Immune response to COVID-19 vaccination in a population with and without a previous SARS-CoV-2 infection

Purpose To evaluate IgG production in a group of vaccinated and unvaccinated subjects previously infected, or not, with SARS-CoV-2. Methods A total of 316 subjects were enrolled at different times after vaccination and/or infection. IgG against target S1 subunit of the spike protein of SARS-COV-2 was assessed by a chemiluminescent microparticle immunoassay. Participant data was collected using a clinical-epidemiological survey. Results A total of 56.2% ( n = 146) of our cohort was vaccinated, with 27.5% ( n = 36) reporting a previous infection. Of these, all were IgG positive at the time of the study, regardless of gender, age category, vaccine type, and elapsed time since vaccination. The vaccinated group without a previous infection (72.5%, n = 95) showed a slightly lower IgG seropositivity and median values, overall, although significantly higher in females and lower with the ChAdOx1 nCoV-19 (AstraZeneca) vaccine. Vaccinated subjects above the age of 65 showed a trend towards higher median IgG values (13,911.0 AU/mL), when previously infected with SARS-CoV-2, but comparatively lower IgG median value (5158.7 AU/mL) in its absence. In all vaccinated groups, IgG antibody production increased at 1–2 weeks, peaking at 4–6 weeks. Afterward, IgG decreased progressively but almost all subjects (97.7%, n = 128) were seropositive for the remainder of our study. Fully vaccinated individuals with a past infection showed a lower IgG rate of decrease versus their uninfected counterparts (17.9 vs 22.6%, respectively). Conclusion Our findings suggest a higher effect of vaccination on the production IgG antibodies, as opposed to natural infection. Nonetheless, in general, antibody titers waned rapidly.