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Background. The optimum age for measles vaccination varies from country to country and thus a standardized vaccination schedule is controversial. While the increase in measles vaccination coverage has produced significant changes in the epidemiology of infection, vaccination schedules have not been adjusted. Instead, measures to cut wild-type virus transmission through mass vaccination campaigns have been instituted. This study estimates the presence of measles antibodies among six- and nine-month-old children and assesses the current vaccination seroconversion by using a non invasive method in Maputo City, Mozambique. Methods. Six- and nine-month old children and their mothers were screened in a cross-sectional study for measles-specific antibodies in oral fluid. All vaccinated children were invited for a follow-up visit 15 days after immunization to assess seroconversion Results. 82.4% of the children lost maternal antibodies by six months. Most children were antibody-positive post-vaccination at nine months, although 30.5 % of nine month old children had antibodies in oral fluid before vaccination. We suggest that these pre-vaccination antibodies are due to contact with wild-type of measles virus. The observed seroconversion rate after vaccination was 84.2% Conclusion. These data indicate a need to re-evaluate the effectiveness of the measles immunization policy in the current epidemiological scenario. © 2008 Jani et al; licensee BioMed Central Ltd.
This study was conducted in Maputo City, the capital of Mozambique, which has an estimated population of 1.5 million people. Health services in Maputo are organized in three districts, each served by several health-centres and a general hospital. Health-centres offer a free health program for all children under the age of five-years. The program includes immunization, growth monitoring and nutritional rehabilitation. Subjects were recruited at the Xipamanine health-centre and the 1° de Junho health-centre located in the urban districts number four and two of Maputo City. The urban districts number four and two have 300,703 and 534,744 inhabitants and reported measles vaccine coverage of 80% and 83%, respectively (district midterm reports to MoH, unpublished document, 2005). The study was performed between June and September 2005, just before the 2005 measles national mass vaccination campaign. According to the EPI schedule in Mozambique, children are routinely immunized with a single dose of standard titre measles vaccine at nine months of age. At the time when the study was conducted, the measles vaccine in use was based on Edmonston-Zagreb strain (E-Z). Mothers of six-month-old children coming for growth monitoring and of nine-month-old children visiting the health-centre for measles immunization were invited to participate in the study. All children and their respective mothers were screened in a cross-sectional study for measles-specific antibodies in oral fluid. Additionally, all children at nine-months of age were invited to donate a second specimen of oral fluid during a follow-up visit 15 days after vaccination. This study was approved by the Mozambican Health Bioethics Committee. Informed consent was obtained by a signature or finger print from mothers after explaining the project aims, the procedures for data and specimen collection, and the need to return for post-vaccination control among those vaccinated. Data was collected using a structured questionnaire. The age of each child was calculated from the date of birth recorded on the Road-to-Health Card. The age of the mother was based on her verbal statement. A birth was considered to be premature if the baby was born at less than 37 weeks of gestation. This information was collected from the mother’s Pregnancy-Monitoring Card. A baby with a birth weight less than 2,500 grams was defined as having low-birth-weight. This information was collected from the Road-to-Health Card. On admission, the children were weighed (grams) and measured (centimetres). The anthropometric indices weight-for-age, height-for-age and weight-for-height were compared with mean Z-scores to access the nutritional status of the children. Minus two Z-scores were used as cut-off values for low height-for-age, weight-for-age and weight-for-height indices. The immunization status of the mothers was confirmed by the Road-to-Health Card when possible. A verbal history of immunization of the mother with no card for confirmation was categorized as a “history of immunization”. Mothers with no knowledge of their past immunization were classified as having an “unknown” status. A past history of measles disease in the infant and in the mother was collected by verbal history using the World Health Organization case definition [18]. The mother’s reproductive history was collected from the last gestation Pregnancy-Monitoring Card retained by the mother. Verbal information was not considered. Oral fluid was collected using the OraSure device (OraSure Technologies, Bethlehem, PA, USA). The collection device consists of a 3 cm × 1 cm flat pad of absorbent material supported by a 10 cm plastic stick. This device is supplied with a tube containing transport buffer and a preservative [19,20]. The absorbent pad was moved gently 4–10 times along the gums and left stationary between the lower gum and buccal membrane for a minimum of two minutes or until the pad was saturated with oral fluid. Thereafter, the collection device was placed in the pre-coded tube containing the buffer. The pads with oral fluid samples were transported to the laboratory at the Instituto Nacional de Saúde every day. There, tubes were centrifuged at 2000 rpm for five minutes. The fluid was then transferred into a screw-capped vial and stored at -20°C until testing. Oral fluid specimens were screened for measles-specific IgG and IgM using the MicroImmune® test (MicroImmune Ltd, UK). Both assays are capture EIAs and clasiify antibody status as positive, negative and borderline. The IgM test has a reported sensitivity and specificity of 100.0% (95% CI 85.2–100.0) and 96.6% (95% CI 90.7–99.3), respectively [17]. The sensitivity and specificity of the IgG assay are 97.5% (95% CI 96.1–98.3) and 86.7% (95% CI 78.4–91.5), respectively [16,21]. Proportions and chi-square were used as a statistical test at a 5% significance level. The differences in mothers’ ages were assessed by one way analysis of variance. Statistical procedures were performed using the Statistical Package for the Social Sciences (SPSS) version 15.0. Seroconversion was expressed as the proportion of seronegative children before vaccination who became positive for measles-specific antibody (IgM and IgG) 15 days after vaccination. The statistical difference in the proportions was compared using chi-square test with a 5% significance level. Samples with borderline results were excluded from this analysis.
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