy of a one-dose versus a two-dose schedule in connection with the implementation of an early two-dose trial in Guinea-Bissau.

Children born from September 1994 to January 1996 were randomized into two groups receiving either two doses of measles vaccine at 6 and 9 months or one dose of inactivated polio vaccine (IPV) at 6 months and measles vaccine at 9 months.

At 6 months of age 86% (1869/2181) of the children participated, and at 9 months of age participation was 87% (1775/2035). The return rate for obtaining a second dose of vaccine was 93% (1647/1773). The main reason for not participating was travelling (78%). Around 50% of those who did not take part in one vaccination took part in the other. When only children participating the first time they were called for a measles vaccination were included, the measles vaccination coverage in the one-dose group was 59% versus 80% in the two-dose group, i.e. a 50% reduction in the risk of not being vaccinated (relative risk [RR] 0.50; confidence interval [CI]: 0.43-0.57). Few measles cases have occurred in the study area since the implementation of the trial making precise estimation of the relative efficacy of the two vaccine strategies difficult, but all seven clinically diagnosed measles cases occurred in the one-dose group making the relative efficacy for the two-dose group compared with the one-dose group 100% (95% CI: 35%-100%; two-tailed P = 0.016). When including maternal reports, the relative efficacy was 90% (95% exact confidence interval; two-tailed P = 25%-97%, P = 0.022).

In this study of a two-dose measles immunization schedule at 6 and 9 months of age there was no sign of low participation or poor return rates. The risk of not being vaccinated was lower in the two-dose group than in the one-dose group, and the relative efficacy of a two-dose versus a one-dose schedule was high. Although our results were obtained within a trial where dedicated personnel informed every participant personally about the study, we believe our results indicate that with thorough information about the population it may be possible to achieve a higher coverage with a two-dose measles vaccination schedule than a one-dose schedule. A two-dose schedule may be a feasible way to resolve the problems of low coverage and severe measles infection among infants.