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Sri Lanka is a dengue (DENV) endemic country which since 1988 has introduced vaccination against Japanese encephalitis virus (JEV). JEV and DENV are both closely related flaviviruses with known cross-reactivity. This cross-reactivity between DENV and JEV is known to affect dengue severity. Antibodies to DENV NS1 are associated with severe dengue. The effect of JEV NS1 on dengue severity has not been evaluated in Sri Lanka and underlying JEV NS1 seroprevalence can potentially be important in dengue vaccine responses. Therefore, a JEV NS1 ELISA was developed as a community assessment tool for testing seroprevalence. An in-house indirect JEV NS1 ELISA was developed and tested in a community cohort of 80 individuals between the ages of 6-80 and with varying JEV vaccine status and past dengue severity. JEV NS1 seroprevalence was higher in individuals who had received the live attenuated vaccine than those immunized with the inactivated vaccine. JEV NS1 responses were higher in older unvaccinated individuals and correlated with higher DENV antibodies. JEV NS1 antibodies were associated with a higher probability of having severe dengue in older unvaccinated individuals. JEV NS1 immunity is probably acquired by cross-reactivity with other flaviviruses such as dengue than through vaccination. The JEV NS1 ELISA is a useful tool to assess seroprevalence in the community and could be potentially useful in community assessment prior to dengue vaccine introduction. Further validation studies with neutralization assays need to be done on a larger number.

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