A population-representative serosurvey estimating vaccine-induced immunity against measles, rubella, hepatitis B and severe acute respiratory syndrome coronavirus 2 in Timor-Leste

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Background Serosurveillance can be used to assess population immunity to vaccine-preventable diseases (VPDs). This study aimed to determine seroprevalence of four VPDs across Timor-Leste and identify immunity gaps. Methods A population-representative three-stage cluster random sample of census-enumerated households were visited between October 2021 and February 2023. Occupants aged above one year were tested for measles immunoglobulin G (IgG), rubella IgG, hepatitis B surface antibody (HBsAb), hepatitis B core antibody (HBcAb) and severe acute respiratory syndrome 2 (SARS-CoV-2) IgG, using serological assays with a priori determined cut-offs. Sample- and response-weighted mixed effects logistic regression models were used to estimate seroprevalence in relevant age-strata. Findings Of 2613 eligible households, 1908 (73.0%) participated. Of 8427 occupants, 4750 (56.4%) participated. Measles IgG seroprevalence was low among children, particularly those aged 10–14 (33.2%, 95% confidence interval (CI) 27.8–38.6%). Rubella IgG seroprevalence was high in all ages (93.2%, 95% CI 92.2–94.2%). SARS-CoV-2 IgG seroprevalence was high, including in young children not eligible for vaccination (74.0%, 95% CI 70.4–77.6%). HBsAb seroprevalence was lowest among adolescents aged 15–19 (12.1%, 95% CI 6.8–17.5%) but higher among younger children, who also had low HBcAb seroprevalence. Interpretation The pattern of measles immunity is consistent with low virus transmission and suboptimal childhood vaccine uptake. These data have informed supplementary immunisation activities. High rubella IgG seroprevalence suggests recent or ongoing virus transmission and a need for congenital rubella syndrome surveillance. Hepatitis B data provide evidence of recent improvements in vaccine-induced immunity and protection. This study demonstrates how serosurveillance can directly influence national vaccine strategies. Funding This study was funded by the Department for Foreign Affairs and Trade, Australian Government (Complex Grant Agreement Number 75889).

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