The number of measles cases in the U.S. has risen to 1,024, according to new Centers for Disease Control and Prevention data published Friday.

Cases have been confirmed in 30 states including Alaska, Arkansas, California, Colorado, Florida, Georgia, Hawaii, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, New Jersey, New Mexico, New York, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas, Vermont, Virginia and Washington.

The U.S. is nearing the total seen in 2019, of which there were 1,274 confirmed infections across the country over the course of a year, CDC data shows.

The CDC says 13% of measles patients in the U.S. this year have been hospitalized, the majority of whom are under age 19.

“The key thing about all of this is that the cases of measles that we’re seeing today and that typically see are nearly 100% in people who have not received the vaccine,” Dr. Roy Gulick, chief of infectious disease at NewYork-Presbyterian and Weill Cornell Medicine, told ABC News.

Among the nationally confirmed cases, CDC says about 96% are among people who are unvaccinated or whose vaccination status is unknown.

Meanwhile, 1% of cases are among those who have received just one dose of the measles, mumps, rubella (MMR) vaccine and 2% of cases are among those who received the required two doses, according to the CDC.

“The thing to know about measles is that it is almost entirely 100% preventable and that’s by receiving [a] measles vaccination,” Gulick said.

The CDC currently recommends that people receive two doses of the MMR vaccine, the first at ages 12 to 15 months and the second between 4 and 6 years old. One dose is 93% effective, and two doses are 97% effective against measles, the CDC says.

Measles was declared eliminated from the U.S. in 2000 due to a highly effective vaccination program, according to the CDC. But vaccination rates have been lagging in recent years.

During the 2023 to 2024 school year, 92.7% of kindergartners received the MMR vaccine, according to data. This is lower than the 93.1% seen the previous school year and the 95.2% seen in the 2019 to 2020 school year, prior to the COVID-19 pandemic.

With vaccination rates declining, “that leaves more of the population susceptible to measles and means that it could be passed in the population more easily,” Gulick said. “Sustained transmission which if it occurs in enough people and for long enough then we will lose status of eradication.”

Dr. Karen Tachi Udoh is an internal medicine resident at Johns Hopkins Hospital and a member of the ABC News Medical Unit.