Men across the world who complain of sickness and are dismissed as having "man flu" have been vindicated by a new study which has found that female hormones actually protect women against the influenza virus.
The study published on Wednesday in the American Journal of Physiology - Lung Cellular and Molecular Physiology, found that the female sex hormone estrogen has antiviral effects against the influenza A virus.
The team of researchers from John Hopkins University found that the antiviral properties of estrogen could protect women from the more aggressive symptoms of the virus, and may even be able to be harnessed to treat infections more broadly.
The findings support the idea that men are more vulnerable to the flu and may experience more severe symptoms of the illness, lead researcher Dr Sabra Klein said.
The John Hopkins team took samples of nasal cells from both men and women, and then exposed those cells to the flu virus, as well as estrogen and estrogen-like compounds used in hormone therapy.
They found that the hormone compounds reduced the flu virus' ability to replicate in the cells of women, but not men.
The researchers were also able to identify the specific estrogen receptor beta, which had the antiviral affect.
The finding that estrogen could fight viruses was no surprise to the researchers, as previous studies in monkeys have found that estrogen in animals can help fight HIV.
According to Dr Klein, "other studies have shown that estrogens have antiviral properties against HIV, Ebola and hepatitis viruses".
"What makes our study unique is two-fold," she said.
"First, we conducted our study using primary cells directly isolated from patients, allowing us to directly identify the sex-specific effect of estrogens. Second, this is the first study to identify the estrogen receptor responsible for the antiviral effects of estrogens, bringing us closer to understanding the mechanisms mediating this conserved antiviral effect of estrogens."
The results, however, may not necessarily help protect women in the wider population, as estrogen levels in women fluctuate. But, Dr Klein suggested women on birth control or hormone replacement therapy may be better protected from seasonal flu epidemics.
"We see clinical potential in the finding that therapeutic estrogens that are used for treating infertility and menopause may also protect against the flu," she said.