DEEP DIVE

New research from the University of Southern California, published in PLOS Biology, argues that men and women age immunologically on completely different timelines, and why almost no drug, vaccine, or treatment guideline currently accounts for it.

There is a strange fact buried in mortality statistics that most people have heard but rarely stopped to think about. Women live longer than men almost everywhere on Earth. And yet they spend more of those extra years sick. Researchers call this the morbidity-mortality paradox, and a recent essay from a team at the University of Southern California, led by Clayton Baker and senior author Bérénice Benayoun, argues this paradox has been sitting in plain sight for decades while medicine has largely failed to ask the obvious question behind it. Why would the sex that lives longer also be the sex that suffers more along the way?

The answer, the researchers argue, lives inside the immune system, and it is more interesting than a simple story about one sex being stronger than the other. Women's immune systems run hotter at baseline. They produce more cytokines, mount stronger antibody responses, and maintain more robust defenses against infection well into old age. This is part of why men are roughly twice as likely to develop severe COVID-19, and why men generally fare worse against infections across the board. But that same heightened vigilance comes with a cost. It is also why autoimmune diseases, conditions where the immune system turns on the body's own tissue, affect women far more often. Women make up roughly 90 percent of people with lupus and Sjögren's syndrome. The same intensity that protects against outside invaders becomes a liability once it starts attacking the self.

What makes this research compelling is that the difference isn't fixed. It shifts dramatically with age, and on two entirely different timelines depending on which body you're in. Women experience menopause, a hormonal cliff that typically arrives around 51 but can occur anywhere from a woman's twenties to her sixties. This isn't just a reproductive event. The essay describes real shifts in immune cell populations around this transition, nudging the immune system into a different mode almost overnight in biological terms. Men experience something far more gradual: testosterone declines roughly one percent a year starting around 30, a slow drift rather than a cliff, with immune consequences that unfold just as slowly.

That single distinction, a cliff versus a slope, explains a surprising amount of variation in health outcomes long chalked up to noise. Take sepsis. Women hospitalized with it tend to have shorter stays and lower mortality than men, but only after a specific age threshold in the mid-forties, suspiciously close to the average age of menopause. The protective edge women carry earlier in life seems to fade right around the moment their hormonal landscape shifts.

The same pattern surfaces again and again once you look for it. Vaccines work differently depending on age and sex, and not in any simple additive way. Younger people and women generally mount stronger antibody responses, but the gap between men and women actually widens with age for things like COVID-19 vaccination, before a booster closes it back up. Drug safety tells a more sobering version of the same story. The essay lists nine drugs pulled from the market or relabeled specifically for toxicity that disproportionately harmed women, including the sleep medication Ambien, which only received a lower recommended dose for women in 2013, more than two decades after approval. Across dozens of FDA-approved drugs reviewed, women showed higher blood concentrations and slower elimination for the majority, and body weight didn't explain the gap.

The researchers trace much of this blind spot to a well-intentioned decision in 1977, when the FDA, responding to the thalidomide tragedy, recommended excluding women of childbearing age from early clinical trials almost entirely. The policy aimed to protect women and potential pregnancies. Its side effect was decades of drug development built almost exclusively on male physiology, with the resulting dosing standards then applied to everyone.

What we can learn from this

The real takeaway here isn't one tip so much as a shift in how you read your own health information. Drug dosing guidelines and general health advice were, for a long time, built primarily on data from men, and this gap hasn't closed nearly as much as most people assume.

If you're a woman taking an older medication, it's reasonable to ask your doctor whether sex-specific dosing guidance exists, especially for sleep medications, heart medications, and anti-inflammatories like NSAIDs, where women face meaningfully higher rates of side effects at standard doses.

If you're approaching menopause, recognize that it's reshaping more than the symptoms usually discussed. It's genuinely altering how your immune system behaves, which is worth flagging to your doctor as relevant medical history, not just a personal milestone.

If you're a man, don't assume your slower decline carries no consequences. The data suggest aging men lean more heavily on a less precise arm of the immune system, which may help explain why men fare worse against new infections as they age, even without an obvious turning point to mark it.

And if you manage a chronic or autoimmune condition, treatment response can genuinely differ by sex and by the age treatment begins, in ways current guidelines often miss. That's not a reason to distrust your plan, but it's a reasonable basis for asking more questions about it.

This research, published in May 2026 in PLOS Biology, ultimately makes a case less about any single fact and more about a pattern worth carrying into your own healthcare decisions. The body you're aging in isn't a generic one, and the more specifically your care reflects that, the better your odds of getting treatment that actually fits you.

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The Vitality Club