A team of researchers used a mathematical simulation to explore what happens when vaccination rates for measles, rubella, polio, and diphtheria drop. Even with today's U.S. coverage, the model predicts as many as 850,000 measles infections and 2,500 deaths each year. If coverage falls by an additional 10 percent, annual measles cases could soar beyond 11 million.
Measles is exceptionally contagious—one infected person can transmit it to 12–18 others, each of whom can repeat that chain, far outpacing diseases like influenza or COVID-19. To interrupt transmission, 95% of the population must be immunized. Yet recent data show coverage slipping worldwide: in England, fewer than 84% of five-year-olds received both MMR doses in 2024.
Measles is more than a rash and fever. About 20% of children require hospital care, 5% develop pneumonia, and 0.1% suffer encephalitis, which can lead to seizures or deafness. Between 0.3% and 0.5% of infected children die.
Other vaccine-preventable diseases also loom large. Rubella infection during pregnancy can cause severe birth defects; polio can result in permanent paralysis; diphtheria carries a 30% mortality rate in unvaccinated kids. Before vaccines, these illnesses were endemic worldwide. Now, pockets of under-immunized communities risk reigniting widespread transmission. Already this year, the U.S. has recorded nearly 900 measles cases and three deaths.
The MMR vaccine provides over 97% protection after two doses. But infants under one year, pregnant women, and immunocompromised individuals cannot be vaccinated, relying on herd immunity to stay safe.
Several factors have driven down vaccination levels. The COVID-19 pandemic caused the steepest global drop in three decades; conflict and natural disasters—such as the recent Yemen outbreak of 10,000 measles cases—have disrupted routine immunizations. In addition, misinformation on social media, including the debunked claim linking MMR to autism, fuels hesitancy. A meta-analysis of 1.25 million children found no connection between the vaccine and autism.
The World Health Organization names vaccine hesitancy among its top ten global health threats. As this new research underscores, maintaining high immunization rates isn't optional—it's our best defense against preventable epidemics. A small decline in coverage is all it takes to turn the embers of near-eradication back into an out-of-control blaze.