Investigating the Seroconversion Patterns of Specific Antibodies Against Various Antigens of SARS-CoV-2 in Hospitalized COVID-19 Patients and Vaccinated Individuals

Background: Inducing a humoral response to SARS-CoV-2 may partially control virus dissemination. However, there is a lack of consistency in the reported kinetics of IgM and IgG responses to SARS-CoV-2. Additionally, the humoral response to SARS-CoV-2 may differ from that elicited by vaccination. Therefore, we were motivated to evaluate the kinetics of antibodies against SARS-CoV-2 in both infected and vaccinated individuals. Objectives: This study aimed to investigate the seroconversion patterns of specific antibodies against various antigens of SARS-CoV-2 in hospitalized COVID-19 patients and vaccinated individuals, focusing specifically on comparing the humoral responses elicited by infection and vaccination. Methods: Serial blood and swab samples were collected from 134 COVID-19 patients at six time points following admission. Real-time RT-PCR specific for SARS-CoV-2, as well as anti-SARS-CoV-2 IgM and IgG, were tested using ELISA. Additionally, 141 serum samples were obtained from vaccinated individuals. Anti-SARS-CoV-2 spike and RBD IgGs, along with neutralizing antibodies (NAs), were assessed using ELISA in both the vaccinated group and 96 COVID-19 patients. Results: Anti-SARS-CoV-2 IgM was found positive in 23.3% of patients at 0 - 7 days after symptom onset, with seropositivity increasing to 71.7% at 15 - 21 days. Subsequently, IgM positivity gradually decreased to 62.7% at > 28 days post-symptom onset. Meanwhile, anti-SARS-CoV-2 IgG was positive in 28.3% of patients at 0 -7 days, rising to 83.7% at 22 - 28 days after symptom onset, and remained constant thereafter. Anti-spike and RBD IgGs, along with NAs, were detected in 89.7%, 87.4%, and 87.9% of vaccinated individuals, respectively, and in 37.5%, 32.3%, and 32.3% of COVID-19 patients, respectively. There was a significant correlation between anti-spike IgG and anti-RBD IgG levels and NAs in both COVID-19-infected and vaccinated individuals. The mean concentrations of anti-spike and RBD IgGs were higher in vaccinated individuals with a history of COVID-19 infection compared to those without prior infection. Conclusions: The antibody profile for IgM and IgG against SARS-CoV-2 suggests that as time passes after the onset of disease symptoms, the seropositivity in COVID-19 patients increases. Furthermore, antibodies against SARS-CoV-2 are produced more efficiently through COVID-19 vaccination than natural infection.