An epidemiological and retrospective study in a cohort qualified for SARS-CoV-2 vaccination in the region of Lower Silesia, Poland.

Background. Since the beginning of the coronavirus disease (COVID-19) pandemic, numerous infections have been observed with various symptoms and degrees of severity. Not all patients have had a confirmation of infection made using reverse transcription polymerase chain reaction (RT-PCR) or antigen tests. It has been observed that some people, including convalescents or those without knowledge of a past infection, perform serological tests to detect anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies. Objectives. We aimed to evaluate the levels of anti-SARS-CoV-2 immunoglobulin G (IgG) antibodies in a cohort of convalescents and in individuals not previously infected, who were willing to get vaccinated. We also aimed to assess several socio-clinical factors associated with participants' humoral responses. Materials and methods. We recruited 298 individuals from the region of Lower Silesia who were willing to get vaccinated for SARS-CoV-2. The participants were divided into 2 groups: convalescents (group I) and participants without a past infection (group II). Several seropositive individuals in group II were identified, and they were transferred to group I, resulting in a final distribution of 171 individuals in group I and 127 individuals in group II. For serological testing, the QuantiVac anti-SARS-CoV-2 (IgG) enzyme-linked immunosorbent assay (ELISA) was used. Results. The results showed the presence of anti-SARS-CoV-2 IgG antibodies in participants from group I, with an average number of 190.3 IU/mL. Twenty-three participants (13.45%) did not have a detectable level of antibodies despite a previous SARS-CoV-2 infection. In 21 participants (12.28%), antibodies were detected despite no previous symptoms of infection (average level: 145.0 IU/mL). Conclusions. Older participants were more likely to experience a symptomatic SARS-CoV-2 infection, and the severity of the symptoms was related to higher antibody titers seen later after COVID-19. Numerous individuals from group II were unaware of past SARS-CoV-2 infections. In several participants, antibodies were not detected despite a previous infection. [ABSTRACT FROM AUTHOR]