Researchers believe men could be more susceptible to the coronavirus, as studies show that the majority of patients infected appear to be older men with pre-existing illnesses.

The virus has killed more than 1,300 people and infected more than 60,000 worldwide, with most cases in mainland China.

In a Lancet medical study published last month, roughly two-thirds of 99 infected patients admitted to a Wuhan, China, hospital were men, with the average age of 55.5 years.

Other recent studies have shown similar results. In a study of nearly 140 coronavirus patients in Wuhan, researchers found the virus was more likely to affect older men with previous health issues. More than 54 percent of the patients in the study were men, with the average age of 56. A third study of more than 1,000 coronavirus cases showed men represented 58 percent of all cases, with a median age of 47.

In a column in the Financial Times, science writer Anjana Ahuja called it an “eye-catching discrepancy,” adding “a picture is emerging of COVID-19 as a novel pathogen that disproportionately affects older men, particularly those with existing illnesses such as heart disease and diabetes.”

Ahuja said reasons for the discrepancy could be due to smoking, a variation of hospital treatment and hormonal differences that could affect men’s immune system response to the disease.

The Financial Times reports women are prone to autoimmune diseases, which causes parts of their immune system to become stronger to compensate. That could result in a stronger response to the coronavirus outbreak.

During the SARS outbreak in 2003, more men were infected than women, and researchers at the University of Iowa carried out a study on mice that found male mice were more susceptible to the virus than female mice. Researchers attributed the results to hormones like estrogen.

“Some scientists are now convinced that these sex difference in clinical data reflect a genuine male vulnerability to coronavirus, rather than a bias in exposure,” Ahuja said. “The observations add to growing evidence that immunologically speaking, men are the weaker sex.”