Induction of Fc-dependent functional antibodies against different variants of SARS-CoV-2 varies by vaccine type and prior infection

SARS-CoV-2 transmission and COVID-19 disease severity is influenced by immunity from natural infection and/or vaccination. Population-level immunity is complicated by the emergence of viral variants. Antibody Fc-dependent effector functions are as important mediators in immunity. However, their induction in populations with diverse infection and/or vaccination histories and against variants remains poorly defined.

Hybrid immunity, from combined natural exposure and vaccination, generates strong Fc-mediated antibody functions which may contribute to immunity against evolving SARS-CoV-2 variants. Understanding determinants of Fc-mediated functions may enable future vaccines with greater efficacy against different variants.

We evaluated Fc-dependent functional antibodies following vaccination with two widely used vaccines, AstraZeneca (AZ) and Sinovac (SV), including antibody binding of Fcγ-receptors and complement-fixation in vaccinated Brazilian adults (n = 222), some of who were previously infected with SARS-CoV-2, as well as adults with natural infection only (n = 200). IgG, IgM, IgA, and IgG subclasses were also quantified.

AZ induces greater Fcγ-receptor-binding (types I, IIa, and IIIa/b) antibodies than SV or natural infection. Previously infected individuals have significantly greater vaccine-induced responses compared to naïve counterparts. Fcγ-receptor-binding is highest among AZ vaccinated individuals with a prior infection, for all receptor types, and substantial complement-fixing activity is only seen among this group. SV induces higher IgM than AZ, but this does not drive better complement-fixing activity. Some SV responses are associated with subject age, whereas AZ responses are not. Importantly, functional antibody responses are well retained against the Omicron BA.1 S protein, being best retained for Fcγ-receptor-1 binding, and are higher for AZ than SV. Conclusions: Hybrid immunity, from combined natural exposure and vaccination, generates strong Fc-mediated antibody functions which may contribute to immunity against evolving SARS-CoV-2 variants. Understanding determinants of Fc-mediated functions may enable future vaccines with greater efficacy against different variants.