2.1 Study population
The study population was subjects who completed 2-year follow-up in the
Jinan Birth Cohort, a baseline population recruited from January 2018 to
December 2019 from 15 vaccination clinics selected in urban areas of
Jinan, Shandong Province, and surveyed at the second vaccination of
infants at 1 month of age for hepatitis B vaccine according to the
national immunization plan. The details of the cohort were described in
the previously published article(Bai et al., 2023; Du et al., 2022).
Each mother of the infant was informed of the purpose and content of the
study and signed an informed consent. The study was approved by the
Ethics Committee of Preventive Medicine of Shandong University (Approved
number: 20170315).