Key takeaways:

• Nearly one-third of all measles cases reported in the U.S. since 2020 were reported in the first quarter of 2024.
• Measles vaccine coverage has remained below estimated population-level immunity for 3 years.

A sharp uptick in reported measles cases in the United States during the first quarter of 2024 has raised concerns that the country could lose its measles elimination status, CDC researchers said Thursday.

Although the risk for widespread measles transmission in the U.S. remains low because of high population immunity, a more than 17-fold increase in measles cases in the first 3 months of 2024 over the mean number of cases in the same time period from 2020 to 2023 is cause for concern, Adria D. Mathis, MSPH, and colleagues from the CDC’s Division of Viral Diseases wrote in a new study published in MMWR.

“We have seen measles on the rise globally over the past few years, so the uptick in cases is unfortunately not unexpected, but the current increase is certainly drastic,” Mathis told Healio.

Although travel-related outbreaks still occur, the U.S. was declared to have eliminated endemic transmission of measles in 2000. It would lose that status if an outbreak persists for more than a year.

“Because of the absence of endemic measles virus transmission for 12 consecutive months in the presence of a well-performing surveillance system, as of the end of 2023, measles elimination has been maintained in the United States,” Mathis and colleagues wrote. “However, the rapid increase in the number of reported measles cases during the first quarter of 2024 represents a renewed threat to elimination.”

Measles vaccination rates, like most childhood vaccination rates, fell during the COVID-19 pandemic. A record 40 million children worldwide missed their first or second dose of measles vaccine in 2021.

In the U.S., roughly 95% of children had received routine, state-required vaccines — including the MMR vaccine before the pandemic — but by the 2022-2023 school year, the vaccination rate had declined to 93%, remaining below pre-pandemic levels, while vaccine exemptions reached an all-time high.

The CDC in January issued an alert for health care providers to be on the lookout for measles amid the ongoing increase in cases.

For their study, Mathis and colleagues analyzed data on confirmed measles cases reported to the National Notifiable Disease Surveillance System or directly to the National Center for Immunization and Respiratory Diseases from Jan. 1, 2020, to March 28, 2024.

Overall, 338 confirmed measles cases with rash onset were reported during the study period in 30 U.S. jurisdictions. Ninety-six percent of cases were associated with importation and 61% occurred among U.S. residents who were eligible for vaccination but who were either unvaccinated or whose vaccination status was unknown.

In 2020, just 13 measles cases were reported — all but one before COVID-19 mitigation efforts started in March 2020. A majority of cases in 2021 and 2022 were related to outbreaks, according to the study.

Of 49 cases in 2021, 47 occurred among Afghan evacuees housed at U.S. military bases during Operation Allies Welcome. In 2022, 86 of 121 cases were linked to an outbreak in central Ohio.

There have already been 97 reported cases this year as of March 28 — nearly one-third of the total number of measles cases reported since January 2020, Mathis and colleagues reported. Nearly all of the cases have occurred among people who either were not vaccinated or whose vaccination status was unknown.

Mathis and colleagues attributed the surge to MMR vaccination coverage remaining below 95% — the mark that is widely recommended for measles elimination — for 3 straight years.

In addition to estimating that roughly 250,000 kindergartners have not received the MMR vaccine in the last 3 years, they noted that vaccination coverage rates in 12 states and the District of Columbia were less than 90% and that vaccine exemption rates among kindergartners exceeded 5% in 10 states during the 2022-2023 school year.

“Clinicians have a crucial role in helping to prevent measles and its complications, and maintain the U.S. measles elimination status,” Mathis said. “Their efforts are needed to help encourage routine vaccination and vaccination before international travel, identify communities at risk for measles transmission and rapidly diagnose and report measles cases.”

PERSPECTIVE

We know that measles outbreaks can occur in pockets where childhood immunization rates with the MMR vaccine begin falling below 90% to 95%. Unfortunately, we are starting to see such outbreaks with increasing frequency in parts of the U.S. where vaccine resistance is accelerating. In 2018, in PLoS Medicine, together with colleagues, we identified many such areas when we look at MMR or other childhood immunization rates at the county level — or even school district level — which is unfortunate given that such information turns out to be a fairly good predictor of where measles might strike next.

As vaccine hesitancy accelerated during the COVID-19 pandemic, especially in my state of Texas and neighboring states in the South, we saw thousands of people needlessly perish during the delta and BA.1 omicron waves of SARS-CoV-2 in 2021 and 2022. This happened because so many refused COVID-19 vaccines, even though they protected at high levels vs. death and severe illness. I reported this in my recent book, The Deadly Rise of Anti-Science. My worry now is that this rise in anti-vaccine sentiments spills over to all childhood or adult immunizations. This is why it becomes especially important to track and monitor it. I am concerned about a sharp return of measles and other childhood illnesses.