Immunological evaluation of an mRNA vaccine booster in individuals fully immunized with an inactivated SARS‐CoV‐2 vaccine

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Immunological evaluation of an mRNA vaccine booster in individuals fully immunized with an inactivated SARS-CoV-2 vaccineDear Editor, More than 5.75 million deaths and 396.55 million cases of coronavirus disease 2019 (COVID-19) have been reported, including those caused by at least five variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).1 This situation has caused public concern about the effectiveness of immune responses to emerging variants elicited by boosters of different vaccines in previously immunized populations.2,3 Our study, based on a cohort immunized with two doses of the inactivated vaccine and boosted with the messenger RNA (mRNA) vaccine at an interval of 6 months, investigates immunity of heterologous vaccination.The inactivated and mRNA vaccines used in this study were developed based upon the sequence of the Wuhan strain.4,5 In total, 288 healthy adult volunteers were immunized with an inactivated vaccine (150 U/dose) at an interval of 14 days; neutralizing antibodies were identified as having a geometric mean titer (GMT) of 2.1 and as negative in 52% of this population at 6 months after full immunization.Among them, 118 and 97 immunized subjects were boosted with the inactivated (150 U/dose) or mRNA (25 μg/dose) vaccine, and 73 immunized subjects were monitored as non-boosted controls.The average ages of the three groups were 43.04 ± 13.05, 31.86 ± 7.62 and 30.05 ± 7.03 years, respectively (Table S1).Immunological observation was until day 180 post-boost, which was available for 60, 61 and 68 subjects in the inactivated vaccine boost, mRNA vaccine boost and nonboosted groups, respectively (Figure S1).Antibody assays performed at days 14 and 180 post-boost showed GMTs of 829 and 455 on days 14 and 180, respectively, against the Wuhan strain in the mRNA boost group, in comparison to GMTs of 39 and 19, respectively, in the inactivated vaccine boost group (Figure 1).Binding antibody titers reached 71864 and 29675 and 3162 and 1993 on days 14 and 180, respectively, in the mRNA and inactivated vaccine boost groups (Figure 1).Further cross-neutralization for the B.1.617.2 and B.1.1.529

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