There was a significant increase in IgG antibodies, which was regularly associated with neutralization of the viruses (in a laboratory test). The IgG antibodies increased after about a week and reached the highest concentration after 28 days. After that there was a slow decline. However, a seroreversion was detected in 3 people by the 90th day.
Cross-reactivity with the antibodies of the 4 “harmless” coronaviruses HKU1, 229E, OC43 and NL63, which usually only cause harmless infections, was not found. This confirms the observations of other researchers that previous infections with the cold virus do not protect against infection with SARS-CoV-2 (but may affect the severity).
The researchers also examined blood samples from 1,548 people from
before the pandemic began. The sensitivity and specificity of the IgG antibodies were high. The AUC value, which combines both, was 0.99 after 14 days and later (95% confidence interval 0.99 to 1.00), i.e. very close to the optimal value of 1.0. This means that the PCR test for virus detection can be combined very well with a later antibody test in order to make a reliable diagnosis. The antibody test should also be suitable for seroprevalence studies.
A team led by Jennifer Gommerman from the University of Toronto has had similar experiences, examining 402 patients with COVID-19 for up to 105 days after the onset of symptoms. Here, too, there was a delay of a few days to an increase in antibodies, which in the case of IgG antibodies had peaked after 16 to 30 days and then slowly fell.
However, the antibody tests remained positive until the end of the follow-up after 105 days, while the titers of IgM and IgA antibodies fell rapidly.
The Canadian researchers have also developed a saliva test that has proven to be reliable in the studies. This test could be suitable for seroprevalence
studies, as participants would then not have to provide a blood sample
