Humoral immunogenicity and reactogenicity of CoronaVac or ZF2001 booster after two doses of inactivated vaccine

At 4–8 months after primary immunization with CoronaVac, neutralizing antibody levels against the three variants are close to the lower limit of detection (8-fold dilution of plasma). Previous studies showed that ZF2001, an RBD-subunit vaccine, could induce humoral immunity that exhibits better tolerance to current VOCs compared to inactivated vaccines and natural infections, suggesting ZF2001 as an ideal candidate for heterologous booster. To assess the impact of a heterologous third dose of ZF2001 or a homologous third dose of CoronaVac on vaccine-induced antibodies against VOCs, we conducted a single-center, open-label, randomized controlled clinical trial among healthcare professionals at Beijing Ditan Hospital who had received two doses of CoronaVac in a 28-day interval 4–8 months earlier(Fig. S1). We assessed the SARS-CoV-2 anti-spike IgG antibody levels and the geometric mean titers (GMTs) against authentic prototype SARS-CoV-2 and Beta, Gamma, and Delta variants on day 0 and 14 after administration of third doses for those vaccinated and for control group subjects. The third dose of either CoronaVac or ZF2001 vaccine rapidly induced a significantly high degree of humoral immunogenicity; the humoral immune response induced by ZF2001 was higher than that from CoronaVac(Fig. 1). Importantly, both of the three-dose regimens were well tolerated. The most common adverse reactions were local injection site reactions, and all adverse reactions were grade 1. In the current pandemic situation with the Delta variant being predominant, analysis of the neutralizing capacity of vaccines against SARS-CoV-2 variants is of utmost relevance. Our results suggest that heterologous boosters, such as RBD subunit vaccines, may also contribute to the protection from SARS-CoV-2 symptomatic illness, and may be more efficient against VOCs.