Tracking epidemiological shifts in hepatitis A in Portugal: a comparison of seroprevalence between two nationwide surveys, 2001 to 2002 and 2015 to 2016

BACKGROUND: Hepatitis A incidence in Portugal declined from 20.1 to 0.4/100,000 population between 1987 and 2023, changing non-vaccinated population susceptibility. This shift has contributed to more frequent outbreaks, including in 2024–25, highlighting the need to enhance surveillance and integrate serological data. AIM: We aimed to describe the exposure profile of the Portuguese population to hepatitis A virus (HAV) over time by estimating and comparing the seroprevalence of hepatitis A in two nationwide surveys. METHODS: Data from two cross-sectional seroprevalence studies performed in 2001–02 and 2015–16 in the population aged ≥ 2 years were analysed. Seroprevalence was weighted for population distribution by age, sex and region, and then analysed by birth cohort (1911 -2014) and compared using Poisson regression. RESULTS: Overall prevalence of anti-HAV IgG antibodies was 67.3% (95% CI: 64.2–70.3) in 2001–02 (n = 1,642) and 56.3% (95% CI: 52.4–60.2) in 2015–16 (n = 2,052), showing an 11-percentage-point decline. Birth cohort analysis revealed consistent seroprevalence within each cohort in both surveys, i.e. seroprevalence for the 1981–90 birth cohort was 16.7% and 18.7%, respectively, suggesting that higher seroprevalence is more closely associated with birth cohort (cohort effect) rather than a specific time point. Additionally, we found that individuals aged ≥ 30 years, born before the 1980s, and those with lower education had higher seroprevalence. CONCLUSIONS: The immunological profile of anti-HAV antibodies in the Portuguese population has shifted over the last decades. High susceptibility and shifting age distribution of Hepatitis A-seropositive individuals highlight the need to revise future vaccination strategies in Portugal.