Effect of wild-type vaccine doses on BA.5 hybrid immunity, disease severity, and XBB reinfection risk.

Vaccination against the wild-type (WT) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus did not produce detectable levels of neutralizing antibodies (NAbs) against the BA.5 strain before it emerged. However, coronavirus disease-2019 (COVID-19) severity varied highly between unvaccinated, partially vaccinated, and fully vaccinated individuals, for unknown reasons. We assessed the severity of BA.5 infection and the risk of XBB strain reinfection and measured serum levels of NAbs against WT, BA.5, and XBB.1.9.1 SARS-CoV-2 strains at varying time points in 1,373 individuals who received zero, one, two, or three WT vaccine doses. We found that two to three WT doses significantly increased WT and BA.5 NAb levels and reduced the incidence of COVID-19-associated pneumonia upon BA.5 strain infection compared to zero to one dose. Regarding XBB reinfection, those who received two to three doses and were infected with the BA.5 variant exhibited a significantly lower reinfection risk compared to those who received zero to one dose. RNA analysis revealed that the differentially expressed genes between the two to three dose and unvaccinated groups were enriched in B cell activation, cytokine-cytokine receptor interaction, complement, and monocyte activation functions-indicating that vaccination increased the antibody response and reduced inflammation. Our results suggest that multiple antigen exposures to either matched or unmatched SARS-COV-2 variants, through vaccination or infection, may be necessary to achieve significant immune imprinting.IMPORTANCEThe administration of coronavirus disease-2019 (COVID-19) vaccines that do not perfectly match the viral strains that individuals become infected with has been found to impact the resultant illness severity-although the precise mechanism underlying this phenomenon remains unclear. We assessed viral clearance, as well as serum levels of inflammatory cytokines and neutralizing antibodies (NAbs) against wild-type, BA.5, and XBB.1.9.1 variants of the severe acute respiratory syndrome coronavirus 2 among individuals who received varying doses of such strain-mismatched vaccines. Notably, vaccination with ≥2 doses of strain-mismatched COVID-19 vaccines appeared to stimulate the production of specific NAbs during infection with new variants, as well as attenuate the inflammatory response and enhance viral clearance. Such vaccination regimens can also reduce the risk of reinfection. These findings may be important for guiding the development of future COVID-19 vaccination strategies that target both matched and mismatched viral variants.