The study, conducted in mice, found that hormone progesterone was protective against the more serious effects of the flu by increasing the production of a protein called amphiregulin by the cells lining the lungs. 

Progesterone not only lessened the inflammation and damage associated with the flu, it also helped induce repair, the study said.

"Understanding the role that progesterone appears to play in repairing lung cells could really be important for women's health," said lead author Sabra L. Klein, Associate Professor at the Johns Hopkins Bloomberg School of Public Health, in the US.

"When women go on birth control, they don't generally think about the health implications beyond stopping ovulation and it's important to consider them," Klein added.

When female mice (and possibly humans) get sick with the flu, their natural levels of progesterone fall. 

Women on hormonal contraceptives -- be it a birth control pill, intrauterine device (IUD) or injection -- get a steadier level of progesterone which overrides what the ovaries make naturally or what the virus takes away during infection, the researchers observed.

The findings suggest that sex hormones have an effect far beyond the reproductive system and that hormone progesterone may one day be a viable flu treatment for women.

More than 100 million young adult women around the world are on progesterone-based contraception, according to the World Health Organisation (WHO). 

And women of reproductive age are twice as likely than men to suffer from complications related to the influenza virus.

"Despite the staggering number of women who take this kind of birth control, very few studies are out there that evaluate the impact of contraceptives on how the body responds to infections beyond sexually-transmitted diseases," Klein noted.

WHO has already listed hormonal contraceptives as an essential medication because of the profound benefits these compounds can have on women's health by widening the interval between pregnancies, including decreased rates of maternal mortality and improved outcomes for babies and children.

For the study, the team placed progesterone implants in female mice and left other mice, also female, without. 

The mice were then infected with influenza A virus. Both sets of mice became ill, but those which had the implants had less pulmonary inflammation, better lung function and saw the damage to their lung cells repaired more quickly.

Klein says there is no scientific data to date showing whether progesterone in humans has any relationship to flu severity. 

"We really want to understand from a therapeutic sense how this could potentially work in humans to keep women from experiencing complications from the flu," Klein stated in the paper, published in the journal PLOS Pathogens.