Effect of Previous SARS-CoV-2 Infection on Antibody Response to a Single Immunization with the Pfizer BNT162b mRNA Vaccine Among Healthcare Workers in Foggia, Italy

Abstract

Introduction: Data have suggested that SARS-CoV-2 infection causes an antibody response at least as strong as one BNT162b2 vaccine dose. Nevertheless, some aspects require further investigation to better understand the immunogenicity of one vaccine dose among infected individuals. Thus, we evaluated how previous SARS-CoV-2 infection may influence the humoral immunity after a single Pfizer BNT162b mRNA vaccine dose in a sample of healthcare workers (HCWs). Methods: As part of the routine surveillance activity conducted among HCWs of the Policlinico Riuniti Foggia Hospital (Apulia region, Italy), we conducted a retrospective serosurvey in the period January–March 2021. We compared specific antibody titres (anti-spike IgGs measured by enzyme-linked immunoassay, ELISA) after SARS-CoV-2 infection and after the first dose of the BNT162b2 vaccine, analysing the impact of sex, age, time since infection, and presence of symptoms on the humoral response. Results: We included in the study 58 HCWs (mean age 44.1 years, 48.2% male) with anti-spike IgG titres available before and after the first BNT162b2 vaccine dose. Among these, we observed higher titres in previously infected cases (N = 21) than in COVID-19-naïve subjects (N = 37) (medians 1510 vs. 0.68; p < 0.001). A statistically significant difference in anti-spike IgG titres was also observed among previously infected HCWs before vaccine dose in comparison with post-dose infection-naïve HCWs (medians 18.37 vs. 0.68, p < 0.001). Among infected individuals, no differences by sex, age, or time since infection were reported (p > 0.05). Post-dose titres of symptomatic and asymptomatic infected HCWs slightly differed (medians = 1900 vs. 1090; p = 0.048). Conclusion: Our data support the viable hypothesis of a single-dose vaccine regimen in individuals with a history of COVID-19, but no conclusion on duration of protection in this group can be drawn from our study.