Effectiveness of Japanese encephalitis vaccine SA 14-14-2 and impact of immunization among adults in Assam, India

Abstract

Background: Japanese encephalitis (JE) is the leading cause of viral encephalitis in India. Assam contributes 2.5% of the country's population, contributing to ∼36% of JE case-load. Vaccination is the only preventive measure available to combat the disease. Live attenuated JE vaccine, SA 14-14-2 is the most widely used JE vaccine in the Asian sub-continent. JE was earlier known as the disease of children. Thus, to combat the disease among the pediatric age-group, the Government of India (GoI) undertook a mass JE vaccination program in campaign mode during 2006 among children (1–15 years). Interestingly, a rising trend of JE among adults became conspicuous since 2007. In response, GoI undertook mass JE vaccination drive among adults in the worst JE affected district of Assam, Sivasagar, on a pilot basis in 2011. Another vaccination drive was carried out in 2014 to cover the missed out adult population. The present study aimed at evaluating vaccine effectiveness (VE) of a single dose of SA 14-14-2 up to seven years post-vaccination and understanding the impact of immunization among adults in Sivasagar. Methods and materials: During 2012–2018, a case-control study was conducted recruiting 261 JE cases and 522 age-sex matched healthy controls. VE was calculated based on the odds ratio. Incidence rate (IR) per 100,000 population during pre (2009–2011) and post-vaccination periods (2012–2018) were compared to assess the impact of immunization. Results: VE among adults was 90% in 2012; it gradually declined to 82% in 2013. Following the second round of vaccination in 2014, a marginal increase in VE was noted (84%). Subsequently (2015–2018), VE stabilized at ∼70%. IR during the pre-vaccination period was 11.5 that came down and maintained at ∼5 (post-vaccination). Vaccine coverage among controls was 39%. Conclusion: In Nepal, the same vaccine showed 96.2% VE among children (5 years’ post-vaccination) with coverage of above 70% that brought down IR <1. Therefore, high vaccine coverage (at least 70%) seems to be a prerequisite for achieving the desired results. As adults are highly mobile population, proper planning and sensitization prior to mass vaccination drive are of paramount importance to get a sizeable population covered.