Vaccine-induced and hybrid immunity to SARS-CoV-2 after three or four doses of BNT162b2 - results from 22 months follow-up of a healthcare workers cohort, Israel, 2020-2022

Objectives: SARS-CoV-2 remains a global health concern 3 years after its emergence. Safe and effective vaccines mitigate the pandemic impact, but the optimal schedule remains unclear, especially in a context where a high proportion of the population is infected. Methods: We periodically measured anti-spike SARS-CoV-2 immunoglobulin (Ig)G titers using a quantitative assay in an Israeli healthcare worker cohort who all received at least two BNT162b2 doses and either received further doses and/or were subsequently infected up to 22 months after dose two, and compared geometric mean concentrations according to number of doses received and infection status using analysis of variance. Results: Among the 993 included participants, infection after dose two led to higher geometric mean concentration IgG titers than a third dose (4285 vs 2845 arbitrary unit/ml 1-2 months after infection/vaccination, P = 0.03). In 16-18 months after dose two, those infected and those who received three or four vaccine doses all had IgG geometric mean concentration levels above 500 arbitrary unit/ml with no significant differences among groups (P = 0.6). IgG levels plateaued 16-22 months after dose two. Conclusion: Three BNT162b2 doses provide long-term immunogenicity comparable to breakthrough infection after dose two. Dose four transiently increases IgG levels and may be especially important for providing additional protection to vulnerable individuals during periods of increased transmission risk.