Baltimore—Women definitely aren’t the weaker sex when it comes to fighting off influenza.

A study published online by the American Journal of Physiology—Lung Cellular and Molecular Physiology demonstrates that estrogen dramatically reduced the amount of flu virus that replicated in infected cells from women but not from men.

Researchers from the Johns Hopkins Bloomberg School of Public Health suggest that estrogen can have a protective advantage, both as the female hormone that naturally circulates in women’s bodies and with artificial forms used in hormone replacement therapy or even estrogen-like chemicals found in the environment.

Background information in the article cites recent studies showing that estrogen also can hamper replication of viruses including HIV, Ebola, and hepatitis. That effect lessens an infection’s severity and makes it less contagious, so the study team sought to determine if estrogen might have the same effect on the flu virus.

For the study, researchers collected cells from the nasal passage from female and male volunteers and then exposed batches of the cells to different types of estrogens, including normal levels of naturally occurring estrogen; different types of selective estrogen receptor modulators (SERMs, synthetic estrogen-like chemicals used for hormone replacement therapy and infertility treatment, among other uses); or bisphenol A, an estrogen-like chemical found in many plastics. The cells then were exposed to an influenza A virus.

Results indicate that the female cells receiving estrogens, including some types of SERMs and bisphenol A, had significant reductions of viral replications, nearly 1,000-fold less, compared to those that hadn't been exposed to the hormones.

Lead researcher Sabra L. Klein, PhD, an associate professor in the Departments of Molecular Microbiology and Immunology and Biochemistry and Molecular Biology at the Bloomberg School, explained that, while men produce estrogen, their cells have far fewer receptors for the hormone.

Klein said the new findings suggest that women taking hormones for contraception, hormone replacement therapy, infertility treatments, or other medical uses are getting an additional advantage. “If women are taking estrogen-like hormones for other reasons, an added benefit might be less susceptibility to influenza during the flu season,” she pointed out in an American Physiological Society press release.

Because estrogens can have other biological effects, ranging from strengthening bones to increasing risks for some types of cancer, study authors don’t urge widespread use of hormone replacement therapy to fight the flu.

The findings could be particularly important for elderly women, however, who are most susceptible to severe influenza. “Being on hormone replacement therapy could be one way to mitigate the severity of this disease, which is exciting, simple and cheap,” Klein noted. “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.”

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