Hepatitis A virus (HAV) remains a common infection among Thai children. Two types of HAV vaccines are available in Thailand: an inactivated vaccine (I-HAV, administered in two doses 6 months apart) and a live-attenuated vaccine (L-HAV, administered as a single dose). However, neither vaccine is currently included in Thailand's Expanded Programme on Immunization (EPI). In 2024, a randomized, active-controlled, open-label, non-inferiority trial was conducted to compare the immunogenicity and safety of the two-dose I-HAV with the single-dose L-HAV in healthy Thai children and adolescents aged 18 months to 18 years. This study aims to evaluate the long-term seropositive rate and immunogenicity of anti-HAV antibodies in this population following a single dose of L-HAV.

Hepatitis A virus (HAV) infection remains one of the most common causes of viral hepatitis among children and adolescents in developing countries, including Thailand. The virus is primarily transmitted through the ingestion of contaminated food or water, or through direct contact with an infected person. HAV typically causes acute hepatitis, ranging from mild illness to severe fulminant hepatitis (acute liver failure), but it does not lead to chronic liver disease. Vaccination is a highly effective strategy to prevent HAV infection and its serious complications.

Currently, two types of HAV vaccines are available in Thailand: (1) the inactivated HAV vaccine (I-HAV), which is administered in two doses six months apart and is approved for use in children aged 12 months and older; and (2) the live-attenuated HAV vaccine (L-HAV), administered as a single dose and approved for children aged 18 months and older. However, neither vaccine has been included in Thailand's Expanded Programme on Immunization (EPI), resulting in suboptimal vaccination coverage across the country.

In 2024, the investigators conducted a randomized, active-controlled, open-label, non-inferiority trial to compare the immunogenicity and safety of the two-dose I-HAV regimen with the single-dose L-HAV in healthy Thai children and adolescents aged 18 months to 18 years. The present study aims to assess the long-term seropositive rate and immunogenicity of anti-HAV antibodies among these participants following a single dose of L-HAV.

Inclusion Criteria:

• Thai children and adolescents who previously participated in the previous RCT study
• Previously randomized to receive one dose of L-HAV vaccine within the past 1 year (+/- 3 months)
• Participants and/or caregivers gives written inform consent/assent form

Exclusion Criteria:

• History of acute illness within 4 weeks prior to study enrollment
• Has a history of illness or a diagnosis consistent with hepatitis A after receiving the live attenuated hepatitis A vaccine as part of participation in a previous research study
• Has a history of receiving any additional hepatitis A vaccine after participating in the previous research study
• Presence of fever (body temperature ≥38.0°C), jaundice, or yellowing of the eyes within 4 weeks prior to study enrollment
• Have any condition that, in the opinion of the site investigator, would compromise the subject's ability to participate in the study

Healthy Thai children and adolescents age 18 months to 20 years who received a single dose of L-HAV vaccine in the previous RCT study.

There is not a plan to make individual participant data (IPD) available to other researchers for this study.