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COVID-19: Protective antibodies still detectable in blood and saliva even after months

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Boston and Toronto - Fears that immunity will be short-lived after infection with SARS-CoV-2 does not seem to be confirmed. In 2 studies from Canada and the USA in Science Immunology (2020; DOI: 10.1126 / sciimmunol.abe0367 and DOI: 10.1126 / sciimmunol.abe5511) the antibody concentration had not dropped again even 3 months after the illness.

In the past few weeks there had been multiple reports that SARS-CoV-2 left behind an unreliable immunity that was only of limited duration. Chinese researchers reported in June that only 30 of 37 asymptomatic people had developed IgG antibodies 3 to 4 weeks after infection, the concentration of which fell rapidly. In 12 infected people there was even a seroreversion: The antibody tests were negative again for them (Nature Medicine, 2020; DOI: 10.1038 / s41591-020-0965-6).

California medical professionals also observed an unusually rapid decline in antibody titers in a group of 34 patients infected with COVID-19 (New England Journal of Medicine 2020; DOI: 10.1056 / NEJMc2025179). Rapidly declining immunity would certainly make the current pandemic more difficult to control. It would also be a bad omen for developing vaccines that rely on long-term antibody production to be effective.

The two studies that have now been published, which are based on a significantly higher number of cases, come to a different assessment. The team led by Richelle Charles from Massachusetts General Hospital in Boston has repeatedly examined the blood samples of 343 patients who were seriously ill with COVID-19 (93% hospitalized; lethality 13%).

Their results show that the immune system reacts to SARS-CoV-2 in a similar way to other viruses, namely with the formation of all 3 essential antibody groups. First, the IgM antibodies rose, the detection of which is also considered an indication of a recent infection in other diseases. These antibodies are only formed temporarily with SARS-CoV-2. After 49 days, they disappeared in half of the patients. The IgA antibodies, which are responsible for immunity on the mucous membranes, were also no longer detectable in the saliva after 71 days.

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 up to the 90th day.

Incidentally, no cross-reactivity with the antibodies of the 4 “harmless” coronaviruses HKU1, 229E, OC43 and NL63, which usually only cause harmless infections, was 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 influence the severity).

The researchers also looked at 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 the two, was 0.99 (95% confidence interval 0.99 to 1.00) after 14 days and later, 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 the 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 the participants would then not have to provide a blood sample. © rme / aerzteblatt.de