Objective:

To evaluate longitudinal humoral immune responses, including anti-spike (anti-S) immunoglobulin G (IgG), anti-receptor-binding domain (anti-RBD) antibodies, and Omicron virus neutralization, following homologous and heterologous COVID-19 booster regimens in Malaysia during the Omicron outbreak.

Approach:

• : Prospective observational cohort study of healthcare professionals and medical students in Malaysia from November 2021 to January 2023. Participants received 1 of 5 booster combinations: homologous mRNA-mRNA, viral vector–viral vector, or inactivated virus–inactivated virus vaccination, or heterologous inactivated virus–mRNA or viral vector–mRNA vaccination.
• : Of 332 participants included in the 24-week follow-up, 268 completed follow-up and 190 were included in the longitudinal immunogenicity analysis after exclusions, including participants with newly diagnosed SARS-CoV-2 infection. Anti-S IgG and anti-RBD antibodies were assessed before boosting and at weeks 2, 4, 8, 12, 16, and 24. Omicron neutralization was evaluated using a surrogate virus neutralization test.

Key Findings:

• Compared with the homologous inactivated virus–inactivated virus regimen, heterologous boosters, particularly inactivated virus–mRNA, produced significantly higher and more sustained anti-S IgG and anti-RBD responses.
• At week 24, the inactivated virus–mRNA group maintained 100% seropositivity for both anti-S IgG and anti-RBD antibodies, whereas seropositivity in the inactivated virus–inactivated virus group declined to 15.63% and 18.75%, respectively.
• At week 24, Omicron neutralization was 95.92% with the heterologous inactivated virus–mRNA booster and 83.87% with the viral vector–mRNA booster, compared with 9.38% with the homologous inactivated virus–inactivated virus regimen.
• Age, sex, and ethnicity were not significantly associated with anti-S IgG responses across time points.

Interpretation:

Heterologous booster strategies incorporating an mRNA vaccine, particularly an mRNA booster after an inactivated-virus primary regimen, generated stronger and more sustained humoral immune responses than homologous inactivated-virus boosting. The findings support the immunologic rationale for mix-and-match booster approaches after initial vaccination with inactivated-virus or viral-vector vaccines.

Limitations:

• Follow-up was limited to 24 weeks, restricting assessment of longer-term booster responses.
• Most participants were aged 20 to 50 years, with relatively few participants older than 50 years, limiting evaluation in older populations.
• The study did not assess longitudinal B-cell or T-cell populations, and additional research is needed to evaluate the influence of body mass index on humoral responses.
• An unvaccinated control group could not be included because vaccination was mandatory for federal government employees and healthcare professionals.
• Participants with severe adverse effects requiring hospitalization and those with autoimmune disease were excluded.

Conclusion:

Heterologous booster vaccination, particularly an inactivated virus–mRNA regimen, produced a superior longitudinal humoral immune response, with sustained anti-S IgG and anti-RBD antibodies and prolonged Omicron neutralization. These findings support mix-and-match booster strategies for individuals initially immunized with inactivated-virus or viral-vector vaccines, while indicating that antibody kinetics alone cannot fully predict protection against emerging Omicron subvariants.