(NewsNation) — U.S. measles cases have spiked to triple digits for the second time in five years, provoking federal health officials to sound the alarm about the need for vaccinations to help limit the spread of the contagious disease.

The 113 U.S. cases reported by the Centers For Disease Control and Prevention so far this year are the highest since 2022, when 121 cases were confirmed. It’s a 50% jump from 2023 just four months into the year.

However, the caseload itself should not be the determining factor in defining an outbreak’s severity, John Sellick, an infectious disease specialist at the University at Buffalo, told NewsNation.

“Just a number is not going to tell the story,” Sellick said. “It’s going to be a number in relation to where the cases are occurring.”

Concentrated measles outbreaks

More than half of measles cases reported by the CDC are in Illinois, linked to an outbreak at Chicago’s largest migrant shelter, where health officials have centered their efforts to limit the spread.

Instead, most measles cases are introduced into the U.S. by unvaccinated U.S. travelers, the CDC said. If those cases are introduced in communities that have low vaccination coverage, large outbreaks can last for months, the spokesman said.

Officials in Chicago point to the low vaccination levels of migrants who arrive in the U.S. from countries like Venezuela. The outbreak has led city health officials to require those living in city-run shelters to be vaccinated, and as of last week, the city had given more than 6,000 MMR shots to new arrivals.

“New arrivals didn’t bring measles to Chicago. It was circulating here, and they are vulnerable to it because many come from countries where they didn’t receive the vaccinations we in the United States do, and they’re living in congregate settings where it can spread more easily,” Chicago Health Commissioner Olusimbo Ige said last month.

Chicago’s cases share similarities with a pair of outbreaks in 2019 when more than 1,000 of the 1,274 measles cases were reported in New York — the highest case total since 1992.

Both outbreaks were concentrated inside communities — one in Brooklyn and the other in Rockland County — with lower-than-average vaccination rates, the New York Times reported, citing local officials.

One of them was tied to a person who traveled to New York from Israel and into a community of Hasidic Jews.

Rockland County Executive Ed Day issued a series of state of emergency orders, which banned unvaccinated children from going out in public and removed more than 6,000 kids from schools.

In 2019, parents of New York City students who were not vaccinated sued the city’s Department of Health over a vaccine mandate that threatened a $1,000 fine to anyone who defied the order to have school children vaccinated against measles.

“We must not allow this outbreak to continue indefinitely or worsen again,” Day said in 2019, according to the New York Times. “We will not sit idly by while children in our community are at risk.”

At the time, the Jewish Public Affairs Council spoke out frequently during the outbreak about the effects of the measles cases seen there. The organization did not return NewsNation’s request for comment.

A Rockland County spokeswoman told NewsNation that vaccine hesitancy remains a growing issue across the board, and officials have worked to increase vaccinations. She declined comment on the specific communities affected by the outbreak.

A big drop in measles vaccinations

CDC data shows measles vaccination levels have dropped each year since 2019, when 95% of school children who were required to be vaccinated were up to date on their shots.

By the 2022-2023 school year, that figure had dropped to 93.1%, below the 95% level needed to obtain herd immunity, CDC officials said.

“When you look at what’s happened with rates of vaccination and you couple that to measles being the most contagious virus that’s known to humankind, we have to expect this to happen,” Dr. Amesh Adalja told “NewsNation Now.”

Hesitation among some Americans to get vaccinated is seen as an extension of the anti-COVID-19 vaccination movement, health officials say. The first two years of COVID-19 also delayed 61 million vaccinations, leaving 250,000 kindergartners at risk, the CDC said.

Currently, Sellick estimates 8 to 10% of young children are not being vaccinated, while 20% of teens scheduled for their second dose of the vaccines have not gotten them.

The drop in childhood immunizations can be linked to communities not having convenient access to vaccines and ongoing questions about vaccines’ effectiveness, the CDC spokesman said.

Measles carries a basic reproduction number of around 15, Sellick said, which means that for each person who contracts measles, up to 15 people can be affected because of the disease’s contagious nature. By comparison, the basic reproductive number for COVID-19 is around 4, Sellick said.

Yet in a country where measles was believed to be eradicated in 2000, measles continues to surface along with diseases like polio. Like measles, polio was believed to be erased from the U.S. until a case was confirmed in Rockland County in 2022.

Sellick said that smallpox is the only spreadable disease the nation has been able to successfully eradicate for good. While measles cases have dropped, the disease will not completely disappear as long as vaccine hesitancy remains an obstacle.

“It is theoretically possible that we could make measles go away,” Sellick said. “We could make it go away, but we’re not going to see it in my lifetime.”

A CDC spokesman told NewsNation that based on modeling, the agency expects the number of cases to be similar to what has been reported in the U.S. in previous years. Since 2019, the U.S. has experienced an average of 60.2 cases per year with just 13 cases being reported in 2020 and 49 in 2021.

The agency says it’s unlikely the outbreak will surge to levels seen in 2019, when 1,274 cases were confirmed across the U.S.

For more information about measles vaccines and how to get vaccinated, visit the CDC website.