Three-month analysis of total humoral response to Pfizer BNT162b2 mRNA COVID-19 vaccination in healthcare workers

Background. Evidence is increasingly provided that post-vaccine immunogenicity against SARS-CoV-2 declines over time. To provide further insights on this important public healthcare matter, we report here an ad-interim analysis of data obtained after a 3-month follow-up in a cohort of healthcare workers who received the Pfizer BNT162b2 mRNA vaccine Methods. The study population consisted of 871 healthcare workers (median age 45 years, 65.7% females, 22.0% baseline seropositive) undergoing voluntary vaccination with Pfizer BNT162b2 mRNA COVID-19 vaccine at the hospital of Peschiera del Garda (Italy). Blood was collected before receiving the first vaccine dose, immediately before the second vaccine dose (i.e., 21 days after the first dose), as well as 50 days and 3 months after the first vaccine dose. Total anti-SARS-CoV-2 antibodies were assayed using Roche Elecsys Anti-SARS-CoV-2 S chemiluminescent immunoassay. Results. Antibodies levels were consistently higher in baseline seropositive subjects throughout the study period (i.e., p<0.001), reached a peak after the second vaccine dose in both seronegative and seropositive cohorts, and then started to decline. Compared to values measured at 30 days after the second vaccine dose, the decline was nearly 2-fold more accentuated in seropositive than in seronegative cohort (i.e., -57% vs. -33%; p<0.001). Although no subject in either cohort displayed total anti-SARS-CoV-2 S antibodies values below the method-dependent cut-off, a reduction in serum levels was observed in 568/679 (83.7%) baseline seronegative subjects and in 173/192 (90.1%) baseline seropositive subjects (p=0.018). In baseline anti-SARS-CoV-2 seronegative subjects, total anti-SARS-CoV-2 S antibodies values were significantly lower at 30 days and at 3 months after the first vaccine dose in those aged >65 years compared to younger people.