Catching a mild cold from a coronavirus may help boost your immune response to SARS-CoV-2, the virus behind COVID-19.

Teaching your immune system to recognise and respond to the SARS-CoV-2 virus is the best way to protect yourself from serious health problems, if you become infected with the virus. And, so far, the only approaches available to boost your immune response to SARS-CoV-2 is through vaccination or by facing (and surviving) the dreaded virus.

But now, a new study found that being exposed to other, milder, coronaviruses can give you some level of protection against SARS-CoV-2.

The study was led by Dr Alexandra Trkola, head of the Institute of Medical Virology at the University of Zurich. Researchers used a custom-made approach to analyse antibody levels in 825 serum samples from people exposed to four different coronaviruses, obtained before the pandemic started. These samples were compared to 389 serum samples obtained from people infected with SARS-CoV-2. Then, using a bioinformatic approach, researchers predicted how well the antibodies from the four coronaviruses would bind to and neutralise invading viruses, such as SARS-CoV-2.

Their results showed that people who caught SARS-CoV-2 tended to have lower levels of antibodies against other coronaviruses, such as those that cause common colds, compared to uninfected people. In contrast, people who had high levels of antibodies against harmless coronaviruses seemed to have some level of protection against SARS-CoV-2.  “Our 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,” Dr Trkola said.

This type of immune response, where immune cells produced against one pathogen also work against a different pathogen, is called cross-reactivity. While this type of immune response is not as effective as that obtained from a vaccine or from surviving an infection, it can still result in beneficial effects.

“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,” Dr Trkola said.

A pending question is whether this is a two-way street and having immunity to SARS-CoV-2 would also confer protection against other coronaviruses. “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,” Dr Trkola said.