Exposure to harmless coronaviruses boosts immunity against Covid: Study

Previous antibody responses to other, harmless coronaviruses may confer immunity against SARS-CoV-2, the virus that causes Covid-19, according to a new study.

The study, published in the journal Nature Communications, showed that people infected with Covid had lower levels of antibodies against coronaviruses that cause common colds compared to uninfected people.

In addition, people with high levels of antibodies against harmless coronaviruses were less likely to have been hospitalised after catching Covid.

aceOur study shows that a strong antibody response to human coronaviruses increases the level of antibodies against SARS-CoV-2. So someone who has gained immunity to harmless coronaviruses is therefore also better protected against severe SARS-CoV-2 infections," said Alexandra Trkola, head of the Institute of Medical Virology at University of Zurich (UZH).

This type of immune response is referred to as cross-reactivity, and it also occurs with T-cell responses, the additional line of the immune system in the defense against infections.

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In the study, the team used a specially developed assay to analyse antibody levels against four other human coronaviruses in 825 serum samples taken before Covid emerged. They also examined 389 samples from donors infected with Covid. Combining these analyses with computer-based models enabled the team to precisely predict how well the antibodies would bind to and neutralise invading viruses.

People get fully protected against Covid shortly after they have recovered from an infection or have received an effective vaccination. This is when antibody levels against the virus are still very high. As these levels drop over time, infection is no longer prevented, but the immunological memory quickly reactivates the body's defenses, the production of antibodies as well as the T-cell defense.

"Of course, immune responses targeting SARS-CoV-2 that are mounted by the memory cells are far more effective than cross-reactive responses. But even though the protection isn't absolute, cross-reactive immune responses shorten the infection and reduce its severity. And this is exactly what is also achieved through vaccination, just much, much more efficiently," Trkola said.

It is, however, not yet known whether this cross-reactivity also works in the opposite direction. Whether immunity to Covid -- achieved through vaccination, for example -- also offers protection against other human coronaviruses still needs to be elucidated.

"If SARS-CoV-2 immunity also offers some degree of protection from infection with other coronaviruses, we would be a significant step closer to achieving comprehensive protection against other coronaviruses, including any new variants," the virologist explains.

This idea is also supported by the fact that cross-reactive protection is not only based on antibodies, but very likely also on T cells.

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