Scientists are puzzled why more men have gotten seriously ill from COVID-19 than women. Right now, two clinical trials are investigating whether hormones may be the reason that men around the world have been more likely to require intensive medical care or die than women.

According to Live Science, men make up 75% of COVID-19 patients in intensive care or on ventilators at Cedars-Sinai Medical Center in Los Angeles. As of early April, infected men in New York City were dying at about twice the rate of infected women, according to Live Science.

While some experts believe the gender disparity is due to the higher prevalence of heart and lung conditions in men, who also generally smoke cigarettes, consume alcohol and are exposed to outdoor pollution at a higher rate than women, others think hormones may play a role.

Women produce estrogen and progesterone, hormones which help regulate the female immune system and may bolster their resistance to infections. Scientists at Cedars-Sinai and the Renaissance School of Medicine at Stony Brook University plan to treat small groups of COVID-19 patients with hormones to see if they make a difference.

The Stony Brook patients who participate in the study will receive an estrogen patch. All men aged 18 and older are welcome to enter the trial if they have at least one serious symptom of COVID-19, according to Live Science.

The Cedars-Sinai volunteers will receive progesterone, rather than estrogen, as progesterone has anti-inflammatory properties that may stave off cytokine storms that can damage the body.

Both trials are hoping that increased amounts of estrogen and progesterone will help men fight off the virus, but not all data supports this theory.

Sabra Klein, a scientist who studies sex differences in viral infections and vaccination responses at the Johns Hopkins Bloomberg School of Public Health, points out that if sex hormones were the primary protective factor for women, then elderly women, whose hormone levels have fallen after menopause, would fare as poorly as men.

But that’s not the case, she told The New York Times.

“We see this bias across the life course,” she said. “Older men are still disproportionately affected, and that suggests to me it’s got to be something genetic, or something else, it’s not just hormonal.”