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.

Automated summary

This study evaluated IgG production in vaccinated and unvaccinated individuals previously infected or not with SARS-CoV-2. The results showed that vaccination led to higher levels of IgG antibodies, which peaked at 4-6 weeks and gradually declined. Individuals previously infected with SARS-CoV-2 showed a slower rate of decrease in IgG antibodies after vaccination.