As an emergency room doctor, Hugo Zeberg has seen first-hand how widely COVID-19 infections can vary in severity. So he started digging for answers in a place that was familiar to him: the genome of Neanderthals.

Zeberg works at the Karolinska Institute in Sweden, and for the last couple of years, has been studying the degree to which Neanderthals - an extinct human species that died out about 40,000 years ago - passed along genes to modern humans through interbreeding.

Scientists think Neanderthal DNA makes up 1% to 2% of the genomes of many people of European and Asian descent. That small fraction of people's genetic codes may hold important clues about our immune responses to pathogens.

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In a study published this week, Zeberg and his colleague Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology suggest that some people may have inherited a genetic advantage that reduces their risk of getting severe COVID-19 by 22%.

The advantage comes from a single haplotype - or long block of DNA - on chromosome 12. The same haplotype has been shown to protect people against West Nile, hepatitis C, and SARS (another coronavirus that shares many genetic similarities with the new one, SARS-CoV-2).

"The protective effect of this haplotype is probably not unique to SARS-CoV-2, but a more general part of our immune system," Zeberg told Insider.

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