I. Adetifa, KEMRI-Wellcome Trust Research Programme, Kenya, Seroepidemiology for monitoring vaccination and informing vaccine policy in Africa

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Although vaccines are essential for preventing many severe and often fatal diseases, they are only beneficial when received so their impact is greatly dependent achieving high vaccination coverage. Consequently, vaccination coverage indices are widely used to monitor vaccination programmes. Vaccination coverage is commonly assessed as the proportion of children aged 12-23 months who received the 3rd dose of a diphtheria-pertussis-tetanus (DPT3)-containing vaccine at age 12 months. DPT3 coverage is typically obtained from administrative records (registers in vaccine clinics and records of total doses of vaccines consumed). It can be also obtained from household surveys of vaccine cards/records or by asking caregivers to confirm vaccination by memory recall. Coverage estimates obtained from administrative records frequently overestimate coverage in low and middle-income countries because of errors in tracking consumption of vaccine doses and wrongly estimated target population. While survey methods provide more accurate estimates, they are disruptive to routine health services and expensive. Other weaknesses are incomplete representation of the target population, underreporting, and missing and/or incomplete information. It is important to for estimates of vaccination coverage to be as accurate as possible especially as receiving a vaccine i.e. vaccination does not always result in immunisation which is defined as vaccine-induced protection against the targeted disease. For example, following measles vaccination, the proportions of children who develop protective antibody levels are approximately 85% at 9 months of age and 95% at 12 months of age. Conventional methods for measuring coverage do not tell us what proportion of the population has immunity against the disease of interest. In high income countries, national serological surveys are conducted to obtain blood samples to measure the proportion of the population that is protected by vaccination. These blood samples are tested for antibodies produced in response to vaccines. As a result, these countries have a longer experience of using these antibody surveys to estimate the burden of diseases before introducing vaccines and assessing the performance of vaccination programmes in reducing disease burden e.g. for hepatitis B. These antibody surveys are also used to identify at-risk groups requiring special vaccination programmes e.g.mass campaigns, and to assess the duration of vaccine induced protection. Serosurveys require reliable vaccination records which are typically available in developed countries. In the developing countries of Africa, I found very limited use of serological surveys to monitor vaccination and to guide revisions of vaccination strategies as seen in high income countries. However, I found that testing for antibodies induced by tetanus vaccine may be useful for this purpose because it overcomes one key limitation of serological surveys i.e. distinguishing between antibodies from natural infection and those resulting from vaccination. I will investigate the possibility of using antibody testing to monitor vaccination and to compare this to vaccine records in Kilifi, extend this experience to samples from a nationwide survey. This project will benefit from the excellent opportunities provided by the availability of serum samples collected during serological surveys in Kilifi from 2009-2017 and reliable vaccine records from the Kilifi vaccine registry. I will apply statistical methods to antibody test results to identify a reliable mathematic model that reliably predicts vaccination coverage when compared to vaccine records. On identifying this model, I will then carry out a nationwide serological survey, apply the methods validated in Kilifi to the antibody test results to estimate national vaccination coverage, identify at-risk populations with gaps in their immunity and make recommendations for interventions to protect these populations.