Serosurvey for Japanese encephalitis virus antibodies following an outbreak in an immunologically naïve population, Victoria, 2022: a cross-sectional study

Objectives: To investigate the distribution and prevalence of Japanese encephalitis virus (JEV) antibody (as evidence of past infection) in northern Victoria following the 2022 Japanese encephalitis outbreak, seeking to identify groups of people at particular risk of infection; to investigate the distribution and prevalence of antibodies to two related flaviviruses, Murray Valley encephalitis virus (MVEV) and West Nile virus Kunjin subtype (KUNV).

Study design: Cross-sectional serosurvey (part of a national JEV serosurveillance program).

Setting: Three northern Victorian local public health units (Ovens Murray, Goulburn Valley, Loddon Mallee), 8 August - 1 December 2022.

Participants: People opportunistically recruited at pathology collection centres and by targeted recruitment through community outreach and advertisements. People vaccinated against or who had been diagnosed with Japanese encephalitis were ineligible for participation, as were those born in countries where JEV is endemic.

Main outcome measures: Seroprevalence of JEV IgG antibody, overall and by selected factors of interest (occupations, water body exposure, recreational activities and locations, exposure to animals, protective measures).

Results: 813 participants were recruited (median age, 59 years [interquartile range, 42-69 years]; 496 female [61%]); 27 were JEV IgG-seropositive (3.3%; 95% confidence interval [CI], 2.2-4.8%) (median age, 73 years [interquartile range, 63-78 years]; 13 female [48%]); none were IgM-seropositive. JEV IgG-seropositive participants were identified at all recruitment locations, including those without identified cases of Japanese encephalitis. The only risk factors associated with JEV IgG-seropositivity were age (per year: prevalence odds ratio [POR], 1.07; 95% CI, 1.03-1.10) and exposure to feral pigs (POR, 21; 95% CI, 1.7-190). The seroprevalence of antibody to MVEV was 3.0% (95% CI, 1.9-4.5%; 23 of 760 participants), and of KUNV antibody 3.3% (95% CI, 2.1-4.8%; 25 of 761).

Conclusions: People living in northern Victoria are vulnerable to future JEV infection, but few risk factors are consistently associated with infection. Additional prevention strategies, including expanding vaccine eligibility, may be required to protect people in this region from Japanese encephalitis.