HELSINKI, Finland -- The female hormone estrogen may protect women from severe heart disease and death caused by COVID-19, said Helsinki University Hospital (HUS) in a press release on Thursday.

The estrogen supplement halved the risk of death due to COVID-19 in women, according to a joint study by HUS, the University of Helsinki, and the University of Umea of Sweden.

The mortality rate for women receiving the estrogen supplement was 2.1 percent, compared with 4.6 percent for women in the control group, according to the study.

The study has been published in BMJ Open, a peer-reviewed open-access medical journal.

The study compared the mortality of women diagnosed with COVID-19 disease at the beginning of the pandemic with the effect of estrogen on mortality.

The study included around 15,000 women aged 50 to 80 who were diagnosed with COVID-19 related heart disease between February and September 2020. The 2,500 women in the study had estrogen replacement therapy for menopausal symptoms, and about 200 women received estrogen-lowering medication after cancer treatment. There were 12,000 women in the control group who did not receive estrogen-increasing or estrogen-lowering medication.

The group receiving estrogen-lowering therapy had the highest risk of death. However, a causal relationship to estrogen could not be established as patients in this group were older than the control group and had received cancer treatment. Age and cancer treatments increase the risk of serious heart disease and death caused by COVID-19.

"Our study does not yet lead to a change in treatment practices, so estrogen therapy should not be started or estrogen-lowering medication should be discontinued based on this study," said Malin Sund, professor and chief physician at HUS, in the press release.

The study was conducted before COVID-19 vaccines were available. Thus, it is not yet possible to deduce from the study how much estrogen reduces the risk of serious illness and death among vaccinated women, said the release.