What an Undervaccinated America Would Look Like

Becoming a public-health expert means learning how to envision humanity’s worst-case scenarios for infectious disease. For decades, though, no one in the U.S. has had to consider the full danger of some of history’s most devastating pathogens. Widespread vaccination has eliminated several diseases—among them, measles, polio, and rubella—from the country, and kept more than a dozen others under control. But in the past few years, as childhood-vaccination rates have dipped nationwide, some of infectious disease’s ugliest hypotheticals have started to seem once again plausible.

The new Trump administration has only made the outlook more tenuous. Should Robert F. Kennedy Jr., one of the nation’s most prominent anti-vaccine activists, be confirmed as the next secretary of Health and Human Services, for instance, his actions could make a future in which diseases resurge in America that much more likely. His new position would grant him substantial power over the FDA and the CDC, and he is reportedly weighing plans—including one to axe a key vaccine advisory committee—that could prompt health-care providers to offer fewer shots to kids, and inspire states to repeal mandates for immunizations in schools. (Kennedy’s press team did not respond to a request for comment.)

Kennedy’s goal, as he has said, is to offer people more choice, and many Americans likely would still enthusiastically seek out vaccines. Most Americans support childhood vaccination and vaccine requirements for schools; a KFF poll released today found, though, that even in the past year the proportion of parents who say they skipped or delayed shots for their children has risen, to one in six. The more individuals who choose to eschew vaccination, the closer those decisions would bring society’s collective defenses to cracking. The most visceral effects might not be obvious right away. For some viruses and bacteria to break through, the country’s immunization rates may need to slip quite a bit. But for others, the gap between no outbreak and outbreak is uncomfortably small. The dozen experts I spoke with for this story were confident in their pessimism about how rapidly epidemics might begin.

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Paul Offit, a pediatrician at Children’s Hospital of Philadelphia and co-inventor of one of the two rotavirus vaccines available in the U.S., needs only to look at his own family to see the potential consequences. His parents were born into the era of the deadly airway disease diphtheria; he himself had measles, mumps, rubella, and chickenpox, and risked contracting polio. Vaccination meant that his own kids didn’t have to deal with any of these diseases. But were immunization rates to fall too far, his children’s children very well could. Unlike past outbreaks, those future epidemics would sweep across a country that, having been free of these diseases for so long, is no longer equipped to fight them.

“Yeah,” Offit said when I asked him to paint a portrait of a less vaccinated United States. “Let’s go into the abyss.”

Should vaccination rates drop across the board, one of the first diseases to be resurrected would almost certainly be measles. Experts widely regard the viral illness, which spreads through the air, as the most infectious known pathogen. Before the measles vaccine became available in 1963, the virus struck an estimated 3 million to 4 million Americans each year, about 1,000 of whom would suffer serious swelling of the brain and roughly 400 to 500 of whom would die. Many survivors had permanent brain damage. Measles can also suppress the immune system for years, leaving people susceptible to other infections.

Vaccination was key to ridding the U.S. of measles, declared eliminated here in 2000. And very high rates of immunity—about 95 percent vaccine coverage, experts estimate—are necessary to keep the virus out. “Just a slight dip in that is enough to start spurring outbreaks,” Boghuma Kabisen Titanji, an infectious-disease physician at Emory University, told me. Which has been exactly the case. Measles outbreaks do still occur in American communities where vaccination rates are particularly low, and as more kids have missed their MMR shots in recent years, the virus has found those openings. The 16 measles outbreaks documented in the U.S. in 2024 made last year one of the country’s worst for measles since the turn of the millennium.

But for all measles’ speed, “I would place a bet on whooping cough being first,” Samuel Scarpino, an infectious-disease modeler at Northeastern University, told me. The bacterial disease can trigger months of coughing fits violent enough to fracture ribs. Its severest consequences include pneumonia, convulsions, and brain damage. Although slower to transmit than measles, it has never been eliminated from the U.S., so it’s poised for rampant spread. Chickenpox poses a similar problem. Although corralled by an effective vaccine in the 1990s, the highly contagious virus still percolates at low levels through the country. Plenty of today’s parents might still remember the itchy blisters it causes as a rite of passage, but the disease’s rarer complications can be as serious as sepsis, uncontrolled bleeding, and bacterial infections known as “flesh-eating disease.” And the disease is much more serious in older adults.

Those are only some of the diseases the U.S. could have to deal with. Kids who get all of the vaccines routinely recommended in childhood are protected against 16 diseases—each of which would have some probability of making a substantial comeback, should uptake keep faltering. Perhaps rubella would return, infecting pregnant women, whose children could be born blind or with heart defects. Maybe meningococcal disease, pneumococcal disease, or Haemophilus influenzae disease, each caused by bacteria commonly found in the airway, would skyrocket, and with them rates of meningitis and pneumonia. The typical ailments of childhood—day-care colds, strep throat, winter norovirus waves—would be joined by less familiar and often far more terrifying problems: the painful, swollen necks of mumps; the parching diarrhea of rotavirus; the convulsions of tetanus. For far too many of these illnesses, “the only protection we have,” Stanley Plotkin, a vaccine expert and one of the developers of the rubella vaccine, told me, “is a vaccine.”

Exactly how and when outbreaks of these various diseases could play out—if they do at all—is impossible to predict. Vaccination rates likely wouldn’t fall uniformly across geographies and demographics. They also wouldn’t decrease linearly, or even quickly. People might more readily refuse vaccines that were developed more recently and have been politicized (think HPV or COVID shots). And existing immunity could, for a time, still buffer against an infectious deluge, especially from pathogens that remain quite rare globally. Polio, for instance, would be harder than measles to reestablish in the United States: It was declared eliminated from the Americas in the 1990s, and remains endemic to only two countries. This could lead to a false impression that declining vaccination rates have little impact.

A drop in vaccination rates, after all, doesn’t guarantee an outbreak—a pathogen must first find a vulnerable population. This type of chance meeting could take years. Then again, infiltrations might not take long in a world interconnected by travel. The population of this country is also more susceptible to disease than it has been in past decades. Americans are, on average, older; obesity rates are at a historical high. The advent of organ transplants and cancer treatments has meant that a substantial sector of the population is immunocompromised; many other Americans are chronically ill. Some of these individuals don’t mount protective responses to vaccinations at all, which leaves them reliant on immunity in others to keep dangerous diseases at bay.

If various viruses and bacteria began to recirculate in earnest, the chance of falling ill would increase even for healthy, vaccinated adults. Vaccines don’t offer comprehensive or permanent protection, and the more pathogen around, the greater its chance of breaking through any one person’s defenses. Immunity against mumps and whooping cough is incomplete, and known to wane in the years after vaccination. And although immunity generated by the measles vaccine is generally thought to be quite durable, experts can’t say for certain how durable, Bill Hanage, an infectious-disease epidemiologist at Harvard’s School of Public Health, told me: The only true measure would be to watch the virus tear through a population that hasn’t dealt with it in decades.

Perhaps the most unsettling feature of a less vaccinated future, though, is how unprepared the U.S. is to confront a resurgence of pathogens. Most health-care providers in the country no longer have the practical knowledge to diagnose and treat diseases such as measles and polio, Kathryn Edwards, a pediatrician at Vanderbilt University, told me: They haven’t needed it. Many pediatricians have never even seen chickenpox outside of a textbook.

To catch up, health-care providers would need to familiarize themselves with signs and symptoms they may have seen only in old textbooks or in photographs. Hospitals would need to use diagnostic tests that haven’t been routine in years. Some of those tools might be woefully out of date, because pathogens have evolved; antibiotic resistance could also make certain bacterial infections more difficult to expunge than in decades prior. And some protocols may feel counterintuitive, Offit said: The ultra-contagiousness of measles could warrant kids with milder cases being kept out of health-care settings, and kids with Haemophilus influenzae might need to be transported to the hospital without an ambulance, to minimize the chances that the stress and cacophony would trigger a potentially lethal spasm.

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The learning curve would be steep, Titanji said, stymieing care for the sick. The pediatric workforce, already shrinking, might struggle to meet the onslaught, leaving kids—the most likely victims of future outbreaks—particularly susceptible, Sallie Permar, the chief pediatrician at NewYork–Presbyterian/Weill Cornell Medical Center, told me. If already overstretched health-care workers were further burdened, they’d be more likely to miss infections early on, making those cases more difficult to treat. And if epidemiologists had to keep tabs on more pathogens, they’d have less capacity to track any single infectious disease, making it easier for one to silently spread.

The larger outbreaks grow, the more difficult they are to contain. Eventually, measles could once again become endemic in the U.S. Polio could soon follow suit, imperiling the fight to eradicate the disease globally, Virginia Pitzer, an infectious-disease epidemiologist at Yale, told me. In a dire scenario—the deepest depths of the abyss—average lifespans in the U.S. could decline, as older people more often fall sick, and more children under 5 die. Rebottling many of these diseases would be a monumental task. Measles was brought to heel in the U.S. only by decades of near-comprehensive vaccination; re-eliminating it from the country would require the same. But the job this time would be different, and arguably harder—not merely coaxing people into accepting a new vaccine, but persuading them to take one that they’ve opted out of.

That future is by no means guaranteed—especially if Americans recall what is at stake. Many people in this country are too young to remember the cost these diseases exacted. But Edwards, who has been a pediatrician for 50 years, is not. As a young girl, she watched a childhood acquaintance be disabled by polio. She still vividly recalls patients she lost to meningitis decades ago. The later stages of her career have involved fewer spinal taps, fewer amputations. Because of vaccines, the job of caring for children, nowadays, simply involves far less death.