PREPRINT: SARS-CoV-2 Omicron triggers cross-reactive neutralization and Fc effector functions in previously vaccinated, but not unvaccinated individuals
The SARS-CoV-2 Omicron variant largely escapes neutralizing antibodies elicited by vaccines or infection. However, whether Omicron triggers humoral responses that are cross-reactive to other variants of concern (VOCs) remains largely unknown. We use plasma from 20 unvaccinated and seven vaccinated individuals infected during the Omicron wave in South Africa to test binding, antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP) and neutralization against VOCs. In unvaccinated individuals, Fc effector function and binding antibodies target Omicron and other VOCs at comparable levels. However, Omicron-triggered neutralization is not extensively cross-reactive to VOCs, with 20 to 43-fold reductions in titer. In contrast, vaccination followed by breakthrough Omicron infection improved cross-neutralization of VOCs, with titers exceeding 1:2,900. This has important implications for the vulnerability of unvaccinated Omicron-infected individuals to reinfection by circulating and emerging VOCs. Further, while Omicron-based immunogens may be adequate boosters, they are unlikely to be superior to existing vaccines for priming in SARS-CoV-2 naïve individuals.