A new study has tried to explain why the flu seems to hit men harder than women.  Researchers who tested various forms of estrogen, which is also present in men, found that the compounds reduced virus replication in the female but not male cells.  The researchers reported their findings in the American Journal of Physiology – Lung Cellular and Molecular Physiology.

Viruses make us ill when they invade our cells and then use their machinery to make copies of themselves.  Copies then spread to other cells, setting up a chain reaction that leads to disease and also infection of other people.  As a general rule, Estrogenthe fewer copies of itself the virus makes, the less severe the disease and the lower the chances of it spreading to new hosts.  The researchers tested the effect of the hormone estrogen on the influenza A virus, one of the types that spreads in people.  They carried out tests in nasal cells, which the virus primarily infects, gathered from men and women.  They then cultured and exposed the cells to the flu virus and various forms of estrogen.

The results of the tests revealed that estradiol, raloxifene and bisphenol A reduced virus replication in the female (although not the male) cultured nasal cells.  The researchers also noticed that the estrogens exerted their antiviral effects through the estrogen receptor beta.  Receptors are “gatekeeper” proteins, which only allow entry to cell-stimulating molecules that can bind to them.

According to the researchers, previous studies have already shown that estrogen has antiviral effects on HIB, ebola and hepatitis.  Yet with this study, the researchers used primary cells directly isolated from patients, which allowed them to directly identify the same-sex effect of estrogens.  This was also the first study to identify the estrogen receptor responsible for the antiviral effects of estrogens, bringing researchers closer to understanding the mechanisms mediating this conserved antiviral effect of estrogens.  These new findings also support those of animal studies that have revealed the antiviral effects of estrogen on the flu virus.

It might be difficult to see this protective effect in premenopausal women, since estrogen levels go up and down in the menstrual cycle.  Yet it could also mean that women on certain kinds of birth control or those receiving hormone replacement for menopausal symptoms could be better protected during seasonal flu epidemics.

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