Immune responses after two inactivated COVID-19 vaccine doses, a heterologous third dose and subsequent boosting with bivalent mRNA in adults

Nov 27, 2025Scientific reports

Immune responses in adults after two inactivated COVID-19 vaccine doses, a different third dose, and later boosting with a two-strain mRNA vaccine

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Abstract

Of 190 participants, the mRNA booster vaccine showed a 29.16 fold rise in neutralizing antibodies against the XBB.1.5 variant.

  • Anti-Spike IgG levels increased at Day 28 after vaccination but decreased by Day 90.
  • Neutralization antibody titers against the ancestral strain peaked at Day 90.
  • The highest increase in neutralization titers against XBB.1.5 was observed in the mRNA vaccine subgroup.
  • T-cell immunity, measured by IFN-γ production, rose comparably across all booster subgroups.
  • Bivalent mRNA vaccination improved both antibody and T-cell responses in individuals primed with inactivated COVID-19 vaccines.

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Key numbers

6.62
Increase in
Fold rise at Day 28 in subgroup after bivalent vaccination.
29.16×
against
Peak fold rise at Day 28 in subgroup.
5,277.24 mIU/mL
Median IFN-γ production
At Day 28 following bivalent vaccination.

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