Tracking anti-SARS-CoV-2 antibodies in blood donors in Brazil across a pandemic year: a serial cross-sectional study

Background The pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has significantly affected Brazil, with more than 700,000 deaths and more than 38 million cases reported. Analyzing vaccine schemes and postvaccination immune status is relevant to reinforcing the importance of immunization. In this serial cross-sectional study, we aimed to evaluate the correlation of the prevalence of IgG anti-SARS-CoV-2 antibodies with coronavirus disease 2019 (COVID-19) waves and the virus reproduction rate (RR). Methods The seroconversion of immunoglobulin G (IgG) against SARS-CoV-2 was evaluated in 29,667 samples from blood donors at the hemotherapy center of Sergipe (HEMOSE) for 1 year. We also assessed the RR of SARS-CoV-2 over this period. Findings The results were grouped into four time points according to COVID-19 waves, with average percentage seroconversion rates of 18.75%, 25%, 14.64%, and 21.39%, respectively. There was a significant decrease in IgG seropositivity between the second wave and the third wave (IgG% = 25.00 vs. 14.64; p<0.001), which was re-established in the fourth wave. This coincided with an increased SARS-CoV-2 RR between the second and third waves ( p = 0.01 ). We observed a decrease in seroconversion and RRs between the periods without (IgG% = 18.81; RR = 0.98) and with vaccination (IgG% = 12.55; RR = 0.87). Conclusions We showed that IgG production is negatively correlated with the virus RR. Moreover, after vaccination, the rate of seropositivity did not exceed 25%, suggesting two possible scenarios: the methodology does not show the effectiveness of vaccine strategies, or the increase in antibody presence in the population can be observed only after the completion of three immunizing doses.