A recent outbreak in Philadelphia has infected eight people so far.
Despite having a very effective and easily available vaccine, measles outbreaks have continued to pop up in the United States over the last two decades.
Most recently, there have been nine cases confirmed in Philadelphia since December 2023, all among unvaccinated individuals. Additionally, a person with measles traveled through D.C.-area airports and cases have been identified in Delaware, New Jersey and Washington state, according to local reports.
In 2023, there were 41 confirmed cases of measles, according to incomplete data from the Centers for Disease Control and Prevention (CDC).
While the number of measles cases in the past few years are not at record highs and the 2023 numbers are lower than recent years, the fact that outbreaks are still occurring is a trend that concerns health officials and experts.
Measles was declared eliminated in 2000 -- meaning the disease "is no longer constantly present in this country." However, the dip in routine childhood vaccinations in recent years as well as travelers bringing measles into the country has resulted in outbreaks.
"The fact that we're seeing sporadic measles cases, to me, says that we probably have pockets in the United States where we're not doing a good job vaccinating and I'm worried that this is a trend that's been getting worse over the years," Dr. Peter Hotez, professor of pediatrics and molecular virology at Baylor College of Medicine, dean of the National School of Tropical Medicine and co-director of the Texas Children's Hospital Center for Vaccine Development, told ABC News.
A CDC report in November found that exemptions for routine childhood vaccination among U.S. kindergartners are at their highest levels ever.
About 93% of kindergarteners received select routine childhood vaccines, including the measles, mumps, rubella (MMR) vaccine, which protects against measles, for the 2022-23 school year, according to the CDC report.
This is about the same as the previous school year but lower than the 94% seen in 2020-21 school year and the 95% seen in the 2019-20, prior to the COVID-19 pandemic. The latter percentage had been the standard for about 10 years.
Hotez said there may be areas of the U.S. where the vaccination exemption rates, both for medical and non-medical reasons, are higher.
"For instance, when we studied this in 2018, looking at the states that allow vaccine exemptions for non-medical reasons, we would find counties that maybe 10 to 20% of the kids were not getting their childhood immunizations and that's what measles exploits," Hotez said. "So, if were uniformly 93%, it's not ideal, but probably that wouldn't be enough to stimulate measles outbreaks."
About one in five people in the U.S. who get measles will be hospitalized. Measles can cause serious health complications especially in children younger than age 5 including ear infections, diarrhea, pneumonia, encephalitis (swelling of the brain) and even death, according to the CDC.
The first measles vaccine, a single-dose vaccine, was introduced in the U.S. in 1963. In the decade prior, there were three to four million cases annually, which led to 48,000 hospitalizations and 400 to 500 deaths.
The CDC recommends that people get two doses with the first dose at 12 to 15 months old and the second dose between ages 4 and 6. One dose is 93% effective and two doses are 97% effective.
Since then, hospitalizations and deaths have dropped dramatically. There were three deaths in the Americas in 2000 and just one in 2022, according to a November 2023 CDC report.
"We can prevent this, we can stop this. Parents should be scared of measles," Dr. Paul Offit, director of the Vaccine Education Center and an attending physician in the division of infectious diseases at Children's Hospital of Philadelphia, told ABC News. "They should be scared of this virus as my parents were. The difference was [my parents] couldn't do anything about it."
"Now you can do something about it, which makes it all the more unconscionable when you see children come into our hospital who could have gotten vaccinated and didn't," he added.
There are a few reasons for a drop in vaccination rates, according to experts. One is a 1998 paper published in The Lancet by Andrew Wakefield claiming the MMR shot caused autism. The paper has since been debunked, subsequent studies have found no link and the journal retracted the paper, but fears still exist.
During an outbreak in Columbus, Ohio that lasted from November 2022 to February 2023, public health officials said many parents of the unvaccinated children infected with measles had chosen not to have their kids receive the MMR shot due to misconceptions that it causes autism.
"Ever since that Anfrew Wakefield article, people have developed important misconceptions from that misinformation and continuing disinformation about the MMR vaccine," Dr. Gregory Poland, head of the Mayo Clinic's Vaccine Research Group, told ABC News. "He claimed there was an association with autism. Some 24 studies have subsequently found none. Not one indicate a hint of autism risk."
"Once you scare people, it's hard to unscare them, so people then then sort of started to back away from that vaccine," Offit added. "So we saw cases again."
Experts said the COVID pandemic caused another problem, Firstly, during the early days of the pandemic, people were scared to go to doctor's' offices, which led to a delay in children being up to date on vaccinations.
Then, after COVID vaccines became politicized, this may have caused a decrease in confidence in vaccination overall.
There has been "an acceleration of anti-vaccine sentiments that we've seen during the COVID 19 pandemic," Hotez said, "And I think what we may be seeing also was a spillover that extended beyond COVID vaccines to all childhood immunizations."
"This rise in cases is a reminder of the ongoing challenges we face with vaccine hesitancy and the need for maintaining high vaccination coverage to achieve herd immunity. We need concerted efforts to address these issues and to ensure that our public health infrastructure is robust enough to handle such outbreaks," added Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hopsital and an ABC News contributor.
Another reason for the decline in vaccination rates may be that, because the diseases have been circulating at low rates due to vaccines, people have forgotten how serious they were before the advent of vaccines, according to the experts.
It's not just a problem in the U.S. Global cases of measles have been on the rise in recent years, increasing 18% from 2021 to 2022, following a drop in vaccinations over the past few years, according to a report from the WHO and CDC released last year.
Deaths also increased globally by 43% from during the same period with a total of 37 countries experiencing large outbreaks in 2022 compared to 22 countries in 2021.
Experts say they are continuing to educate parents about the safety of vaccines and even advocate for starting vaccine education in adolescence.
"It's a dangerous game we're playing by leaving a critical percentage of children unvaccinated," Offit said. "It is a dangerous and unnecessary game we're playing. This is a safe and effective vaccine. This is a virus that can cause considerable suffering and hospitalization and occasional death. Don't play around with this virus or we will pay an even bigger price than we're paying now."