When it comes to fending off the flu, women may have an advantage over men, new research suggests.
The study found that the female sex hormone oestrogen helps keep the flu virus somewhat at bay, which may help explain why flu appears to be harder on men than women.
Less severe symptoms
The findings may also lead to new flu treatments, the researchers said.
Read: Surviving a summer cold
In experiments with nasal cells from women and men, the researchers found that oestrogen seems to limit the ability of the flu virus to replicate.
Less replication of the virus means that an infected person has less severe symptoms and is less likely to spread the flu to others, said lead investigator Sabra Klein, of Johns Hopkins University in Baltimore.
"Other studies have shown that oestrogens have antiviral properties against HIV, Ebola and hepatitis viruses. What makes our study unique is twofold," Klein said in a news release from the American Physiological Society.
Read: How far does a sneeze really travel?
"First, we conducted our study using primary cells directly isolated from patients, allowing us to directly identify the sex-specific effect of oestrogens," she said.
Oestrogen levels vary
"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," she added.
It's possible this effect is hard to see in the general population because oestrogen levels vary throughout the month in women who haven't gone through menopause, Klein suggested.
Read: Flu myths and facts
"But, premenopausal women on certain kinds of birth control or post-menopausal women on hormone replacement may be better protected during seasonal influenza epidemics," Klein said.
Therapeutic oestrogen used to treat infertility and menopausal symptoms – may also offer some protection against flu, she added.
The study was published online recently in the American Journal of Physiology – Lung Cellular and Molecular Physiology.