Antibody levels in 41 Indian girls were measured 6 years after measles-mumps-rubella (MMR) vaccination. Rates of seropositivity were 88% (measles antibodies), 95% (mumps antibodies), and 100% (rubella antibodies). The MMR vaccine induces long-term immunity in a majority of vaccinees; however, due to the observation of some seronegative vaccinees, the policy of administering a second dose of the MMR vaccine seems appropriate.Measles outbreaks in immunized populations have been reported previously (7,13). Similarly, mumps outbreaks in highly vaccinated populations (4, 10), including the recent large epidemic in the United States (5), have occurred. Both primary vaccine failure and waning immunity are responsible for the outbreaks in vaccinated populations. The infective dose and the vaccine strain are important factors for long-lasting immunity (6).In India, the measles-mumps-rubella (MMR) vaccine is manufactured by the Serum Institute of India Ltd. at Pune. The vaccine contains Edmonston-Zagreb measles virus, Leningrad-Zagreb mumps virus, and RA 27/2 rubella virus. Studies of Indian children using this vaccine have shown over 95% seroconversion against measles and rubella and 90% seroconversion against mumps (2, 12). The antibody persistence study would indicate the likely duration of protection afforded by the vaccine. Therefore, the present study was undertaken to assess the antibody titers persisting in a previously vaccinated pediatric population.In November and December 1999, 99 healthy children aged 1 to 10 years from Kusumbai Motichand Mahila Seva Gram, Karve Road, Pune, an organization for the rescue and rehabilitation of women and children, were given a single dose of an MMR vaccine (Serum Institute of India Ltd.) for the first time. The present study was conducted between April and August 2005 to assess the persistence of antibodies. The serological tests were conducted in the quality control department of the Serum Institute of India Ltd.The study was approved by the ethics committee of the Serum Institute of India Research Foundation. Informed written consent from the legal guardian of each child or the warden of the institute and signed assent from the subjects were obtained. Forty-one children from this group were available for follow-up in 2005. Acute febrile illness, any other infection, conditions associated with immunosuppression, the receipt of immunosupressive therapy, and participation in any other clinical trial within 1 month before and during the course of the study were criteria for exclusion.In 1999, the batch number of the vaccine used was 320-V (expiration date, August 2001). A 0.5-ml dose of reconstituted vaccine was given subcutaneously. In 2005, measles and rubella immunoglobulin G (IgG) antibodies were assayed by the enzyme-linked immunosorbent assay technique using Trinity Biotech kits. Mumps IgG antibodies were assayed by using a Calbiotech Inc. kit. For measles and rubella antibodies, immune status ratios of Ն1.1 were interpreted as seropositive. For data analysis, values below ...