Why the Coronavirus Seems to Hit Men Harder Than Women

The coronavirus that originated in China has spread fear and anxiety around the world. But while the novel virus has largely spared one vulnerable group — children — it appears to pose a particular threat to middle-aged and older adults, particularly men.

This week, the Chinese Center for Disease Control and Prevention published the largest analysis of coronavirus cases to date. Although men and women have been infected in roughly equal numbers, researchers found, the death rate among men was 2.8 percent, compared with 1.7 percent among women.

The figures were drawn from patient medical records, and the sample may not fully reflect the scope of the outbreak. But the disparity has been seen in the past.

A number of factors may be working against men in the current epidemic, scientists say, including some that are biological, and some that are rooted in lifestyle.

When it comes to mounting an immune response against infections, men are the weaker sex.

“This is a pattern we’ve seen with many viral infections of the respiratory tract — men can have worse outcomes,” said Sabra Klein, a scientist who studies sex differences in viral infections and vaccination responses at the Johns Hopkins Bloomberg School of Public Health.

“We’ve seen this with other viruses. Women fight them off better,” she added.

Women also produce stronger immune responses after vaccinations, and have enhanced memory immune responses, which protect adults from pathogens they were exposed to as children.

“There’s something about the immune system in females that is more exuberant,” said Dr. Janine Clayton, director of the Office of Research on Women’s Health at the National Institutes of Health.

But there’s a high price, she added: Women are far more susceptible to autoimmune diseases, like rheumatoid arthritis and lupus, in which the immune system shifts into overdrive and attacks the body’s own organs and tissues.

One hypothesis is that women’s stronger immune systems confer a survival advantage to their offspring, who imbibe antibodies from mothers’ breast milk that help ward off disease while the infants’ immune systems are still developing.

A stew of biological factors may be responsible, including the female sex hormone estrogen, which appears to play a role in immunity, and the fact that women carry two X chromosomes, which contain immune-related genes. Men, of course, carry only one.

The male mice developed SARS at lower viral exposures, had a lower immune response and were slower to clear the virus from their bodies. They suffered more lung damage, and died at higher rates, said Dr. Stanley Perlman, a professor of microbiology at the University of Iowa who was the senior author of the study.

When researchers blocked estrogen in the infected females or removed their ovaries, they were more likely to die, but blocking testosterone in male mice made no difference, indicating that estrogen may play a protective role.

“It’s an exaggerated model of what happens in humans,” Dr. Perlman said. “The differences between men and women are subtle — in mice, it’s not so subtle.”

Health behaviors that differ by sex in some societies may also play a role in disparate responses to infections.

In unpublished studies, Chinese researchers have emphasized that patients whose diagnoses were delayed, or who had severe pneumonia when they were first diagnosed, were at greatest risk of dying.

One study of 4,021 patients with the coronavirus emphasized the importance of early detection, particularly in older men. And men have been turning up in hospitals with more advanced disease.

But in areas of China outside Hubei Province, the disease’s epicenter and where the majority of those affected are concentrated, the patterns are different: The disease appears to have dramatically lower mortality rates, and men are being infected at much higher rates than women, according to the Chinese C.D.C. analysis.

Gathering and analyzing data about the new virus by sex is important both for the scientists studying it and for the general public, experts said.

Since the start of the outbreak, for example, public health officials have emphasized the importance of washing hands well and often, to prevent infection. But several studies have found that men — even health care workers — are less likely to wash their hands or to use soap than women, Dr. Klein said.

“We make these broad sweeping assumptions that men and women are the same behaviorally, in terms of comorbidities, biology and our immune system, and we just are not,” Dr. Klein said.

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