Receiving hormone replacement therapy within six months of a recorded diagnosis of COVID-19 was associated with a reduction in mortality from the viral disease, according to a study.

The research, published in the journal Family Practice, analysed the association between hormone replacement therapy or combined oral contraception use, and the likelihood of death in women with COVID-19.

The researchers from the University of Oxford and the University of Southampton in the UK investigated combined oral contraception, which contains estrogen because some recent observational data suggests that women taking oral contraceptives have a lower risk of acquiring COVID-19.

They used a retrospective cohort with medical records from the Oxford-Royal College of General Practitioners Research and Surveillance Centre primary care database. The team identified a group of 1,863,478 women over 18 from 465 general practices in England.

There were 5,451 COVID-19 cases within the cohort. Hormone replacement therapy was associated with a 22 per cent reduction in all-cause mortality in COVID-19, the researchers said. This suggests that estrogen may contribute a protective effect against COVID-19 severity.

This may explain why fewer women compared to men have been hospitalised, admitted to intensive care, or died due to COVID-19 during the pandemic, they said. "This study supports the theory that estrogen may offer some protection against severe COVID-19," said Christopher Wilcox, one of the authors of the research paper.

"We hope that this study can provide reassurance to patients and clinicians that there is no indication to stop hormone replacement therapy because of the pandemic," Wilcox added.

The researchers noted that although men and women are equally susceptible to the SARS-CoV-2 virus, men tend to have more severe infections. A recent review of sex differences in COVID-19, using data from 38 countries, found mortality in men was 1.7 times higher than in women, they said.

Younger women or those with higher estrogen levels are less likely to experience COVID-19 complications, according to the researchers. Earlier studies have also shown that women have faster and greater immune responses to viral infections, they said.

Researchers have observed similar data in previous pandemics, including the Severe Acute Respiratory Syndrome Corona Virus (SARS-CoV) and Middle East Respiratory Syndrome Corona Virus (MERS-CoV) outbreaks.

"The reason for these sex differences is uncertain. Limited recent observational data suggest that estrogen may reduce the severity of COVID-19 disease," the authors of the study noted.