One of the mysteries of the new coronavirus is that it is deadlier for men than women.
White House coronavirus coordinator Dr. Deborah Birx recently pointed out this “concerning trend” after looking at statistics in Italy, where footage of hospital intensive care units showed bed after bed of older men breathing with the help of ventilators.
“The mortality in males seems to be twice in every age group of females. This should alert all of us to continue our vigilance to protect our Americans that are in nursing homes,” Birx said Friday during a briefing of the Trump administration's coronavirus task force.
Full coverage of the COVID-19 outbreak and how it impacts you
When Italy recently reported the country’s death toll, 72% of those who had died were men, according to the BBC. One study put the number even higher, with men making up 80% of people who had passed away in Italy.
Experts have suggested a number of factors that may help explain the disparity, including immune system differences between men and women, the protective effect of estrogen, lifestyle habits and the tendency for men to have more risk factors.
For example, men are more likely to smoke — with 40% of men smoking cigarettes worldwide compared to 9% of women, according to the World Health Organization — which puts them at higher risk of lung disease and a tougher battle when a respiratory virus strikes.
Men also drink more alcohol and may put off going to a doctor when feeling unwell.
Meanwhile, women may have the edge when fighting a viral infection.
“There’s something about the immune system in females that is more exuberant,” Dr. Janine Clayton, director of the Office of Research on Women’s Health at the National Institutes of Health, told The New York Times.
Women mount higher innate and adaptive immune responses than men, which can result in "faster clearance of viruses,” one study found.
They also may have an immune advantage from having two X chromosomes, which “could contribute to an immunological advantage for females in many infections,” according to another paper.
Still, the full explanation remains elusive as the coronavirus outbreak grows.
This story first appeared on TODAY.com. More from TODAY: