Measles vaccine-induced immunity in children age nine months after vaccination and measles-vaccinated children age five years in Nairobi
Brown, Arthur N.
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Measles is the leading cause of vaccine-preventable deaths among children under 5 years globally, despite the availability of measles vaccines. Despite national and international efforts in the coverage of measles vaccination in Kenya, the cases of measles continue to rise in recent years. Measles vaccination coverage was about 86% as reported in 2010, in Kenya following the 2009 Supplementary Immunization Activities (SIAs). In Nairobi, as of 2012 the measles vaccination coverage was over 100%. Yet in 2012, the number of measles cases was 2462 up to the 5th of November, with 767 cases reported from September to the 3rd of October 2012, an indication that the disease infects even the measles vaccinated children. The aim of this study was to evaluate the level of measles-specific maternal antibodies (mIgG) prior to vaccination in children, at age nine months; immunity induced by measles vaccine in children after initial vaccination; and the immune status of children aged 5 years who had been vaccinated at age 9 months in Nairobi. The study also investigated factors that lead to measles outbreaks in Nairobi. 2ml of blood was collected from 66 children scheduled for vaccination before and two weeks after vaccination. Another 2ml of blood was obtained from 62 children aged 5 years, who visited the child welfare clinic at the Mama Lucy Kibaki Hospital (MLKH) from August to November 2013. Each mother of the enrolled children was asked to voluntarily fill out a structured questionnaire. The blood samples were analyzed for the levels of measles specific IgM and IgG using ELISA. The results showed that 97.0% of the children had lost maternal IgG to measles before vaccination at the age of 9 months. The results also revealed that 77.5% of the children sampled for post-vaccination analysis had measles-specific IgM levels greater than 15 IU/ml, which is considered positive. 88.7% of the children aged 5 years had IgG levels greater than 200 mIU/ml, which is conventionally considered to be immune. This study concluded that maternal measles antibodies are lost before 9 months in most children in Nairobi, Kenya. It is also concluded that the measles vaccine (Schwarz strain) that is administered at 9 months is immunogenic and that the antibodies (IgG) produced by measles vaccination persist up to 5 years after vaccination. It was also noted that mothers who had secondary level education and above, had a better understanding of measles and the importance of vaccination; therefore they were most likely to have their children vaccinated. The age of the mothers had no significance to the vaccination of the children. It is suggested that initial vaccination be given earlier than age nine months. In addition children could be given a booster vaccination at age 12 months in order to cater for the small fraction (3%) whose maternal IgG were detected at 9 months old. Since the present study was conducted in a relatively small population, a further study is recommended in a larger population.