LONDON—Doctors began noticing the change last month as the U.K.’s new coronavirus variant spread. In hospital critical-care units, they were seeing more younger people and women with serious cases of Covid-19, a disease that previously inflicted its worst symptoms largely on older patients and men.
The shift—backed up now by statistical studies—is part of an urgent puzzle preoccupying British public-health officials as they race to understand a strain of the virus that is more transmissible and, some studies indicate, could be deadlier than earlier versions.
One early hypothesis, scientists say, is that one of the virus’s genetic mutations makes it better at invading a greater number of cells inside the body, leading to serious sickness in people who with previous Covid-19 variants would only have had mild or no symptoms.
David Strain, a physician and instructor at the University of Exeter’s medical school who is also treating Covid-19 patients, said initial research paints a picture of a virus variant that is like a burglar that has become better at breaking and entering—into vulnerable cells in this case, not houses.
“We think that’s why it’s making people sicker as well as being more transmissible,” Dr. Strain said.