Estimating hepatitis A immunity and outbreak risk among MSM in New South Wales, Australia, 2017-2018.

In 2017-2018, New South Wales (NSW), Australia, experienced a hepatitis A virus (HAV) outbreak predominantly affecting men who have sex with men (MSM). We aimed to estimate immunity and identify immunity gaps among MSM in NSW. We triangulated four data sources. We tested 409 residual sera collected from males undergoing syphilis testing between August and October 2017 for anti-HAV IgG. We surveyed all publicly funded sexual health clinics about HAV testing and vaccine provision for the preceding financial year. We conducted a convenience behavioural survey of 50 men at a sex-on-premises venue in October 2017, and we analysed community survey data from 2018 to 2019 (n = 5,924). Descriptive statistics and logistic regression ere used to examine associations with immunity and vaccination. Overall serological immunity was 62.8% (257/409) and increased with age, from 50.0% in those aged under 26 years to 79.0% in those over 45 years. Clinic immunity and community survey estimates were higher (74.5% and 76.8%). Immunity gaps were identified among bisexual men, those less connected to the gay community, and people born outside Australia. NSW Health implemented targeted vaccination and community engagement in response. Multisource assessment provided actionable evidence to guide targeted vaccination and outbreak response.