Interpretation of Serologic Test Results and Evaluation of Humoral Immune Response After Vaccination Following Infection with COVID-19 Disease

SARS-CoV-2 virus infection induces a cellular and humoral mediated immune response and produces antibodies against viral antigens such as nucleocapsid (N) protein and spike (S) protein. Anti-protein S antibodies also target the spike protein subunit S1 and the receptor binding domain (RBD). Serologic tests can detect the presence of these antibodies in blood serum within a few days to a few weeks after an acute infection. However, these tests should not be used to diagnose acute SARS-CoV-2 infection. Serologic tests can identify people who are currently infected with SARS-CoV-2 or had been infected in the past, and thereby help public health scientists and specialists better understand the epidemiology of SARS-CoV-2. Although the link between humoral immunity and protection against the disease is not yet fully understood, evidence suggests that the production of antibodies following infection may provide immunity against re-infection for up to about 6 months. However, it is unclear to what extent these antibodies can protect against emerging viral variants. Some serologic tests do not detect antibodies produced by COVID-19 vaccines. Since these vaccines induce antibodies to viral proteins, if the applied test does not detect vaccine-induced antibodies, the results of the post-vaccination antibody test in people who have no previous infection may be negative. Therefore, it seems that all eligible people should be vaccinated, including unvaccinated people who already have been infected and possess detectable antibodies. Unvaccinated people, including those who were previously tested positive for antibodies, should follow current recommendations for preventing SARS-CoV-2 infection.