In the COVID-19 pandemic, individuals may be asymptomatic, with mild or severe symptoms associated with respiratory tract infections. Patients with COVID-19 have IgG antibodies on average two weeks after infection and persist at stable levels for a few months. The influence of the levels of these antibodies and the time in circulation, protection, and severity of the disease in reinfections, has not yet been completely elucidated. The objective of this work was to evaluate the titers of anti-SARS-CoV-2 IgG antibodies in asymptomatic patients or with mild symptoms in a period of six months.

In this longitudinal study, we selected 62 individuals (median age 42.5 years; IQR 33.3‒52.0; 59.7% female) tested positive for SARS-CoV-2 Immunoglobulin G (IgG) antibodies using a Fluorescence Immunoassay (FIA) (iChroma II, BioSys + Kovalent) from July 2020 (peak of the first wave) to March 2021 (begging of the second wave associated with transmission of the Gamma variant) in the state of Sergipe, Northeast Brazil. All participants included in the present study had a history of asymptomatic or mild COVID-19, were not vaccinated against the disease, and were selected from the EpiSERGIPE Project conducted in the state of Sergipe. An in-house indirect Enzyme-Linked Immunosorbent assay (ELISA)-based method was used to evaluate the serum titers of anti-nucleocapsid SARS-CoV-2 antibodies in three moments: baseline (D0), 90 days (D90), and 180 days (D180). Briefly, 96-microwell plates (Nunc, Thermo Fisher Scientific).

In D0, 79% (49 of 62) of individuals had a positive result for the presence of anti-nucleocapsid SARS-CoV-2 antibodies. In D90 and D180, the percentage of positive results was 69.3% (43 of 62) and 53.2% (33 of 62), respectively. We found a progressive decline in the levels of anti-nucleocapsid SARS-CoV-2 antibodies over time, ranging from 26.2 (Interquartile Range [IQR] 12.4‒37.7) in D0 to 11.7 (IQR 5.6‒18.2) in D180 (p<0.001). In addition, we found that individuals over 40 years old had higher levels of IgG antibodies to the SARS-CoV-2 nucleocapsid in D90, regardless of sex. However, an influence of sex and age was not observed on D0 and D180.

Studies describe that IgG levels remain for only 3‒4 months in the body of individuals who have had previous contact with SARS-CoV-2, however in this study it was observed that most individuals had a significant and gradual decrease in anti-nucleocapsid SARS-CoV-2 antibodies circulating, even before completing three months after exposure to the virus.

This study showed a progressive decline in the levels of anti-nucleocapsid SARS-CoV-2 antibodies during the first six months after SARS-CoV-2 infection among individuals with asymptomatic or mild COVID-19. Approximately 50% of individuals have no detectable antibodies six months after infection. Moreover, we found a potential influence of age on the humoral response against SARS-CoV-2.