Serum and saliva anti-SARS-CoV-2 display similar temporal kinetics. IgA antibodies are produced in mucosa, and they are responsible for the early humoral immunity against SARS-CoV-2, neutralizing the virus. ![]() In the oral cavity, class IgG and IgM antibodies mainly diffuse from blood circulation into gingival crevicular fluid and further into the saliva. Mucosal immunity is crucial in limiting respiratory infections. A stronger exposure and the severity of the COVID-19 infection are associated with higher antibody levels and a longer durability of antibodies. Serum IgM and IgA levels elevate synchronously to or slightly earlier than IgG, and their seroconversion occurs between days 6 and 15. Over 90% of subjects start to develop IgG antibodies to SARS-CoV-2 antigens 10–11 days after the onset of symptoms. Most SARS-CoV-2 vaccines are designed to use the viral spike glycoprotein or part of it as the immunogen. Most serological tests are based on the detection of the IgG and IgM antibodies that recognize SARS-CoV-2 nucleoprotein (N), viral spike glycoprotein S1 subunit, or its receptor-binding domain (RBD), but some applications have also been developed to detect IgA antibodies against these antigens. Especially after the implementation of vaccination programs, serological surveys of large populations are essential in evaluating the level and duration of antibody responses. Serological assays are useful in investigating an individual’s immune status and response to vaccinations as well as in perceiving epidemiological information on the spread of the infection. Saliva IgG has high potential to monitor vaccination response wane, since the sample is non-invasive and easy to collect. Self-collected dry blood and saliva spot samples combined with the GSP/DELFIA technique comprise a valuable tool to investigate an individual’s immune response to SARS-CoV-2 exposure or vaccination. Both blood and saliva IgG-seropositivity proportions followed similar trends to the exposures reported in the questionnaires. While the blood IgG assay had a 99.5% sensitivity and 75.3% specificity to distinguish participants with two vaccinations from all other types of exposure, the corresponding percentages for saliva IgG were 85.3% and 65.7%. ![]() The level of exposure was the strongest determinant of all blood antibody classes and saliva IgG, increasing as follows: (1) no exposure (healthy, non-vaccinated), (2) exposed, (3) former COVID-19 infection, (4) one vaccination, (5) two vaccinations, and (6) vaccination and former infection. Anti-SARS-CoV-2 IgG, IgA, and IgM levels were determined from both samples using the GSP/DELFIA method. Participants filled in a questionnaire on their COVID-19 exposures, infections, and vaccinations. We analyzed 1231 self-collected dried spot blood and saliva samples from healthcare workers. ![]() We examined the usefulness of dried spot blood and saliva samples in SARS-CoV-2 antibody analyses.
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