Blood type and other genetic factors may be linked with risk of infection and severity of coronavirus, according to European researchers looking for further clues about why COVID-19 hits some so much harder than others.

The findings, published in the New England Journal of Medicine, suggest people with type A blood have a higher risk of being infected and developing worse symptoms.

At the epidemic’s peak in Europe, researchers analysed the genes of more than 4000 people to look for variations that were common in those who became infected and developed severe COVID-19.

A cluster of variants in genes involved with immune responses was more common in people with severe COVID-19, they found.