Determinants of protection against SARS-CoV-2 Omicron BA.1 and Delta infections in fully vaccinated outpatients

Abstract

AbstractObjectivesWe aimed to evaluate the association between the humoral and cellular immune responses and symptomatic SARS-CoV-2 infection with Delta or Omicron BA.1 variants in fully vaccinated outpatients.MethodsAnti-RBD IgG levels and IFN-γ release were evaluated at PCR-diagnosis of SARS-CoV-2 in 636 samples from negative and positive patients during Delta and Omicron BA.1 periods.ResultsMedian levels of anti-RBD IgG in positive patients were significantly lower than in negative patients for both variants (p< 0.05). The risk of Delta infection was inversely correlated with anti-RBD IgG titres (aOR = 0.63, 95% CI [0.41; 0.95],p= 0.03) and it was lower in the hybrid immunity group compared to the homologous vaccination group (aOR = 0.22, 95% CI [0.05; 0.62],p= 0.01). In contrast, neither the vaccination scheme nor anti-RBD IgG titers were associated with the risk of BA.1 infection in multivariable analysis. IFN-γ release post-SARS-CoV-2 peptide stimulation was not different between samples from patients infected (either with Delta or Omicron BA.1 variant) or not (p= 0.77).ConclusionsOur results show that high circulating levels of anti-RBD IgG and hybrid immunity were independently associated with a lower risk of symptomatic SARS-CoV-2 infection in outpatients with differences according to the infecting variant.