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Brief Reports Indian Pediatrics 2001; 38: 1286-1289 |
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Study of Measles Antibody in Children |
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Measles and its associated morbidity and mortality cause the greatest burden on the health and lives of the children in the developing world(1). The advent of immunization against measles has led to a sharp decrease in the incidence of the disease in all the countries where it was introduced(2,3). However, serological analysis remains important because of causes which occur due to missed vaccination or failure of seroconversion(4). The existing strategies for measles control in India have not been able to achieve the target of 90% coverage with measles vaccination by 1995 and measles and measles outbreaks continue to occur in the community. This communica-tion is the result of a study into the prevalence and titers of measles antibody in children aged 12 to 24 months. This age group was chosen because it is expected that most of the immunized children would have sero-converted by this age, since the measles immunization age is nine months. Subjects and Method One hundred and sixty-five healthy children in the age group of 12 to 24 months attending the outpatient department were include. An attempt was made to elicit the history of measles infection. The immuniza-tion status of these children was assessed as per records available. Children having malnutrition (grades 3 and 4 according to the IAP classification), history of any chronic illness such as tuberculosis, nephrotic syndrome, immunological disorders and those on steroid therapy were excluded from the study. The blood samples were collected by venepuncture and allowed to clot. Serum was separated, labeled and stored at –20º C. Microhemagglutination inhibition (HAI) tests were performed on the sera according to the method of Katz and Enders. Results and Discussion The children were divided into 3 sub-groups according to their immunization status and history of measles (Table I). Group A comprised of 55 immunized children, Group B comprised of 55 unimmunized children with no previous history of measles. Among the 55 children with a clinical history of measles (Group C), all were positive for antibody to measles with titers clustering around 64, 128 and 256 in the majority of children. The lowest titre was 8, the highest going up 1024. This shows excellent correlation of a history of clinical measles with positive antibody titers. It is interesting to note that most of these children had relatively high titers.
Table I__Distribution of Serum HAI Titers in Groups A, B and C
Among the 55 unimmunized children (group B), there were two clusters. The first consisted of 47 sera without detectable antibody. All these children gave no history of measles infection and had obviously lost the maternal antibody. It has been seen that in developing countries maternal antibody is usually lost by the age of six to seven months(5). The second cluster consists of three children with titers of 2 and 4, and five children with titers of 16 and 32. The ages of children in this cluster ranged from twelve to eighteen months. Two explanations are possible. These titers of 2 or 4 could be the result of residual maternal antibodies. Studies on the speed of loss of maternal measles antibodies suggest that this depends on qualitative difference in the nature of maternal antibodies, quantitative differences in maternal antibody titers, differences in effi-ciency of placental transport and differences in the rate of degradation of passive antibodies by the infant(6). The five children with antibody titers of 16 and 32 could have suffered from subclinical measles at some time. Even the three children with titers of 2 and 4 could have had subclinical measles during their infancy. There is some evidence to suggest that subclinical measles occurs more often than previously thought(7) and low antibody response to subclinical measles in infants has been seen(8). The most interesting and significant find-ings are among the 55 immunized children (group A). Eleven (20%) were seronegative, inspite of documented immunization at the age of nine months. This clearly indicates primary vaccine failure, which could be due to persistence of the maternal antibody(9) or improper storage and handling of the vaccine(10) and emphasizes the need for surveillance and improvement of the vaccination program. It was also seen that in group A 14 children (25.5%) had very low antibody titers. All of these children had received the measles vaccine at nine months. This suggests that a second dose of measles vaccine before the age of two years should be seriously considered in order to reduce the number of primary vaccine failures and to boost titers in all children. Several developed countries including the United States have achieved a substantial reduction in the incidence of measles due to initiation of a two-dose measles vaccine strategy. The observations of this study suggest that more such seroepidemiological studies on a larger scale should be conducted to determine the prevalence of measles infection, to identify high-risk pockets, provide data needed to design and evaluate alternate methods of disease control and to assess immunization programs and vaccines. Contributors: AC conceptualized and designed the study and will act as the guarantor for the paper. DR collected, analyzed and interpreted the data. SY critically revised the article for important intellectual content. Funding: None. Competing interests: None stated.
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