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. 

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.

Florida has also reported 10 cases 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 help 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.

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.

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. 

Cases then virtually disappeared nationwide amid the restrictions imposed during the initial months of the COVID-19 pandemic, with a record low 13 cases reported in 2020.

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

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.

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.