Measles could become endemic again in the United States

Childhood vaccination rates have been falling in the United States, especially since the start of the COVID-19 pandemic. Lower levels of immunity have resulted in a resurgence of measles cases, including a recent outbreak in western Texas that infected more than 620 people, leading to 64 hospitalizations and the deaths of two children.

If immunization rates drop further over a prolonged period of time, measles and even other wiped-out diseases - such as rubella and polio - could one day make a comeback in the United States, according to a new study by researchers at Stanford Medicine and other universities.

The study, which was published in the Journal of the American Medical Association on April 24, used large-scale epidemiological modeling to simulate the spread of infectious diseases in the United States at various childhood vaccination levels. Even at current immunization rates, researchers predict that measles may become endemic again - circulating in the U.S. - within two decades; with small declines in vaccination, this could happen more quickly. However, small increases in vaccine coverage would prevent this.

Lead author Mathew Kiang, ScD, assistant professor of epidemiology and population health, and senior author Nathan Lo, MD, PhD, assistant professor of infectious diseases, hope the study will provide useful data for decision makers setting vaccine policy.

We spoke with Kiang and Lo to learn more. This interview has been edited for clarity and length.

Why did you think this research was important?

Nathan Lo: We've seen a worrisome pattern of decreasing routine childhood vaccinations. There was a disruption to health care services during the pandemic, but declines preceded this period and have accelerated since then for many reasons. People look around and say, "We don't see these diseases. Why should we vaccinate against them?" There's a general fatigue with vaccines. And there's distrust and misinformation about vaccine effectiveness and safety.

Mathew Kiang: As vaccinations decline, the effect won't be immediate. We wanted to know: When will we see the impact of decisions being debated and made now?

Lo: Specifically, we wanted to look at some key diseases that have been eliminated from the U.S. through vaccination, which means they're not spreading within the country on an ongoing basis. These include measles, polio, rubella and diphtheria, which can have awful complications, like lifelong paralysis, birth defects and death.

How did you model what would happen?

Lo: We used a large-scale epidemiological model to simulate all individuals living in the U.S. and assigned them an age, vaccination status, immunity, state of residence, etc. We then simulated how infections would spread under different vaccine conditions. The model assumes that diseases are introduced by someone who travels abroad and brings them back, most commonly a U.S. citizen who isn't vaccinated. Right now, so many people are immune through vaccination that diseases don't spread far. But if vaccinations decline over a longer period, you start to see outbreaks increase in size and frequency. Eventually you see sustained, ongoing transmission, meaning these diseases become endemic - they become household names once again.

What is likely to happen with these diseases if vaccinations stay at current levels?

Kiang: For current rates, we took a conservative approach by using average vaccination levels between 2004 and 2023. With measles, we found that we're already on the precipice of disaster. If vaccination rates remain the same, the model predicts that measles may become endemic within about 20 years. That means an estimated 851,300 cases over 25 years, leading to 170,200 hospitalizations and 2,550 deaths. The other diseases are not likely to become endemic under the status quo.

Why is measles likely to become endemic and not the others?

Lo: Measles is one of the most infectious diseases that exists, so the number of people who have to be immune to prevent it from spreading is extremely high. Polio, diphtheria and rubella are still far more infectious than COVID-19, for example, but measles is in a different ballpark, with one person infecting up to 20 others (though our model took a conservative estimate of 12) in a fully susceptible population. Also, the MMR (measles, mumps and rubella) vaccine has become particularly controversial, partly due to a history of fraudulent medical research that raised safety concerns; it has been conclusively shown that there is no link with autism. Measles is also more common around the world, so travelers are more likely to bring it back.

Kiang: Travelers importing a disease are like matches, and U.S. under-vaccination is the tinder. With measles, you're throwing a lot of matches in, and eventually something is going to happen.

What do you predict will happen if fewer people get their kids vaccinated?

Kiang: If vaccination were to fall by even 10% today, measles cases would skyrocket to 11.1 million over the next 25 years. If vaccination rates were cut in half, we'd expect 51.2 million cases of measles, 9.9 million cases of rubella, 4.3 million cases of polio and 200 cases of diphtheria over 25 years. This would lead to 10.3 million hospitalizations and 159,200 deaths, plus an estimated 51,200 children with post-measles neurological complications, 10,700 cases of birth defects due to rubella and 5,400 people paralyzed from polio. Measles would become endemic in less than five years, and rubella would become endemic in less than 20. Under these conditions, polio became endemic in about half of simulations in around 20 years.

What differences did you find at the state level?

Kiang: Massachusetts has high vaccination rates and was consistently low risk. Both California and Texas were higher risk, even after accounting for larger population size, because vaccination rates in both have dropped and there's a lot of travel to those states. Our model assumed there was no spillover of infections across state lines, so the numbers could be an underestimate.

If these diseases become more widespread, who is in danger?

Lo: Folks who are unvaccinated are most at risk of infection and the awful complications. That includes babies, especially those between the ages of 6 and 12 months, who have waning antibodies from their mothers but are not yet eligible for their first dose of an MMR vaccine. Others are also at risk. People who are immunocompromised, which is a sizable segment of the U.S. population, can also be at risk. While the effects of declining vaccination won't be immediate, we could eventually see the return of awful complications from diseases that most clinicians today have not encountered thanks to decades of successful immunization.

Which of your scenarios is most likely to play out?

Lo: One that's conceivable is that vaccine coverage continues to drop, measles outbreaks become larger and more frequent, and eventually measles becomes endemic again. Hopefully, some fraction of the unvaccinated population seeks vaccination as a result, and state and local public health departments continue to do their vital work in outbreak response, allowing us to reestablish elimination. We might get into that kind of pattern. If we start to see major changes to the childhood vaccination schedule and policy and coverage really drops, you get into a world where you worry about diseases like polio and rubella, but that would likely take well over a decade or more. If that were to happen, you can't just flip a switch - once these diseases get unleashed, it would take time eliminate them again.

What should parents, providers and others do based on these results?

Lo: I would encourage parents who aren't sure about vaccination to discuss this with their pediatrician and believe in our health care providers. We hope our research also provides the data for federal and state officials, vaccine guideline committees, and others to understand what will happen if decisions are made that lead to declines in vaccination.

With measles, we're right on the cusp. Increasing vaccination levels by just 5% brings the number of measles cases down, safely away from returning to endemic levels. These are the kinds of small percentages that can really be a tipping point. It's empowering that a small segment of the population can make a difference here.

Kiang: It's worth emphasizing that there really shouldn't be any cases at this point, because these diseases are preventable. Anything above zero is tragic. When you're talking about potentially thousands or millions, that's unfathomable.

Researchers from Baylor College of Medicine, Rice University, and Texas A&M University contributed to the study.

A grant from the National Institutes of Health (T32AI007502) supported the research.