ABSTRACT
Background: Four vaccines that have been authorized in the European Union offer different levels of protection against SARS-CoV-2 by generating immune responses against the spike receptor-binding domain (RBD) of the virus. Monitoring the levels of IgG antibodies against the SARS-CoV-2 is important during the coronavirus disease 2019 (COVID-19) pandemic to plan an adequate and evidence-based public health response. Methods: We compared the levels of serum IgG antibodies against SARS-CoV-2 spike protein in three groups: i) individuals without evidence of prior infection with SARS-CoV-2 who received one or two doses of either an mRNA-based (Comirnaty BNT162b2/Pfizer-BioNTech or Spikevax mRNA-1273/Moderna) or an adenoviral-based vaccine (Vaxzervia ChAdOx1 nCoV-19 /Oxford-Astra Zeneca) (n=227), ii) unvaccinated individuals with evidence of prior infection with SARS-CoV-2 (n=109), and iii) individuals with evidence of prior infection with SARS-CoV-2 who received at least one dose of a vaccine (n=30). Unvaccinated individuals without evidence of prior infection with SARS-CoV-2 were used as a control group (n=211). Results: The levels of specific SARS-CoV-2 IgG antibodies in all three groups were significantly higher (p<0.001) compared to the control group. The highest levels of virus-specific IgG antibodies were observed in individuals who were infected with SARS-CoV-2 and received at least one dose of a vaccine. Our analyses also revealed that the levels of specific anti- SARS-CoV-2 IgG levels were higher in individuals who received two doses of a licensed vaccine, regardless of the vaccine technology (mRNA-based and adenoviral vector-based). Furthermore, anti- SARS-CoV-2 IgG levels were higher in previously uninfected participants who received at least one dose of a vaccine compared to the unvaccinated individuals with evidence of prior infection. Conclusions: Our results indicate that vaccine-induced responses lead to higher levels of IgG antibodies compared to those produced following infection with the virus. In agreement with previous studies, our results suggest that among individuals previously infected with SARS-CoV-2, even a single dose of a vaccine is adequate to elicit high levels of humoral immunity.