With five months still to go, the number of U.S. measles cases reported so far this year already triple that of all the cases seen in the country last year, federal health officials report.

A total of 188 cases have been confirmed in 26 states and Washington, D.C., new data released Friday by the U.S. Centers for Disease Control and Prevention show. At this point, 93 people have been hospitalized, most of them children under the age of 5. For comparison, there were only 58 cases reported in al of 2023.

This year's tally is the highest recorded since 2019, when the country nearly lost its measles elimination status. Most of the 1,200-plus cases confirmed that year were linked to outbreaks in Orthodox Jewish communities in New York.

“Having this year be even worse than 2019 — there’s definitely potential for that,” Dr. David Hamer, a professor of global health and medicine at the Boston University's School of Public Health, told NBC News.

However, CDC modeling suggests that’s not likely.

What is fueling the rise in cases this year? There have been 13 measles outbreaks, the largest of which raced through a migrant shelter in Chicago in March and spawned more than 60 cases.

But beyond that, experts point to vaccination rates in the United States that are declining even as measles cases are climbing worldwide.

Around 85% of the people who got measles this year were either unvaccinated or had an unknown vaccination status, according to the CDC. Many of the cases have been linked to international travel, meaning American travelers were infected in other countries.

“We live in a global community where vaccination rates everywhere affect diseases that are transmitted in the United States,” Dr. Erica Prochaska, a pediatric infectious diseases specialist at Johns Hopkins Children’s Center in Baltimore, told NBC News. “But the main issue is that in the United States, our population isn’t at the threshold of vaccination that we should be.”

What is that threshold? Because measles spreads so easily, public health officials recommend at least 95% vaccination coverage.

But only 93% of U.S. kindergartners had received two doses of the measles, mumps and rubella (MMR) vaccine for the 2022-23 school year,, down from 95% in 2019-20. And 12 states and Washington, D.C., had rates below 90%, according to the CDC.

“To me, what’s surprising is that the outbreaks aren’t more extensive,” given those statistics, Dr. Gregory Poland, founder and director of the Mayo Clinic’s Vaccine Research Group, told NBC News.

It wasn't that long ago that the United States had measles under control: In 2000, measles were effectively eliminated in this country.

Because of that, “it’s sort of dropped out of people’s minds as an important issue,” Hamer noted.

Unfortunately, vaccine hesitancy — exacerbated by vaccine misinformation that spread during the pandemic — has only made things worse, he added.

Hamer said lockdowns disrupted vaccination services in many low- and middle-income countries.

“The end result was that many countries that were sort of borderline in terms of having adequate coverage fell down on their coverage for routine childhood vaccinations, including measles,” Hamer explained.

The end result? From 2021 to 2022, measles cases increased 18% and measles deaths went up 43% worldwide, a joint report from the CDC and World Health Organization found.

Common symptoms of measles include a high fever, cough, conjunctivitis (pink eye), runny nose, white spots in the mouth and a rash that spreads from head to toe. Around 1 to 3 out of every 1,000 children infected with measles die from complications such as pneumonia or swelling of the brain, according to the CDC.