Optimal timing of infant post-vaccination serological testing for hepatitis B virus immunity

The authors would like to thank Professor Halil Haldun Emiroglu and colleagues for their letter regarding the 2025 Canadian Association for the Study of the Liver (CASL) and Association of Medical Microbiologists of Canada (AMMI) management of hepatitis B guidelines (1). The authors discussed the optimal timing of post-vaccination serological testing (PVST) of infants who receive the standard passive-active hepatitis B virus (HBV) immunoprophylaxis regimen comprising birth dose HBV vaccine and hepatitis B immune globulin (HBIG), as well as two additional HBV vaccine doses by 6 months of age. In their letter, Emiroglu et al recommend following the 2018 American Association for the Study of Liver Diseases (AASLD) guidelines (2), which advise testing between 9 and 15 months of age, as the potential presence of passive HBV surface antibody (HBsAb) from HBIG may be detected if testing is done earlier.

We agree with AASLD guidelines that testing should not occur any earlier than 1 to 2 months after the final HBV vaccine dose. However, the 2025 CASL/AMMI management of hepatitis B guideline recommendations (1) remain evidence based and valid, recommending PSVT no earlier than 1 month after the last HBV vaccine dose and by 12 months of age. Thus, infants should be tested for hepatitis B surface antigen (HBsAg) and HBsAb between 1 and 4 months after the last dose of vaccine (and by 12 months of age) to confirm they are not infected and are immune to HBV. (Strong recommendation; Level 1) (1).

This recommendation is supported by published evidence. A large meta-analysis by Huang et al, involving 982 infants who received passive-active immunoprophylaxis concluded that PVST should be conducted at 7 months of age or at least 1 month after the final vaccine dose to identify non-responders as early as possible (3). While testing at 9 months may increase the likelihood of detecting late HBV infections, it also risks delaying the identification of non-responders who may be susceptible to infection.

Furthermore, studies in liver transplant recipients have shown that the half-life of HBIG is approximately 3 to 4 weeks, with the anti-HBs levels declining by 50% every 21 days and typically becoming undetectable by 3 months post-transplant (4). Therefore, anti-HBs detected at 7 months of age are most likely due to active vaccine-induced immunity, not residual passive antibodies from HBIG. Delaying PVST beyond 12 months as allowed by AASLD's 15-month upper limit may result in a false impression of vaccine failure due to waning antibody levels.

The 2025 CASL/AMMI guidelines were developed within the context of the Canadian health care system, aiming to reduce ambiguity and avoid unnecessary revaccination or retesting. The recommendation to perform PVST between 1 month after the final vaccine dose and 12 months of age allows for flexibility based on real-world clinical settings. This approach is also supported by Canada's National Advisory Committee on Immunization as well as regional public health authorities in Manitoba and British Columbia (5,6).

The 2025 CASL/AMMI HBV guidelines for PVST provide a clear, evidence-based time frame that balances early detection of vaccine failure with minimizing of false results from residual antibodies.