Expanding on previous findings by a genome wide association study of severe COVID-19, specifically with respiratory failure which had found that a gene cluster residing on chromosome 3 had a significant association with severe acute respiratory failure post infection, the authors showed that:
1) Chromosome 3 in those patients is specifically populated by a 50,000 DNA nucleotides long segment that entered the human population by gene flow from Neanderthals or Denisovans.
2) This long haplotype entered the Neanderthal population, and was transmitted by Neanderthals to present day humans about 40,000-60,000 years ago.
3) This specific genomic segment is carried by about 50% of people in South Asia, is almost absent in East Asia and is carried by about 16% of European humans overall.
4) The authors posited that this genomic cluster was maintained in the genome most likely as the result of positive natural selection in Neanderthals because it probably contributed to the species chances of survival and reproductive success.

Order of authorship in the original publication: Zeberg, Pääbo. Available from nature.com at this link.

(Thanks to Juan Weiss for this reference and its interpretation.)