In a study from The University of Hong Kong and elsewhere, scientists found that the timely third vaccination of either CoronaVac or BNT162b2 strongly reduces omicron reinfection and severe symptoms.
They found neutralizing antibody potency against BA.2.12.1 and BA.4/5 was enhanced by the BA.2 breakthrough infection and bivalent booster vaccination among people who had received prior three doses of vaccines.
The majority of pre-vaccinated people, however, experienced mild clinical symptoms after the BA.2 breakthrough infection.
Since the immunogenicity of CoronaVac (CorV) is weaker than that of BNT162b2 (BNT), it was essential to determine how vaccine-induced immune responses protected omicron BA.2 breakthrough infections.
In the study, the team examined the vaccine efficacy against the omicron BA.2 breakthrough infection among 470 local public servants.
The research team found that the timely third-dose vaccination provided better protection with lower incidence rates of breakthrough BA.2 infections.
They also found that the third-dose vaccination-activated spike (S)-specific memory B cells and omicron cross-reactive T cell responses correlated with reduced frequencies of breakthrough infections and disease severity rather than with types of vaccines.
Moreover, the frequency of S-specific activated memory B cells was much lower in infected vaccinees than in uninfected vaccinees before vaccine-breakthrough infection whereas the amount of S-specific CD4 T cells was negatively associated with viral clearance time.
Critically, BA.2 breakthrough infection boosted cross-reactive memory B cells with enhanced cross-neutralizing antibodies to omicron sub-lineages, including BA.2.12.1 and BA.4/5, in all vaccinees tested.
These findings highlight the importance of activated memory B cells and omicron cross-reactive T cell responses for prevention and disease protection against omicron BA.2.
The team says since BA.2 breakthrough infections mainly recalled vaccine-induced antibodies against ancestral strains, emphasize the importance of monitoring cross-protection against newly emerged viral variants induced by breakthrough infection and the new bivalent COVID-19 vaccine.
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The study was conducted by Dr. Zhou Runhong et al and published in The Lancet Regional Health—Western Pacific.
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