Signs at the Gaines County Courthouse in Seminole, Texas, inform people about measles. (Photo credit: Justin Rex for The Texas Tribune/Handout/Reuters via CNN Newsource)

(CNN) — Half of states in the US are now reporting measles cases, with the national total surpassing 800 cases so far this year, according to a CNN tally.

Most of the cases are concentrated in a multistate outbreak involving Texas, New Mexico, Oklahoma and possibly Kansas that reached 709 cases Friday, according to state health department updates.

Texas has reported 597 outbreak-associated cases, New Mexico reported 63 cases, and Oklahoma reported 12 cases – nine confirmed and three probable – as of Friday. New Mexico and Oklahoma’s numbers have stayed the same since Tuesday.

Cases in Kansas, which the state health department says may be linked to the larger outbreak, have reached 37 as of Wednesday.

Experts say these numbers are all a severe undercount because many cases are going unreported.

The multistate outbreak has put 69 people in the hospital across Texas, New Mexico and Kansas, five more since Tuesday’s update.

Nationally, the US Centers for Disease Control and Prevention reports that at least 85 people have been hospitalized, accounting for 11% of confirmed cases. CDC data shows that only 3% of cases this year have been reported in people who have gotten one or two doses of the measles-mumps-rubella (MMR) vaccine.

Local health officials are scrambling to increase vaccinations, especially in heavily undervaccinated communities. Clinics in Lubbock, Texas, near the epicenter of the outbreak, just expanded their hours.

These Lubbock clinics have administered 450 more doses of the measles-mumps-rubella (MMR) vaccine than typically would have been given by this time of year, Katherine Wells, director of Lubbock Public Health said this week. Two doses of the MMR vaccine are 97% effective against the measles virus.

Measles was declared eliminated in the United States in 2000. Experts warn that growing case numbers could threaten the country’s elimination status if spread of the outbreak continues.

“That … would happen after 12 months of ongoing circulation of the same sequence,” Dr. David Sugerman, a senior scientist at the CDC, said Tuesday at a meeting of the agency’s vaccine advisers.

That date would be around January 20, 2026, he said.

Two children in Texas have died in the current multistate outbreak, and a third death in New Mexico is under investigation. They were all unvaccinated.

The World Health Organization said that cases in Mexico have also been linked to cases in Texas. Three cases in Colorado are being investigated for connection to the outbreak after two of the people traveled to Mexico. Another case was identified after a Pennsylvania resident traveled to Texas, though it is unclear what part of the state the person visited. In a news release last week, the Arkansas health department said that the second case of measles identified in the state this year was “exposed during out of state travel,” but no further details were provided.

Many cases in other states have been linked to international travel.

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