(NewsNation) — A measles outbreak in Chicago that produced 33 confirmed cases, the majority of which are tied to a migrant shelter, has also made the city a national leader in a virus that was declared eliminated from the United States for nearly two decades.

Chicago has seen nine new confirmed cases this week alone, and officials in two suburban counties reported measles cases linked to the Chicago outbreak. So far in 2024, there have been 35 cases reported in Illinois — just three fewer than all the cases reported in the eight years prior, according to state health department data.

The Windy City outbreak represents half of all cases reported nationally, according to recent data from the Centers For Disease Control and Prevention. As of March 21, the agency reported 64 confirmed cases had been confirmed in 17 states across the country.

The 64 cases in the first three months of this year have already surpassed the 58 cases confirmed in the U.S. in all of 2023 — on pace for an 800% increase. The CDC did not specify how many cases were found in each of the 17 states where measles cases have been reported. However, Florida health officials have reported 11 confirmed cases, many of which were tied to an outbreak at an elementary school.

A CDC spokesperson did not return an email from NewsNation Wednesday seeking updated case numbers.

In Illinois, the majority of cases reported have been at a Chicago migrant shelter housing more than 1,800 people. A CDC team was sent to Chicago at the start of the outbreak to help local officials deal with the health emergency. Alderperson Byron Sigcho-Lopez, who represents the district where the shelter is located, previously criticized the lack of health protocols for new arrivals after they are sent from the southern border.

Sigcho-Lopez also said that many of the migrants sent to Chicago arrive with a hesitancy to be vaccinated. City health officials have since made vaccination for both measles and COVID-19 a requirement to live in city-run shelters.

In an online discussion posted on the American Medical Association’s website, Andrea Garcia, the AMA’s vice president of science, medicine, and public health, said measles outbreaks have increased since 2010 “because it’s easily imported by unvaccinated travelers and (the virus) can spread in under-immunized communities.”

Wednesday, GOP Florida Gov. Ron DeSantis defended his state surgeon general’s response to the outbreak in Florida, which included advising parents to continue to send unvaccinated children to school. He claims to have set the public health standard by providing “evidence-based leadership” that focuses on “personal responsibility and parents’ rights.”

Earlier this month, DeSantis said Florida had received disproportionate attention for “political reasons.” On Wednesday, the red state governor took aim at blue Chicago.

“Now you see 30 cases in Chicago with illegal aliens. I don’t hear the same carping from the media,” he said, according to a report by the Florida Phoenix. “In fact, they’re not talking about it really very much at all. So, it just goes to show you, you know, the phony narratives that get put out all to drive a phony agenda.”

The CDC reported that between 2020 and 2022, however, more than 61 million measles-containing vaccines were postponed or missed due to COVID-19-related delays.

In Chicago’s current outbreak, 22 of the 33 cases found in the city have been found in children four years and younger, which has prompted city health officials to change their policy on how it vaccinates people at the shelter.

The Chicago Department of Public Health announced that people who were vaccinated at the shelter would receive the second dose of the measles, mumps and rubella (MMR) vaccine 28 days after their first shot.

Typically, children are eligible to receive the MMR vaccine at 12 months of age and then a second dose between the ages of 4 and 6 years, Chicago health officials said. However, the second dose of the vaccine can be received as soon as 28 days after the first was administered.

Health officials say that the policy change will prevent the spread of the virus in the facility. The expedited second dose will provide the best protection for preschool children until their immunity to measles is fully developed, the health department said.

The new policy is expected to affect about 50 children at the shelter, where more than 900 people were vaccinated in just a matter of days after the first confirmed case was reported earlier this month. Another 700 shelter residents were found to be immune from the virus after previously being vaccinated or having exposure to measles.

“While the MMR vaccine is the best protection against the virus, children are at highest risk for contracting breakthrough measles after receiving one dose of the vaccine, especially those less than 5 years old. We’re seeing some of these cases at the Halsted shelter, which isn’t surprising,” Chicago Department of Public Health Commissioner Olusimbo Ige said in a statement released this week.

“I understand this will be a challenge for families, but we want to do everything to protect young children from contracting measles by ensuring two doses of the MMR vaccine.”