Men could be more susceptible to flu, a new study has found, which could prove why "man flu" is often as bad as it's made out to be.
Scientists discovered that oestrogen has anti-viral effects against the flu, which can help make women better equipped to fight the illness.
Lead author of the study, Sabra Klein, from Johns Hopkins University, said that the female sex hormone can help reduce the replication of the influenza virus in cells, which is great news for women - but not so much for men.
The researchers now hope to identify whether oestrogen used for treating infertility and menopause may also protect against the flu.
When a virus enters a cell, it makes copies of itself within a host cell. These infected cells then release the virus, which spread across the body.
According to Sabra Klein, the amount that a virus has replicated determines its severity. If a virus has less replication in a person, this means they might be less prone to illness and are less likely to spread the disease.
A research team from Johns Hopkins University gathered nasal cells, the cell type that the flu virus primarily infects, from male and female donors.
They wanted to investigate whether oestrogen reduced replication of the influenza virus in cells.
Experts exposed the cells to a combination of the virus, oestrogen, the environmental oestrogen bisphenol A and selective oestrogen receptor modulators, also known as SERM. The latter of which is made up of compounds that act like oestrogen and are used for hormone therapy.
They found that oestrogen reduced replication of the flu virus in nasal cells from women, but not men. They also discovered that it has antiviral effects.
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"Other studies have shown that oestrogens have antiviral properties against HIV, Ebola and hepatitis viruses," Klein told Science Daily.
"What makes our study unique is two-fold. First, we conducted our study using primary cells directly isolated from patients, allowing us to directly identify the sex-specific effect of oestrogens.
"Second, this is the first study to identify the oestrogen receptor responsible for the antiviral effects of oestrogens, bringing us closer to understanding the mechanisms mediating this conserved antiviral effect of oestrogens."
Researchers believe the new study supports earlier evidence from animal studies, which showed the benefits of oestrogen in fighting the flu.
Klein explained that because oestrogen levels cycle in premenopausal women, "it may be difficult to see this protective effect in the general population".
But she did say that women on certain kinds of birth control or post-menopausal women on hormone replacement may be better protected during peak flu season.
She added: "We see clinical potential in the finding that therapeutic oestrogens that are used for treating infertility and menopause may also protect against the flu."
The research was published in the American Journal of Physiology - Lung Cellular and Molecular Physiology.
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[H/T Daily Mail]