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As measles outbreaks increase, CDC sounds the alarm

By Elaine K. Howley
April 3, 2024
Young girl receiving vaccination from provider

2024 has so far seen measles outbreaks in 18 states. Here’s what public health officials are saying and why vaccination is critical.

On March 28, the Centers for Disease Control and Prevention (CDC) published new data showing that more cases of measles had been confirmed in the first three months of 2024 in the United States than in all of 2023. The CDC also recently tweaked its guidance on travel to the 49 countries around the globe that are now facing large measles outbreaks, advising travelers to check with their health care providers at least six weeks before visiting places where measles outbreaks are active and ongoing.

It’s a far cry from 2000, when the World Health Organization officially declared measles had been eradicated in the United States.

“While the exact number of outbreaks varies, reports indicate a significant increase compared to previous years,” says Christi Wojewoda, MD, FCAP, director of clinical microbiology at the University of Vermont Medical Center and professor at the Robert Larner, M.D., College of Medicine at the University of Vermont.

Declining immunization rates, both at home and abroad, have resulted in increasing numbers of measles cases, says Brian Labus, PhD, MPH, assistant professor in the University of Nevada Las Vegas School of Public Health.

Wojewoda, who also chairs the College of American Pathologists’ microbiology committee, notes the recent outbreaks have been observed in many parts of the country, including areas that have not experienced measles outbreaks in recent memory. “This spread to new regions is particularly alarming as it suggests potential gaps in vaccination coverage,” she says.

Origin and Spread

The vast majority of the cases recorded in the United States in 2024 have been among international travelers or those exposed to people who have traveled internationally, says Shruti Gohil, MD, MPH, associate medical director, epidemiology and infection prevention, at UCI Health and assistant professor of infectious diseases at University of California, Irvine, School of Medicine in Southern California.

Although the United States succeeded in eradicating measles, many other countries have not. Kazakhstan is currently experiencing a large measles outbreak, with 21,740 cases, while Azerbaijan, Yemen, and India have each recorded more than 13,000 cases.

As of March 28, 97 cases of measles had been confirmed in 18 U.S. states, with Illinois and Florida leading the way.

Chicago is the site of the largest outbreak to date. As of April 1, 53 cases had been reported in Chicago, including in 10 residents at a migrant shelter.

In Florida, at least 11 cases have been reported with 9 of those stemming from an outbreak at an elementary school in Broward County. Federal officials also have confirmed measles cases in three other states that are linked to visits to Florida.  

Because measles is so highly contagious, public health officials fully expect to see additional cases emerge in the coming weeks, says Tammy Lundstrom, MD, JD, an infectious disease specialist and chief medical officer for Trinity Health, a large health system that operates 101 hospitals including Trinity Health Oakland, a teaching hospital in Pontiac, Michigan.

“90% of unvaccinated people who are exposed [are] likely to catch it,” says George Rust, MD, MPH, FAAFP, FACPM, professor in the Department of Behavioral Sciences and Social Sciences and director of the Center for Medicine and Public Health at Florida State University College of Medicine in Tallahassee. “If you’re in the same room within two hours of when a measles patient was coughing in that room, you’re likely exposed.”

An infected person also can be contagious for up to four days before the recognizable measles rash shows up, and by then, they may have exposed dozens or even hundreds of people to the virus.

“Frequently, people will have a fever and not feel good during those first four days,” says Ashley Drews, MD, an infectious disease specialist with Houston Methodist in Texas, “but you’ll be unlikely to suspect measles” until the rash shows up.

Even then, it may not be identified properly as many health care workers have never before encountered measles in the clinical setting.

“A lot of doctors, residents, and medical students may have read about it or attended a lecture but still haven’t seen it up close,” says David Nguyen, MD, a pediatric infectious disease specialist with Rush University Medical Center and associate professor at Rush Medical College in Chicago.

What’s more, many medical textbooks skew toward showing the measles rash on Caucasian or fair skin, rather than darker complexioned individuals. “Measles rash looks different on dark skin, but we’re seeing so many African American, Latino, and other darker-skinned patients here,” he says.

If the rash isn’t identified and the person returns to the community, that can launch an outbreak.

It’s worth noting that an outbreak can involve “as few as three or four cases,” Rust says. The key to defining an outbreak is that infections have risen “above the baseline rate (usually zero) and involve local person-to-person transmission.”

Immunization Rates Declining

Measles is still endemic in many other parts of the world, and as world populations shift due to economic, environmental, and social reasons, that can bring the virus into places where it previously had not been circulating. “Internationally, the major reason for low vaccination where we are seeing outbreaks is lack of health care access,” Gohil says. “In the U.S., lower vaccination rates seem more commonly due to anti-vaccine sentiment coupled with pandemic-associated disruptions in vaccine coverage.”

For the 2022-2023 school year, the CDC reported national vaccination coverage for measles, mumps, and rubella had dropped to 93.1%, down from the target level of 95%, last seen during the 2019-2020 school year.

Immunization levels have slipped due to many factors, including because many people lost access to health care or were delayed in receiving health care during the worst of the COVID-19 pandemic. The CDC estimates that more than 61 million doses of measles vaccines were postponed or missed from 2020 to 2022 due to COVID-19 related delays. “Some kids and even some adults may have missed some of their important vaccines, including the measles vaccine,” Nguyen says.

Mistrust of vaccines is also a big contributor. “We’ve had people against vaccines since they were first developed,” Labus says, but it’s more prevalent now because “it’s easier for people who have the same views to find common places to discuss this than in the past.”

Labus adds that where unvaccinated folks tend to cluster also plays a role. “If all the people who are against vaccines are in the same school, even if the community rate is 95%, that place is going to be at much higher risk for an outbreak.”

He uses an analogy to help paint the picture: “It’s like if you’re cleaning up the brush from around your house to prevent fires. If you stack all the brush in one place, sure, you’ve removed the risk from where it was before, but you’ve created a bigger hazard in another place. So, 95% is meaningful if that 95% is equally distributed. But we know that’s not the case.”

Sporadic outbreaks can occur when “immunization levels drop even a little bit,” he notes.

Curbing This and Future Outbreaks

While this outbreak is concerning, there are public health and clinical tools and strategies that can help bring it to heel, and quickly, including:

  • Vaccinating everyone who’s eligible.
  • Identifying and isolating new cases.
  • Communicating the facts.
  • Traveling mindfully.
  • Taking a community-minded approach to personal responsibility.


The good news is there’s a safe and highly effective vaccine readily available. “Vaccinated people are 97% protected, 93% after the first dose,” Rust says.

If the COVID-19 pandemic disrupted patients’ routine immunizations, Drews urges vaccination without focusing on the lapse in care. “Really, no judgment. Just let them come and get caught up where they need to be and safe.”

Those who cannot take the vaccines — infants under the age of 6 months and certain immunocompromised individuals — should isolate if they reside in an area with an active measles outbreak, Drews says.

Identifying and Isolating New Cases

Diagnosing and quarantining new cases is also important for curbing a potential outbreak. Many hospitals and public health agencies have invested a lot of time tracing contacts of known measles cases to identify and isolate those who may have been exposed.

“Quarantining the susceptible exposed individuals is crucial, because if they get measles, they can be contagious for up to four days before they show any symptoms,” Rust says.

He notes that this is currently happening in Chicago. “Chicago Department of Public Health is currently modeling an effective response, having now vaccinated every single person who is vaccine-eligible who might have been exposed at a temporary housing shelter.”

Masking can also help curb onward transmission, Drews says. When someone with a diagnosed case of measles must leave the house, such as going to a doctor’s appointment, they should wear a mask.

Communicating the Facts

Communication around the outbreak is critical so clinicians know what’s happening. Gohil notes that the CDC, many state health departments, and local public health groups “have been sending regular communications to alert health care professionals about the outbreaks and how to recognize measles.”

Efforts to help clinicians understand when and how to test for measles is also important, Wojewoda says.

Many public health departments around the country have launched revived campaigns to encourage vaccination and are staging vaccine clinics in high-risk communities. All of these efforts can help slow the spread.

Traveling Mindfully

While immunity from the measles vaccine is thought to endure for life, getting a measles booster if you’re headed to an international location where the virus is active and spreading can be beneficial, Drews says.

“Travel medicine clinics are very in tune to where measles is active and can make appropriate recommendations. And when in doubt, it does not hurt to get an extra booster. It’s not going to hurt you.”

She recommends scheduling that appointment at least six weeks prior to your trip to ensure your body has enough time to build immunity.

Taking a Community Mindset to Personal Responsibility

Lastly, curbing the virus’s onward transmission involves personal decision-making with the community in mind. The vast majority of individuals must make the responsible choice to vaccinate themselves and their children on schedule. 

“Herd immunity protects us all. That’s why getting the vaccine is not just about my benefit and my risk but is a ‘we-all’ practice that protects my neighbors,” Rust explains.  

Emphasizing the safety and effectiveness of vaccines while addressing misinformation head-on is critical, Wojewoda adds. “Fostering a culture that values and prioritizes vaccination as a cornerstone of public health is crucial in preventing future outbreaks.”