BACKGROUND: Limited research has explored the immunological effects of a second COVID-19 vaccine booster in older adults within Western Pacific countries. METHODS: This study involved healthcare workers aged ≥65 years who received two doses of ChAdOx1 nCoV-19 (A), mRNA-1273 (M), or MVC-COV1901 (Mc), followed by a booster dose of mRNA-1273. Younger adults served as controls. Humoral and cellular immune responses were measured before and after the vaccination. RESULTS: Younger adults exhibited the highest fold-rise in anti-spike IgG levels (13.20, p <
0.001), followed by the AAMM (9.93, p <
0.001), McMcMM (6.97), and MMMM (4.9, p <
0.05) groups. Cellular response increased most in the McMcMM group (3.77), followed by AAMM (3.04, p <
0.001), MMMM (2.24), and the younger group (1.08). Neutralizing activity against the D614G variant was highest in younger adults (7.77, p <
0.001), followed by AAMM (4.07, p <
0.001), MMMM (2.89, p <
0.05), and McMcMM (2.76). Against the XBB.1.16 variant, titers were significantly lower (17.33-29.08 times less than wild type). The highest fold-rise was observed in the McMcMM group (8.59), followed by AAMM (7.66, p <
0.001), MMMM (4.16), and younger adults (2.26). CONCLUSIONS: A second mRNA COVID-19 booster increased neutralizing antibodies and enhanced T cell responses against SARS-CoV-2 variants. Adapted vaccines targeting new subvariants are critical to strengthen immunity, particularly for older adults facing vaccine-resistant strains.