A team of researchers led by the University of Zurich (UZH) has discovered another component that contributes to SARS-CoV-2 immunity – previous antibody responses to other, harmless coronaviruses. “People who have had strong immune responses to other human coronaviruses also have some protection against SARS-CoV-2 infection,” says Alexandra Trkola, head of the Institute of Medical Virology at UZH. In their study, the researchers used a specially developed assay to analyse antibody levels against four other human coronaviruses in 825 serum samples taken before SARS-CoV-2 emerged. They also examined 389 samples from donors infected with SARS-CoV-2. Combining these analyses with computer-based models enabled the team to predict how well the antibodies would bind to and neutralize invading viruses. The researchers were able to demonstrate that people who caught SARS-CoV-2 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 hospitalized after catching SARS-CoV-2. 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 defence against infections.