A new study suggests a protective advantage to estrogen, the quintessential female hormone that naturally circulates in women’s bodies, as it was proven to dramatically reduce the amount of flu virus that replicated in infected cells from women. In addition, artificial forms given for hormone replacement therapy and estrogen-like chemicals found in the environment were also found to have the same effect on cells in women, but not in men.

Researchers at the Johns Hopkins Bloomberg School of Public Health collected cells from the nasal passage from both female and male volunteers, according to a press release. Cells in the nasal passage are typically the first to be infected with the flu.

They then exposed the cells to different types of estrogens, including normal levels of naturally occurring estrogen, different types of selective estrogen receptor modulators including synthetic estrogen-like chemicals used for hormone replacement therapy and infertility treatment, and an estrogen-like chemical found in many plastics. According to the press release, researchers then exposed cells to the influenza A virus.

Tests showed that female cells that received estrogen had nearly 1,000-fold less viral replications, compared to those that hadn’t been exposed to the hormones. Researchers also noted that the hormones behind the reduction act on estrogen receptor beta, one of two types of receptors for estrogen inside cells.

Study leader Sabra Klein said in the press release that even though men produce estrogen, their cells have fewer receptors for the hormone, which may be why estrogen did not have the same protective effects against flu virus replication in men.

Researchers sought a mechanism behind estrogen’s protective effect and found that binding to estrogen receptor beta decreased the activity of more than 30 genes involved in cell metabolism, slowing the metabolic activity of the cells and potentially preventing them from manufacturing viral particles.

Klein noted in the press release that since hormone levels cycle in pre-menopausal women, it’s unlikely there’s a population-wide effect in protecting this group against the flu. But, the new findings suggest that hormones that women may already be taking for contraception, hormone replacement therapy, infertility treatments or other medical uses can play a role in reducing infection.

“If women are taking estrogen-like hormones for other reasons, an added benefit might be less susceptibility to influenza during the flu season,” Klein said in the release. However, she doesn’t recommend women seek hormone therapies for this purpose, as other side effects could include increased risk of cancer.

“Being on hormone replacement therapy could be one way to mitigate the severity of this disease, which is exciting, simple and cheap,” Klein said. “While the decision to take hormone therapy should always depend on a patient’s history and include discussion with their care providers, our study shows another potential benefit to this hormone.”

Researchers noted that elderly women might benefit most from the findings, as the population is most susceptible to influenza.

The study was published in the American Journal of Physiology – Lung Cellular and Molecular Physiology,