The U.S. has now tallied at least 59 confirmed or suspected measles cases investigated so far this year by authorities in 17 states — more than the number of cases reported nationwide in all of 2023. It comes as health officials are grappling with multiple major outbreaks of the highly contagious virus around the world. 

Now with spring break travel looming, health officials have ramped up pleas for Americans to double check whether they are up to date on the highly effective vaccines used to protect against measles.

"This is why it's urgent for us to sort of address vaccination so that we can really decelerate sort of the march of measles, given the global scenario as well as what we're seeing with some vaccination rates in the country," said Dr. Demetre Daskalakis, director of the CDC's National Center for Immunization and Respiratory Diseases. 

Friday's tally of measles cases is up from 45 counted by the CDC last week. Additional infections have since been announced in Arizona, California, Illinois and Ohio. 

While infections have climbed, Daskalakis said counts still remain small enough to make it difficult for officials to navigate privacy concerns in releasing additional demographic trends of cases so far. Investigations are also still ongoing to collect that information.

Daskalakis said the majority of cases so far have been linked to unvaccinated Americans returning to the U.S. with the virus.

"Measles is a easily preventable disease with a readily available and safe vaccine. And so, as we are seeing more cases in the U.S., we have the technology and the ability to be able to prevent measles," he said.

The CDC updated its guidance Wednesday to counsel Americans who are unsure if they're up to date on their shots to seek out a doctor at least six weeks before their trip.

Here's what we know about the outbreaks so far this year.

Which states have reported the largest measles outbreaks in 2024? 

So far this year, 17 states – Arizona, California, Florida, Georgia, Illinois, Indiana, Louisiana, Maryland, Michigan, Minnesota, Missouri, New Jersey, New York, Ohio, Pennsylvania, Virginia, and Washington – have reported at least one case.

Illinois has reported the most measles cases in recent weeks, with 12 total infections reported in Chicago. Of them, 10 were residents at a migrant shelter. Two cases are not related to the shelter outbreak, a spokesperson for the city's health department said.

The CDC deployed a team to Chicago on March 12 to work with the city to investigate and curb the spread of the virus, and support the city's vaccination campaign.

Daskalakis praised the Chicago team responding to the cases. The city was the first to invite CDC to help in a measles outbreak response so far this year. 

He said the agency would likely gather lessons learned from Chicago's response to help other cities too, citing his previous experience as a top-ranking health official in New York City during past large outbreaks there.

"I could see the lessons from the New York experience being built into this response. So the answer is yes, we always iterate and I'm sure we'll learn some things from Chicago that will be valuable in other settings as well," Daskalakis said.

Florida has reported the next most cases, with 10 infections so far this year. All but one of the cases was reported in Broward County, after a Miami-area elementary school reported an outbreak. 

CDC laboratories were tapped to help in investigating the genotype of the virus behind that outbreak, which can aid in narrowing down leads for the outbreak's origin.

Most of the cases nationwide have been confirmed through laboratory testing. Two cases remain suspected, in Ohio's Clermont County and Arizona's Coconino County. 

Why are measles cases on the rise in 2024?

Most outbreaks this year in the U.S. have been blamed on unvaccinated travelers bringing the virus back with them and exposing others who don't have immunity. 

Measles is extremely contagious, and a person who has the infection can spread it for four days before developing symptoms. Symptoms often do not appear until 11 days after exposure.

Thousands of U.S. schools at risk of measles outbreak, data shows 02:18

At least six cases this year were in people who were first exposed to the virus abroad, according to CDC data updated Thursday. Last year ended with 13 of these "imported" cases reported, as of the agency's preliminary tally from the end of December.

Parents may not be aware that babies are recommended to get a vaccine for measles before international travel as early as six months old, officials have said, while older travelers may not realize that their destinations are facing outbreaks.

Cases have been on the rise around the world, blamed on a wide gap in immunity in many countries resulting from missed vaccinations during the COVID-19 pandemic. 

Records obtained by CBS News through a Freedom of Information Act request show the CDC had launched investigations of measles exposures through Feb. 20 in arriving international flights that departed from Saudi Arabia, Egypt and Turkey. 

Health officials in Georgia and California have also confirmed at least one of their cases this year was linked to travel through the Middle East. Other outbreaks in recent months have also been linked to travel in other parts of the world like Europe, which has also been facing a resurgence of infections.

"These fires are popping up all across the world, really creating the sparks that can kindle these small outbreaks in the U.S., with our goal of not letting that spark go beyond a small outbreak and kindle like a larger fire," said Daskalakis.

However, some recent clusters have also not been tied to international travel. 

Louisiana's health department said their cases this year were exposed in another U.S. state. A spokesperson for Arizona's Coconino County said their recent cases had not been linked to recent international travel or the other cases reported previously in the state this year.

When was the last time the U.S. saw a surge of measles cases?

The most measles cases seen in the U.S. since the COVID-19 pandemic was in 2022, with 121 cases reported in just six states. 

Many of the infections in 2022 resulted from low immunity among the thousands of evacuees airlifted from Afghanistan that year, prompting a mass vaccination campaign to curb further spread. 

Before the pandemic, the last peak in infections came in 2019, when 1,274 measles cases were reported across 31 states.

This marked the largest number of measles infections on record nationwide since 1992, driven in part by outbreaks that continued for months among large, close-knit Orthodox Jewish communities in New York. Other countries around the world also saw major outbreaks that year.

At the time, federal health authorities worried the outbreaks could threaten the U.S. status of having eliminated the virus — a status it officially achieved in 2000.

But Daskalakis says at this point "we are nowhere close to" seeing measles elimination being threatened.

Losing measles elimination status requires months of sustained transmission of the virus in a community, Daskalakis said, which has yet to be seen.

"We're being, with our local and state health officials, being really persistent on the issue that we address these outbreaks, as well as the bigger picture of catch up vaccination. But I think that this is the time to act so we don't get anywhere close to losing our measles elimination," he said.

What are the symptoms of measles?

Around two weeks after first being exposed to someone else infected with measles, the earliest symptoms of the virus tend to be a high fever alongside at least one of what experts call "the three C's" of measles: 

  1. Cough
  2. Runny nose, which doctors call coryza
  3. Pink eye, which doctors call conjunctivitis

Daskalakis stressed that fever was a key part of the early signs of measles that set it apart from other causes of these common symptoms.

"We don't want every runny nose to say, I need to get tested for measles. It is allergy season, after all," he said.

A few days after that begins the distinctive rash caused by measles, which usually starts around the face before spreading to the upper body. Measles rashes are typically not itchy. Common complications include ear infections and diarrhea.

Tests are usually most sensitive to detecting measles infections when administered by a doctor at least three days after the rashes begin. 

Health authorities say people worried they may be sick with measles should call ahead to their doctor or hospital before visiting, given precautions that providers need to take to avoid spreading the highly contagious virus to other patients.

What are the complications of measles?

The CDC says about 1 in 5 unvaccinated Americans who catch measles are hospitalized. 

During 2019's record outbreak, 5% of the hospitalized patients had infections in their lungs, which doctors call pneumonia, and one developed encephalitis, or brain swelling. No deaths were reported. 

Measles can be fatal, especially in young children. Before the disease was eliminated in the U.S., thanks to the widespread adoption of the vaccine in the 1960s, around two to three deaths occurred for every 1,000 cases reported.

The last U.S. death from measles was in 2015, a CDC spokesperson said. Daskalakis said that measles fatalities are rare in U.S. patients, but cautioned even mild cases can face longer-term issues.

"There's a pretty good body of evidence that measles itself can cause reduced immunity to other pathogens. So even if we are seeing folks who are having mild cases, there's other things that could be further downstream that worry me," he said.

    In:
  • Vaccine
  • Measles
Alexander Tin

Alexander Tin is a digital reporter for CBS News based in the Washington, D.C. bureau. He covers the Biden administration's public health agencies, including the federal response to infectious disease outbreaks like COVID-19.

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