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Back in 2000, health experts declared measles had been eliminated from the United States thanks to an ambitious measles-mumps-rubella (MMR) vaccination program for children. But as vaccination rates decline, measles is making a troubling comeback. As of Friday, the CDC says that nearly 900 measles cases have been reported in at least 29 states this year.

Now, a study from Stanford suggests measles could become endemic again (meaning bigger than an outbreak but not quite as big as a pandemic).

Researchers used epidemiological modeling (the use of health data to predict how a disease will behave) to simulate the spread of infectious diseases in the United States at various childhood vaccination levels.

The results, published Thursday, are drawing lots of attention.

Here’s what you need to know.

Measles Could Become Endemic in the US Within Two Decades

Right now, the MMR vaccination rate is around 93%. If that rate were cut in half, the US could expect 51.2 million cases of measles over the next 25 years, the study predicted — with the disease becoming endemic. Even a small drop in vaccination rates would be dangerous, too. If rates fell by just 10%, measles cases would jump to 11.1 million over the next 25 years, the study found.

Why it matters: That could result in 10.3 million hospitalizations and 159,200 deaths, plus an estimated 51,200 children with post-measles nerve-related problems, the study authors said.

The reason is because measles is so contagious — one of the most infectious diseases in the world. Estimates show that someone with measles will infect 12-18 others, on average. (By comparison, someone with COVID-19 can infect about three people, on average.)

It Wouldn’t Take Much to Prevent That Rise

Even a small increase in the vaccination rate would greatly reduce the endemic risk, the authors say. If the number of people receiving the MMR vaccine went up just 5%, there would only be 5800 cases over 25 years.

Why it matters: Every vaccinated person counts. “It’s empowering that a small segment of the population can make a difference here,” senior study author Nathan Lo, MD, PhD, an assistant professor of infectious diseases at Stanford University, said in a news release.

Knowing Your Vaccination Status Is Crucial

Children should get two doses of MMR vaccine, starting with the first dose at 12-15 months of age, and the second dose at 4 through 6 years of age. (But if you live near a measles outbreak, your baby may be able to get vaccinated as early as 6 months.)

Some adults may need to get vaccinated too. You’re generally considered fully vaccinated if you’ve received two doses of MMR or MMRV. If you were born before 1957, you’re presumed to be immune due to widespread measles exposure during that time. If you received the inactivated measles vaccine between 1963 and 1967, you may need a booster, so consult your healthcare provider.

Why it matters: Being informed helps protect not just your own health, but public health, too.

“I would encourage parents who aren’t sure about vaccination to discuss this with their pediatrician and believe in our healthcare providers,” Lo said. “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.”

Sources:

CDC: “Measles Cases and Outbreaks.”

The Journal of the American Medical Association: “Modeling Reemergence of Vaccine-Eliminated Infectious Diseases Under Declining Vaccination in the US.”

Stanford University: “Measles may be making a comeback in the U.S., Stanford Medicine-led research finds.”