Men with COVID-19 three times more likely to need intensive care: study

By early June 2020, researchers had begun to notice something striking in the data: men were dying from COVID-19 at rates that far outpaced women. A team of scientists analyzing more than three million confirmed cases across 46 countries and 44 American states—data collected between January and June of that year—found a pattern so consistent it demanded explanation. The virus did not discriminate in who it infected; roughly half of all confirmed cases were men, half were women. But once hospitalization became necessary, the picture shifted dramatically. Men were nearly three times as likely as women to require intensive care, and they faced a 39 percent higher risk of death.

The finding was significant enough that researchers published their work in Nature Communications, and it raised an immediate question: why? The answer, they concluded, lay not in behavior or circumstance but in biology itself. The human immune system, it turned out, was not neutral ground between the sexes. Women possessed natural advantages that men lacked, advantages written into their cellular machinery.

Women's bodies produce type I interferon proteins at higher levels than men's—proteins that act as a brake on the runaway immune response known as a cytokine storm. That storm, a cascade of inflammatory signals spiraling out of control, appeared to be one of the mechanisms driving severe COVID-19. Additionally, the female hormone estradiol seemed to offer protection by strengthening the response of T cells, the immune cells that hunt down and destroy infected tissue, while also boosting antibody production. Testosterone, by contrast, appeared to suppress immune function rather than enhance it. The researchers noted this was not new science; sex-based differences in both innate and adaptive immunity had been documented before. But the pandemic had thrown these differences into sharp relief.

Kate Webb, a researcher at Cape Town University and co-author of the study, told the press that the findings should serve as a reminder to clinicians. "Sex is a risk factor for severe disease," she said, and doctors needed to recognize it as such when deciding how to manage individual patients. She also flagged something she saw as a broader problem in medical research: sex was routinely overlooked or underreported as a variable in studies. This work was meant to correct that oversight.

The researchers acknowledged gaps in their own analysis. While they could point to biological mechanisms that favored women, they could not fully account for the role of other medical conditions—comorbidities like hypertension and diabetes that made severe COVID-19 more likely. The prevalence of these conditions appeared roughly equal between men and women globally, yet men still fared worse. That discrepancy suggested other factors were at play, factors the available data could not fully illuminate.

Looking ahead, Webb and her colleagues raised a question that would shape pandemic response for years to come: would the same sex-based differences appear in vaccine responses? Previous vaccines against other infections had shown measurable differences in how men and women responded immunologically. Whether COVID-19 vaccines would follow that pattern remained unknown. But the authors made a clear case that sex needed to be treated as a serious variable in vaccine research, not an afterthought. The pandemic had revealed that ignoring biological sex differences came at a cost. Future preparedness, they suggested, depended on building that knowledge into the foundation of research from the start.