Community-based, prospective cohort study evaluating susceptibility to measles during the first year of life among infants in India

Abstract

Background: Measles is a significant public health problem in India, despite infant vaccination at 9 months of age since 1985. The age of 9 months was chosen to balance the disappearance of maternal (transplacental) antibodies and ability of infants to mount an immune response. However, the duration and quality of transplacental protection has not been studied in the present era, wherein majority of mothers have antibodies acquired through vaccination rather than natural infection. This community-based, prospective study was undertaken to determine measles susceptibility in infants followed from birth to 12 months of age, in 30 anganwadi clusters in Chandigarh, India. Methods & Materials: Using systematic random sampling, 420 infants and mothers were recruited at birth. The infants were followed up at 3, 6 and 9 months (pre-vaccination). At each visit, history of measles (WHO case definition) was inquired about and blood drawn for quantitative estimation of anti-measles IgG antibodies using ELISA kits. The proportion of susceptible infants at each time point and the ratio of maternal:infant antibody level at birth were calculated. Results: A total of 305 mothers and 305 infants completed the study protocol. Of these, 90% mothers had protective levels of antibodies (defined > 12mIU/ml), 7% were unprotected and 3% had borderline status. The proportion of protected, unprotected and borderline infants were 92%, 6% and 2% respectively. The proportion of infants that was susceptible to measles at 3, 6 and 9 months was 67%, 97%, and 99% respectively. Only one infant had clinical and laboratory confirmed measles prior to 9 months of age. At birth, only 54 (17.7%) pairs had mother:infant antibody levels in the ratio of 1.0 + 0.1. The remainder had either higher (n = 111, 36.4%) or lower (n = 140, 45.9%) antibody ratios. Conclusion: These data confirm that the majority of Indian infants are susceptible to measles well before the expected age of 9 months. The inconsistent maternal: infant antibody ratio suggests that transplacental antibody transfer cannot always be relied on for infant protection. Taken together, this study confirms an urgent need to lower the age of infant measles vaccination in India.