Why does the US keep experiencing measles outbreaks?

As the number of measles cases increases across the country, Chicago has become the latest U.S. city to be at the center of an outbreak.

As of Thursday morning, eight measles cases have been confirmed, with at least seven among children and adults at a new arrivals shelter, according to the Chicago Department of Public Heath.

The Centers for Disease Control and Prevention has sent a team to help public health officials respond to the outbreak, with staff arriving Tuesday, according to the federal health agency.

At least 45 measles cases have been reported across 17 states so far this year, according to CDC data. Due to delays in reporting national data, this number does not include the current outbreak in Chicago. Numbers are now close to the total number of cases -- 58 -- reported for all of last year.

Measles was considered eliminated in 2000 because most Americans were vaccinated against the disease or had some level of immunity. Over the last several years, however, vaccination rates have dipped and pockets of unvaccinated and undervaccinated communities have led to sporadic outbreaks across the U.S.

Public health experts told ABC News these localized outbreaks can be hard to contain.

"As an infection control practitioner and a physician, I just feel like ... you're kind of always waiting for the next measles outbreak," Dr. Jennifer Grant, an infectious disease physician and system medical director for infection prevention and control at Endeavor Health in the greater Chicago area, told ABC News. "It just feels like we have a perfect storm of declining vaccination, that there is always a possibility that a measles outbreak could happen anywhere."

Measles cases in Chicago

The first measles case in Chicago was confirmed on March 7 in a city resident whose source of infection is unknown, according to the CDPH.

Since then, seven cases have been confirmed at a new arrivals shelter in Pilsen, in the lower west side of the city, health officials said.

Because it can take about 11 to 12 days from someone being exposed to developing symptoms, experts said it might take some time to see the full impact of the outbreak.

"I think it is safe to say that the majority of those who have come down with cases of measles are unvaccinated," Dr. Aniruddha Hazra, an associate professor of medicine in the section of infectious diseases and global health at UChicago Medicine, told ABC News. "I think the big concern is that this is not telling us the fuller picture of the current outbreak in Chicago and likely there are potentially more cases and ... likely more exposures in the city."

A CDC team arrived in Chicago this week to support local officials by providing guidance for monitoring symptoms and quarantine or isolation practices as well supporting the ongoing vaccination campaign, the CDPH said.

Hazra said the CDC has its own epidemiological intelligence service that goes to places around the country where outbreaks are occurring and offers both financial support and manpower support.

"Contact tracing is backbreaking work; it takes a lot of time, a lot of manpower to get done," he said. "There is significant benefit for contact tracing and case investigation with measles, just given the prolonged incubation period and the time between when someone is exposed to when someone actually presents with symptoms and then can be contagious or transmissible to others."

"Being able to capture folks in that window, making sure they're able to quarantine and self-isolate, that is so important to help reduce the spread of outbreak," Hazra added. "So, the CDC being able to deploy additional folks to come here to assist and collaborate with our public health department is really critical."

If I get the measles vaccine, am I protected?

The CDC currently recommends that everyone receive two doses of the measles, mumps and rubella (MMR) vaccine with the first dose between ages 12 and 15 months and the second dose between ages 4 and 6.

One dose of the measles vaccine is 93% effective at preventing infection if exposed to the virus. Two doses are 97% effective.

If someone has had two doses of the MMR vaccine, they do not need to receive a booster dose and are essentially protected for life, public health experts told ABC News.

For any adult not sure of their immunization status, the experts recommended speaking to a primary care provider. Antibody levels may be drawn to assess immunity, and if someone is not immune, they may be eligible for at least one MMR dose.

It's currently not recommended for pregnant women to receive the MMR vaccine if they are not immune to measles.

"We do recommend discussing with your doctor whether or not [antibody levels] can be drawn and to make sure that you're not in any situation where you might be presented to measles because that could be a very concerning situation" if pregnant, said Dr. Nicholas Cozzi, EMS medical director at Rush University Medical Center in Chicago.

In the case of the measles outbreak at the shelter, experts have stressed not conflating the outbreak with anti-migrant sentiment because many people in these shelters have likely not had access to vaccines or have had barriers to access in their native countries.

"This is really a call for us to be providing better medical care to these populations," Dr. Gary Reschak, a pediatrician at Northwestern Medicine Huntley Hospital, told ABC News. "A lot of these people who are coming here ... did not have access to the same health care and the easy access to vaccines that we take for granted."

So why is measles coming back?

Public health experts said there has been a "perfect storm" in the U.S. of vaccine hesitancy, vaccine access and vaccine fatigue.

Grant said during the COVID-19 pandemic, countries around the world, including the U.S., lost a lot of ground when it came to maintaining rates of routine childhood vaccinations due to lack of access and people being fearful of accessing health care settings during the early days of the pandemic due to risk of COIVD exposure.

She added that people may be experiencing fatigue from receiving the original COVID-19 vaccine and subsequent boosters, which may have spilled over into feelings about routine vaccines.

Cozzi also pointed to a now-debunked paper from the U.K. in 1998, which allegedly found that MMR vaccines cause autism. The paper was discredited, retracted from the journal where it was published, and its primary author lost his medical license. More than a dozen studies have tried to replicate the findings and failed to find a link.

Despite no evidence that vaccines cause autism, some parents may still be hesitant for their children to receive the MMR vaccine.

"That was debunked, it was proven to be false, but we see the rise of medical misinformation, and that was one of the earliest signs that that was occurring," Cozzi said.

Outbreaks in unvaccinated groups be hard to contain

"Measles as a virus is something that I think a lot of [people] just don't appreciate how extremely contagious it is," Reschak said. "It goes beyond like your colds and your flus."

Measles is highly contagious and can spread through the air when an infected person coughs or sneezes, according to the CDC.

It is so contagious that if an infected person comes into contact with people who aren't protected, 90% of them will also become infected. In a congregate setting, like a shelter, this can mean quick spread.

"So, with this level of contagiousness, it really does spread like wildfire, and that's why when you have outbreaks like this, you have to do very good tracking of the cases and trying to isolate people who may have been exposed and are at risk for spreading measles," Reschak said.

These outbreaks can potentially put a strain on the healthcare system because measles can be deadly.

According to the CDC, one in five unvaccinated people with measles are hospitalized, one in every 20 children with measles contracts pneumonia and one in every 1,000 children with measles suffers encephalitis, or swelling of the brain.

These complications can lead to ear infections, deafness, neurological complications or even death. Nearly one to three of every 1,000 children with measles will die from respiratory and neurologic complications.

"These are just huge, huge numbers that put a major strain on the health care system as well as the families themselves," Reschak said. "So, keeping it as isolated and keeping these outbreaks as sequestered as possible is in everyone's interest."

Will measles outbreaks keep happening?

A November 2023 CDC report found that for the 2022-23 school year, about 93% of children in kindergarten had met the vaccination requirements, about the same number seen for the previous school year but lower than the 94% rate in 2020-21 and 95% rate in 2019-20.

Additionally, exemptions from school vaccination requirements increased to 3% during the 2022-23 school year, which is the highest vaccination exemption rate ever reported in the U.S., according to the CDC.

"Unfortunately, I believe that if we don't start getting more children vaccinated more consistently, we will start seeing more of these outbreaks pop up," Reschak said. "We are really approaching that threshold where, I can't say this with certainty, but I'm concerned that it's possible measles can become endemic to the United States again."

This is why experts say it's so important to receive the MMR vaccine to both protect yourself and those most at risk of severe disease in your community.

"Measles is an equal opportunity virus, irrespective of your legalization status," Cozzi added. "It just concerns itself if you're under-vaccinated or unvaccinated. Measles, if you're unimmunized, is incredibly dangerous."