A cohort event monitoring to determine the adverse events following administration of mouse brain derived, inactivated Japanese Encephalitis vaccine in an endemic district in Sri Lanka

• JE cases dramatically declined in Sri Lanka following annual JE vaccination.
• Over the years, low vaccine uptake was observed due to perceived fear of AEFI.
• We determined AEFI consistent with causal association to incriminated vaccine.
• AEFI incidence was several folds higher than routine surveillance figures.
• Serious AEFI rates were very low with no neurological disorders.

Introduction of human immunization reduced Japanese Encephalitis (JE) cases dramatically in Sri Lanka. However, the increased reporting of adverse events following immunization (AEFI) affected vaccine acceptance by the community. Against this background, we describe the incidence of overall AEFI and incidence and profile of AEFI, thought to be causally related to the mouse-brain derived JE vaccine.A follow-up of 9798 vaccine recipients was performed for a period of two weeks post-vaccination. Parents self-recorded observed signs and symptoms. The self-records were collected by trained supervisors. All monitored children who manifested symptom/s were investigated in details by medical officers experienced in AEFI investigations within two weeks after ending the follow-up period. Using the results of the investigation, the causality assessment was performed.The estimated cumulative incidence rate of overall AEFI was 8.6 children per 100 immunizations. The same for observed AEFI consistent with causal association to the inactivated JE vaccine was 4.3 children (95% CI—3.9–4.7%) per 100 immunizations. The most frequent AEFI was fever (81%). The frequency of high fever (>102 °F) was 26%. Other major AEFI were body ache (22%) vomiting (21%), urticaria (19%), pruritus (5%), and headache (5%). Though 83% of children with AEFI thought to be causally related to the vaccine sought medical care, only 6.6% required hospitalizations.The incidence rate of AEFI in the cohort event monitoring was several-fold higher than that reported through the national AEFI surveillance system. The incidence rate of allergic manifestations among Sri-Lankan children approached what was reported for non-endemic settings and was higher than in other JE endemic populations elsewhere. Contrary to the belief of medical practitioners and the general public, incidence of seizures was low and vaccine related other neurological manifestations were absent.