In 2000, after a decadeslong public health battle and a Herculean vaccination program, the US won a coveted status: measles elimination. The designation means that the extremely infectious measles virus is no longer endemic in the US—defined as continuous transmission in the country over 12 or more months while in the presence of an effective disease monitoring system. The country went from having 3 to 4 million children fall ill with the severe infection each year, to tallying just dozens of mostly travel-linked cases.

But in an alarming turn, the country's elimination status is now at risk. Measles cases in the first quarter of 2024 have increased more than 17-fold over the cases seen in the first quarters of 2000 to 2023. Measles vaccination rates among kindergarteners have slipped in that time, too, with vaccination coverage in the last three consecutive years below the 95 percent target that is needed to prevent sustained transmission. Outside the US, measles cases are exploding in the wake of pandemic-related disruptions to routine childhood vaccination programs. Altogether, the conditions are prime for measles to regain its foothold in the country—and the Centers for Disease Control and Prevention is clearly anxious.

"The rapid increase in the number of reported measles cases during the first quarter of 2024 represents a renewed threat to elimination," CDC researchers write in a new analysis of the country's measles cases and surveillance system. The analysis was published Thursday in the CDC's Morbidity and Mortality Weekly Report.

The analysis takes a deep look at the country's current and recent measles cases: how they got here, how and why they spread once here, and how good our public health systems were at tracking them and breaking the chains of transmission. The good news is that our surveillance systems appear to be working well. "A variety of transmission chain sizes were detected, including isolated cases, suggesting that sustained measles transmission would be rapidly detected," the CDC notes.

But there are many reasons to worry that that is not enough. In 2019, the country nearly lost its elimination status amid two prolonged outbreaks in New York and New York City. While the COVID-19 crisis kept measles at bay since then, the conditions for a resurgence have only worsened.

How it could happen

If the US loses its status, the new analysis paints a clear picture of how it's likely to happen: An unvaccinated US resident will pick up an infection while abroad and bring it home to an under-vaccinated community. Unvaccinated but vaccine-eligible children in that community contract the virus and create a chain of sustained transmission.

The CDC noted 338 measles cases between January 1, 2020, and March 28, 2024. The median age of the cases was 3 years old. Of the 338 cases, 97 were in 2024. (The year's current tally now stands at 113.) Among the 338 cases, 326 (96 percent) were linked to an importation (12 cases—4 percent—had an unknown source). The 326 include 93 cases that were directly imported into the country, of which 59 (63 percent) were in US residents. Of those 59, 53 (90 percent) were vaccine-eligible but unvaccinated or had an unknown vaccination status.

Of the 338 total cases, 309 cases (91 percent) were either unvaccinated (68 percent) or had an unknown vaccination status (23 percent). Of the 309 cases, 259 (84 percent) were vaccine eligible, with the remainder being in babies 6 to 11 months who are too young for routine vaccination (13 percent) or babies less than 6 months who are too young for vaccination altogether (3 percent).

In the past years, the most common places from which measles importations originated were in the Eastern Mediterranean and African regions. But, so far in 2024, six of 16 importations (37.5 percent) came from Europe (three) and East Asia (three), which represents a 50 percent increase in importations from these areas in the past three years. In other words, measles is increasing globally, increasing the risk of importation to the US.

And as such: "The US measles elimination status will continue to be threatened by global increases in measles incidence and decreases in global, national, and local measles vaccination coverage," the CDC concludes. Overall, vaccination is still high enough in the US to prevent large-scale outbreaks. But to keep measles from becoming endemic once more, the US must improve measles vaccination rates, encourage vaccination before international travel, identify vulnerable communities that are under-vaccinated, and swiftly investigate suspected measles cases, the CDC researchers advise.