Effect of first and second doses of measles vaccine on immunity to measles among infants in Sana'a city, Yemen

ABSTRACT

To investigate the effects of first and second doses of measles vaccine on immunity to measles among infants in Sana'a city,. A total of 518 infants, aged 2-24 months were enrolled. Of these 376 infants were unvaccinated and were either under the age of vaccination or had already passed the age of vaccination [age 2 to 12 months], 71 infants who were given the first dose and 71 infants who were given two doses of measles vaccine. Serum samples were tested for measles specific IgG. Two hundred and seven out of 376 [55.1%] unvaccinated infants were seronegative for measles antibodies. With increasing age, the rates of seronegativity increased and the mean antibody levels among seropositive infants decreased. In the unvaccinated cohort [n=129] who aged 9-12 months [the age at which all infants are due for vaccination], 109 [84.5%] were seronegative. Compared to unvaccinated infants 56 of 71 [78.9%] who were given the first dose of vaccine seroconverted [p<0.0001] developing a significantly higher mean antibody level [p<0.001]. However, 65 of 71 [91.5%] who had received the second vaccine dose seroconverted [pwho only received the first dose. However, the antibody levels decreased significantly with time intervals after the first dose [p=0.044] but insignificantly decreased after the second dose [p=0.18]. The relatively high rate of non-responders along with the decline of antibody concentrations after the two doses of vaccine could be due to vaccine failure. This demands improving the vaccination coverage and supplementary vaccination campaigns