Pre-vaccination immune response to COVID-19 in a population in Northeast Portugal.

Purpose: To study the immunization status and IgM and IgG antibody behavior against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in an unvaccinated population of Northeast Portugal (including RT-PCR diagnosed and undiagnosed individuals).

Methods: Application of a clinical-epidemiological survey, and analysis of IgM and IgG SARS-COV-2 antibodies (against N core protein) in 362 participants that voluntarily sought the laboratory for testing.

Results: At the time of the analysis, 31.7% (n = 114) of the study population had a previous SARS-CoV-2 diagnosis, 48.3% of which were asymptomatic, and 71.9% IgG seropositive. Of these, 83.3% and 60% were, respectively, IgM and IgG seropositive within 2 weeks after the initial diagnosis. Both antibodies peaked in the 3rd week post diagnosis, with titers decreasing over the following weeks, until a state of seronegativity was achieved after the 6th week for IgM, and the 21st for IgG. Symptomatic patients showed higher IgM and IgG values, when compared to asymptomatic ones. Fever, the most reported symptom, was found to be positively associated with IgM values. Ages of ≤ 18-year-old and ≥ 65-year-old exhibited the highest median values for both IgM and IgG, with the former being statistically significant. In the undiagnosed group, 13.9% and 11.1% were seropositive for IgM and IgG, respectively.

Conclusion: IgM and IgG displayed a similar initial increase (within 1/2 weeks), with IgG having a significant decrease after the 21st week post-diagnosis, translating a loss of immunity at this point. The youngest and oldest symptomatic age groups were found to be the highest responders. Antibody assays enabled the identification of previously undiagnosed participants.