"Man flu" aside, scientists have known for a while that colds and the flu may take a bigger toll on women than men, even when virus levels are the same. Now, new research from Johns Hopkins Bloomberg School of Public Health indicates that men are also more likely to recover faster from the flu, and it could be because of a specially produced key protein.

The fact that women tend to have more severe flu symptoms than men was previously believed to be caused by their higher levels of lung inflammation during infections. However, a lab test showed that both male mice and human cells infected with a non-lethal dose of H1N1 showed a higher production of a lung-healing protein called amphiregulin. Not only did male mice recover quicker but those genetically engineered to lack amphiregulin recovered at similar rates to those of females.

"The novel finding here is that females also have slower tissue-repair during recovery, due to relatively low production of amphiregulin," says senior study author Sabra Klein in a statement. 

Every year, as much as 20 percent of the US population will get the flu, resulting in an average of 200,000 hospitalizations and as many as 49,000 deaths. Researchers say their work could lead to new flu treatments that aim to boost the production of amphiregulin in women.

"We found in that study that by increasing amphiregulin production in females we could expedite their recovery from flu," said Klein.

Of course, the study was conducted on mice not men, and only human cells. While both male and female mice had similar levels that cleared out in about the same time, females suffered a “greater loss of body mass and greater lung inflammation” and were slower to recover.  

Why amphiregulin, which is known to promote epithelial cells in the skin and lungs, increases in men when they are in the recovery phase of the flu is still a mystery; researchers initially believed it could be because of testosterone levels, but found the sex hormone doesn’t control amphiregulin levels. They hope to now investigate how testosterone might provide a “protective effect”, as well as other factors that could boost amphiregulin production.

The study was published in the Biology of Sex Differences.