Immune response to measles vaccine in 6 month old infants in Papua New Guinea

Abstract

Objective: To assess the efficacy of the current measles immunization schedule in Papua New Guinea, which is to give the first dose at 6 months of age and the second at 9 months.

Methods: Humoral immune response study of 140 Papua New Guinean infants at 6 months of age, measuring measles IgG antibodies by enzyme immunoassay before and 85 days after the 6-month dose of measles vaccine.

Results: After vaccination at 6 months, 35.7% of infants developed a level of measles antibodies consistent with protection (IgG >330 IU/ml); 17.7% had an antibody response (150-330 IU/ml) that is likely to afford some protection; 46.8% had no detectable antibody response (IgG <150 IU/ml). Among 53 infants with no antibody response, 37 (69.5%) developed an antibody response, while 42.4% (37/87) of those with maternal antibodies sero-converted (P = 0.002).

Conclusions: Antibody response to measles vaccine was lower than expected at 6 months. While the presence of maternally derived antibodies accounted for some of the limited seroconversion in young infants, other factors are involved. Issues to be considered in determining the value of the first dose of measles vaccination in mid infancy in poor countries are complex and antibody responses are only one factor. Others, such as cell mediated immune responses, the non-specific protective effect of measles vaccine in preventing illness and death and the practicalities of uptake of vaccines at different ages, are also important.