Horizontal transmission might be a common route of hepatitis B virus exposure in highly endemic areas.

Hepatitis B virus (HBV) is a common viral pathogen that infects more than a third of the world's population; however, the transmission route remains to be further defined. The 18-year implementation of the free HBV vaccine for children has greatly changed the prevalence of HBV infection in China, which presents a unique real-world model for assessing the pattern of HBV transmission. Cross-sectional data of HBV seromarkers between July 2019 and April 2020 were collected from 53 371 individuals aged 1-60 years in four areas of North to South in Eastern China. Longitudinal data of HBV seromarkers between 2007 and 2020 were collected from 177 adults in an area of South China. The regional- and age-specific changes in HBV seromarkers were analyzed. Overall, positive rates of HBV surface antigen (HBsAg; from 3.44% to 15.1%) and antibody against HBV core antigen (anti-HBc; from 7.6% to 44.0%) significantly increased from North to South. Among persons aged ≤18 years, the positive rates of antibody against HBsAg (anti-HBs) and anti-HBc (+) remained at low levels in the North, while they were increasing among persons aged >12 years in the South, despite higher positive rates of anti-HBs (+). Among persons aged >18 years, the anti-HBs (+) rates remained relatively stable (~60%), while anti-HBc (+) rates increased significantly with age. Up to ~80% of the anti-HBs (+) adults in the South was anti-HBc (+) while it was 13.6% in the North. In the longitudinal cohort, the anti-HBc (+) rate among adults in the South increased by 14.2% during 10 years of follow-up. Horizontal transmission might be a common route in highly endemic areas, and may help to explain the high HBV exposure worldwide. The risk of horizontal transmission among children without seroprotective anti-HBs should be notified in highly endemic areas.