U.S. health officials are putting all they have into extinguishing measles outbreaks, many of them raging in cities throughout the country.

The reality, though, is that there is a growing amount of tinder afoot, a fact that will make it increasing difficult to battle these blazes, experts fear.

In recent years, the percentage of children who have received one or more doses of measles-containing vaccine has remained relatively stable, according to the Centers for Disease Control and Prevention. But that stability masks the fact that, over the past few decades the overall size of the population that is either unvaccinated or undervaccinated has been growing.

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With each cohort of kids born to parents who distrust or fear vaccines, the number of people susceptible to the measles virus expands. At the same time, there is a growing population of adults who were children in the early days of measles vaccination whose immunity may have worn off. (During their youth, only one dose of vaccination was recommended, instead of the standard two-dose treatment today.)

Introduce the spark of the highly contagious measles virus into these populations and the tinder will be set ablaze.

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A combination of factors makes this virus a veritable infecting machine. Here’s how it works. Alex Hogan/STAT

“I’m very concerned where this is all heading,” said Dr. Peter Hotez, professor of pediatrics and dean of the National School of Tropical Medicine at Baylor College of Medicine, who describes this year’s measles activity as “a wakeup call.”

“At what point do all of these pockets of measles transmission start becoming confluent? I think that’s the big worry,” he said.

On Monday the CDC announced that the number of cases so far this year has topped 700, the highest rate in a quarter century. With the year only a third of the way through and measles spreading in a number of outbreaks across the country, this year’s total could well top the 1994’s 963 cases.

The United States is hardly the only country struggling with a resurgence of measles. More than 112,000 measles cases were recorded globally in the first three months of the year, with major epidemics in countries including Ukraine, Israel, and the Philippines.

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But when the U.N. Children’s Emergency Fund recently surveyed 10 high-income countries, the United States topped the list of nations in terms of percentage of children who were not vaccinated against measles from 2010 to 2017, with an estimated 2.6 million children who didn’t receive their first dose of measles vaccine during that period.

(It takes two doses to achieve maximum protection. The first is given around the time of a child’s first birthday, the second between the ages of 4 and 6.)

The vaccine is one of the most effective available, protecting 97% of people who get both doses. But that means 3% of fully vaccinated people — people who have every reason to believe they are protected — remain susceptible to the virus. Through no fault of their own, they are part of the accumulating tinder.

Dr. Saad Omer has been worrying about the growing number of measles-susceptible people in this country for a while. In 2016 he and colleagues from Emory University published a study estimating the number of children and teenagers who are unprotected against measles.

Using data from 2008 to 2013, they calculated that 12.5% of children and adolescents were not protected against measles, with the biggest portion being children 3 and younger. At the time they submitted their study for publication, in June 2015, they estimated that 8.7 million children aged 17 and younger were vulnerable to the virus.

Omer’s study estimated that in 2013 alone, 4.5% of teens aged 13 to 17 had not received a single dose of measles-containing vaccine, years after they should have received both doses. That’s more than 940,000 children who are now young adults.

The authors were concerned their study would appear alarmist, so they “cushioned” their message a bit,” Omer admitted. But to them, the future the data foretold was clear.

“We predicted that if we don’t change the trajectory we’ll start seeing larger and more frequent outbreaks. And this is exactly what we are seeing,” he said.

Nationally, the percentage of children who received one or more doses of measles-containing vaccine remained pretty stable between 2013 and 2017, hovering between 91.1% and 91.9%. Still, that’s near the lower end of the vaccination rates — between 90% and 95% — needed to maintain herd immunity, the phenomenon where enough people in a population are immune to the pathogen that it cannot effectively spread within it.

Omer’s study calculated that even a 2% drop in immunization rates — taking the national rate down to 90% — would add another 1.2 million children to their estimate, bringing the percentage of children 17 years and younger who aren’t protected against measles to 14.2%.

And the national figures hide a key fact. The people vulnerable to measles are not evenly dispersed across the population. Instead, they tend to cluster, creating pockets where rates of protection are far lower than the national average. That was the case in an outbreak in Washington state that was recently declared over. That’s the case in the ongoing outbreaks in Brooklyn and in Rockland County, north of Manhattan, where transmission is largely occurring among unvaccinated families of Orthodox Jews.

Some of the clusters are among people who share religious beliefs, but other groups are harder to define. California, for instance, has seen low vaccination rates among children in some affluent, highly educated families, said Dr. Karen Smith, director of the California Department of Public Health.

“Overwhelmingly the schools that have very low vaccination rates tend to be the Waldorf schools, the Montessori schools, but also then a number of charter schools. Most of those are small schools that cater to a very specific community,” she said.

Hotez, who has done research mapping out where rates of non-vaccinated children are highest, finds it frustrating to hear the overall national figures cited as a reason for comfort.

“That’s a really blunt instrument,” he said. “It really obscures the pockets where large numbers of kids are not getting vaccinated.”

This fact of clustering increases the stress the growing number of susceptible people put on herd immunity, Omer said.

“It’s all a game of probability — the likelihood that someone travels and brings back the infection, then there’s a higher level of probability to start these outbreaks the lower the overall protection level goes down,” he said.

Hotez, who like Omer has seen the trend towards increasing susceptibility, has been predicting rising measles cases may have silver lining: They may finally persuade vaccine-hesitant parents that the threat measles poses to their children is real and vaccination rates may rise again. He’s dismayed it hasn’t happened yet.

“It’s unfortunate that even with 700 measles cases in the U.S. that’s still not good enough for an auto-correction,” he said.