Women have twice as much protection written into their cells
Across nations and age groups, men have faced COVID-19's severest consequences at a rate that could not be explained by behavior or circumstance alone. Researchers at the University of Alberta have traced this disparity to the X chromosome — a genetic inheritance that grants women doubled copies of two critical protective genes, ACE2 and Toll-like receptor seven. In the long human story of biological difference, this finding reminds us that vulnerability and resilience are sometimes written before a single choice is made.
- A persistent, global pattern — men dying from COVID-19 at disproportionately higher rates — demanded a biological explanation beyond lifestyle or social behavior.
- The answer lies in a chromosomal asymmetry: women's double X inheritance allows two key immune genes to escape silencing, flooding their cells with twice the protective protein men can produce.
- ACE2, the very enzyme the virus hijacks to enter cells, doubles as a shield for the heart, lungs, and kidneys — and women carry twice as much of it, turning the virus's own entry point into a defensive advantage.
- A second gene, Toll-like receptor seven, also escapes X-chromosome inactivation in women, supercharging the innate immune system's first response to viral invasion — a mechanism that may explain why even the common cold hits men harder.
- The stakes are sharpened by occupation: with 70% of healthcare workers being female and facing elevated exposure daily, women's superior outcomes despite greater risk underscore just how powerfully genetics can outpace circumstance.
When the pandemic revealed that men were dying at consistently higher rates than women — across countries and age groups — a team at the University of Alberta set out to find the biological mechanism behind the gap. What they discovered was rooted in chromosomes.
Women carry two X chromosomes; men carry one X and one Y. On the X chromosome lives ACE2, the enzyme SARS-CoV-2 uses to enter human cells. ACE2 also protects the heart, lungs, and kidneys — the organs COVID-19 attacks most severely. Normally, one of a woman's X chromosomes is silenced to prevent genetic redundancy, but ACE2 escapes that silencing. The result: women's cells produce roughly twice as much ACE2 protein as men's, offering greater defense against the virus's most damaging effects.
Senior author Gavin Oudit and his team, publishing in the American Journal of Physiology-Heart and Circulatory Physiology, identified a second advantage: Toll-like receptor seven, also on the X chromosome, also escaping inactivation. This receptor anchors the innate immune system's first response to infection. Women carry double the active copies, which Oudit suggests explains a long-observed phenomenon — that women simply tolerate viral infections better than men, a reality colloquially known as the "man-cold."
Perhaps most striking is the occupational dimension. Healthcare workers are 70 percent female, placing them at far greater exposure to the virus than the general population. Yet their outcomes remain better than men's. The genetic advantage appears to outweigh the occupational risk — a finding that reframes the pandemic's gender gap not as a social artifact, but as something written into biology itself.
When the pandemic arrived, a pattern emerged that epidemiologists couldn't ignore: men were dying from COVID-19 at higher rates than women. The gap was consistent across countries and age groups. A team of researchers at the University of Alberta decided to look for the mechanism—the biological reason why the virus seemed to hit harder in male bodies.
What they found pointed to chromosomes. Women carry two X chromosomes; men carry one X and one Y. That difference, seemingly small, cascades through the immune system in ways that matter when a virus arrives. The culprit and the protector both live on the X chromosome: a gene called ACE2.
ACE2 is the enzyme that SARS-CoV-2 uses to break into human cells. It's the lock, and the virus has the key. But ACE2 does something else too. It guards against damage to the heart, lungs, and kidneys—the very organs that COVID-19 attacks most viciously. Men have one working copy of the ACE2 gene. Women have two. Because of how X-chromosome genetics work, one of a woman's X chromosomes gets switched off in most cells to prevent genetic duplication. But ACE2 escapes that silencing. It stays active on both copies. This means women's cells are churning out twice as much ACE2 protein as men's cells are—more protection, more defense.
The research team, led by senior author Gavin Oudit, published their findings in the American Journal of Physiology-Heart and Circulatory Physiology. They identified a second genetic advantage working in women's favor: a gene called Toll-like receptor seven, also located on the X chromosome, also escaping inactivation. This receptor is part of the innate immune system—the body's first-line defense against invaders. Women have roughly double the active copies. Oudit noted that this genetic difference explains something people have observed for years: women tend to tolerate viral infections better than men do. The common cold hits men harder. The phenomenon is real enough that it has a name: the man-cold.
What made the finding particularly striking was the occupational context. Healthcare workers are 70 percent female. These women face higher exposure to SARS-CoV-2 than the general population—they're in rooms with infected patients, handling specimens, breathing the same air. By raw exposure alone, you might expect them to get sicker. They don't. Their outcomes are better than men's, not worse. Oudit emphasized the significance: women face more risk due to the nature of their work, yet their biology appears to be protecting them. The genetic hand they were dealt—two copies where men have one—is doing the work.
The study doesn't erase the reality that women face other health disparities, or that gender shapes access to care and treatment. But it does explain one piece of the puzzle: why, across the world, men were ending up in intensive care units and dying at higher rates. The answer wasn't behavioral or social alone. It was written into their DNA.
Citações Notáveis
The highlight of our study is how the sex differences in COVID-19 are linked to ACE2— Gavin Oudit, University of Alberta
Due to gender issues, women face more risk, so it's reassuring to know that their outcomes are not any worse; in fact, they are clearly better than men's— Gavin Oudit
A Conversa do Hearth Outra perspectiva sobre a história
So women have two copies of this ACE2 gene and men have one. That seems like it should be obvious—why did it take a pandemic to notice?
Because ACE2 wasn't famous until the virus arrived. It was just another enzyme doing its job quietly. The pandemic made us look at it closely, and suddenly the math became visible.
And the X-chromosome inactivation thing—that's the part where one X gets turned off but ACE2 doesn't listen?
Right. It's an escape. Most genes on the second X chromosome get silenced to balance out the genetic load. But ACE2 ignores that signal. It keeps working on both copies. Women end up with double the protein.
Does that mean women are just better at fighting off viruses in general?
Not necessarily all viruses. But for respiratory infections especially, yes. The Toll-like receptor seven gene does the same thing—it escapes inactivation, doubles up in women. That's part of the innate immune system. It's why men tend to get hit harder by colds and flu.
The study mentions that 70 percent of healthcare workers are female. Isn't that a confounding variable?
It would be, if women were getting sicker. But they're not. They're exposed more, yet their outcomes are better. That's the point. The genetic protection is outweighing the occupational risk.
Does this mean men are just biologically worse at surviving COVID?
Not worse—different. They have one copy of ACE2 where women have two. That's a real disadvantage against this particular virus. But it's not a character flaw. It's genetics.
What happens next? Does this change how we treat men versus women?
That's the question researchers are asking now. If you understand the mechanism, maybe you can intervene. Maybe you can boost ACE2 in men, or find other ways to compensate. The study is a map. What we do with it is still being written.