Estrogen, given as part of hormone replacement therapy or infertility treatment, has been shown to help protect women from influenza, in new research by Johns Hopkins University Bloomberg School of Public Health.

Researchers found the female hormone dramatically reduced the amount of flu virus that replicated in infected cells from women, but did not have the same effect on those taken from men.

The findings, reported online in the American Journal of Physiology - Lung Cellular and Molecular Physiology, suggest a protective advantage the hormone confers that naturally circulates in women's bodies — as well as artificial forms given for hormone replacement therapy and estrogen-like chemicals found in the environment.

Past studies have suggested estrogen can hamper replication of viruses including HIV, Ebola, and hepatitis, which can lessen an infection's severity and make an infection less likely to spread to other people.

But the new study's leader, Sabra L. Klein, an associate professor in the Departments of Molecular Microbiology and Immunology and Biochemistry and Molecular Biology at the Bloomberg School, says it is the first to show estrogen might have the same effect on the flu virus.

To reach their conclusions, she and her colleagues collected cells from the nasal passages of female and male volunteers. They exposed them to different types of estrogens and to the influenza A virus.

Tests showed that female cells that received estrogens had marked reductions in the rate at which the flu virus replicates and grows — nearly 1,000-fold less compared to those that hadn't been exposed to these hormones.

Klein notes that because hormone levels cycle in pre-menopausal women, it's unlikely that there's a population-wide effect in protecting this group against the flu. She adds that hormones women take for contraception, hormone replacement therapy, infertility treatments, or other medical uses may also play a powerful 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 says.

"Being on hormone replacement therapy could be one way to mitigate the severity of this disease, which is exciting, simple and cheap. 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."