In addition, the investigator will make a monthly telephone call to the family of enrolled infants asking for specific history compatible with the World Health Organization (WHO) case definition for measles, in the infant or any household member. In case such history is elicited, the infant and affected individual will be invited for clinical examination and a sample of blood will be obtained. Serum will be separated as described and immunoglobulin M (IgM) anti-measles antibody will be measured using commercially available ELISA kits as described below.