Pre-vaccination immune response to COVID-19.

Dear Editor,

We would like to share ideas on the publication “Pre-vaccination immune response to COVID-19 in a population in Northeast Portugal [1].” Duro et al. concluded that “IgM and IgG displayed a similar initial increase … the identification of previously undiagnosed participants. [1].” We agree that the pre-vaccination antibody study might be useful. A diagnosis of previous asymptomatic infection can help gather correct epidemiological data. Additionally, the investigation might be useful for determining vaccine efficacy and surveillance of safety of vaccination. In a previous report, immune response to COVID-19 vaccine differs in cases with and without previous COVID-19 [2]. Additionally, a COVID-19 vaccine recipient with previous COVID-19 has a higher blood viscosity after vaccination, which might lead to hyperviscosity-associated adverse effect of vaccination [2].