The deadly coronavirus that originated in Wuhan, China, has infected more men than women, and scientists are divided about why that is.

The COVID-19 virus, as it’s now known. has killed 1,384 people and infected more than 64,000, with the vast majority of cases in mainland China. (For the latest case total and death toll, see Business Insider’s live updates here.)

A recent study of nearly 140 coronavirus patients at a Wuhan University hospital offers one of the broadest pictures of how the virus operates in humans so far.

The researchers found that the virus was most likely to affect older men with preexisting health problems. More than 54% of the patients in the study were men, and the median age of patients was 56.

Other recent studies have produced similar results. A study of 99 coronavirus patients at Wuhan Jinyintan Hospital showed that the average patient was 55.5 years old, and men represented around 68% of the total cases. A third study of nearly 1,100 coronavirus patients (which is still awaiting peer review) identified a median age of 47, with men representing around 58% of the cases.

This data has led some researchers to suspect that men have certain biological conditions that make them more susceptible to the virus. But Chinese men also smoke more than Chinese women, which increases their risk of respiratory problems.

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SARS affected mostly men, too

In the absence of much reliable, broad data about the new coronavirus, scientists have turned to a similar outbreak – the SARS pandemic from 2002 to 2003 – for clues.

SARS was also a coronavirus that jumped from animals to people in wet markets. It shares about 80% of its genome with the novel coronavirus, and like the current outbreak, it infected more men than women.

Customers wearing face masks shop inside a supermarket following an outbreak of the novel coronavirus in Wuhan, Hubei province, China February 10, 2020. China Daily via REUTERS

Foto: Shoppers wear masks at a supermarket during the outbreak of the novel coronavirus in Wuhan, China.sourceReuters

In 2017, researchers at the University of Iowa infected male and female mice with SARS in order to investigate why that was. Mouse studies don’t necessarily have definitive implications for humans, but the researchers did find that male mice were more susceptible to the virus than female mice.

The team attributed those results to genes on the X chromosome and hormones such as estrogen that may keep a virus from spreading throughout the female body.

The researchers at Wuhan Jinyintan Hospital gave a similar explanation for why more of their coronavirus patients were men, suggesting that women may have a “reduced susceptibility” to viral infections. But they also said many patients with severe cases had chronic illnesses such as cardiovascular disease or diabetes. Those illnesses tend to affect middle-aged men more than middle-aged women.

Chinese men tend to smoke more than Chinese women

On Friday, the executive director of the WHO Health Emergencies Program, Michael Ryan, said smoking was “an excellent hypothesis” for why the virus has affected mostly men. A 2010 national survey of smoking in China found that 62% of Chinese men had been smokers at some point, while only 3% of Chinese women had ever smoked.

“There is a marked difference between male and females in this outbreak in terms of severity. And there’s certainly a marked difference in those habits in China,” Ryan said on a press call. “I think it should be relatively straightforward to establish the science.”

men smoking china

Foto: Men smoke cigarettes outside of an office building in Beijing on October 8, 2015.sourceMark Schiefelbein/AP Photo

He added that smoking is a risk factor any type of lower respiratory-tract infection.

“We would expect it to be no different here,” he said, referring to the outbreak.

Saskia Popescu, an epidemiologist at the Honor Health medical group in Arizona, told Business Insider anyone with a history of smoking would be more vulnerable to this coronavirus.

“Since COVID-19 is a respiratory disease and often causes pneumonia, having a history of smoking could increase the risk of more severe respiratory distress or pneumonia,” she said.

More data is needed to confirm the theory, though.

The outbreak started among mostly male workers

Aaron Milstone, an epidemiologist at Johns Hopkins University, told Business Insider that he didn’t think one subset of the population was necessarily more vulnerable than another.

“When we see any new virus, the whole population is susceptible,” he said.

The new study from Wuhan University suggested that the higher number of male patients could be due to the outbreak’s origin point: the Huanan Seafood Wholesale Market in Wuhan, which had mostly male workers before it was shuttered.

The research also showed that the share of male and female coronavirus patients in the Intensive Care Unit was about the same as the share of male and female coronavirus patients in other parts of the hospital. That suggests the men’s symptoms weren’t more severe than women’s overall (though many patients are still hospitalized, so their conditions could change over time).

But Milstone said any dataset from this coronavirus outbreak is still inherently limited.

“There can be underreporting because of under-testing, especially because this is not a commercially available test,” Milstone said. “We don’t know enough yet about what’s happening.”

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