(CNN) — Three hundred and fifty-five cases have been reported in the ongoing measles outbreak in Texas, New Mexico and Oklahoma, but local officials say that it’s an undercount and that the virus could continue to spread for months.

Texas has reported 309 outbreak-associated cases, New Mexico has reported 42 cases, and Oklahoma has reported four probable cases.

Forty-two people have been hospitalized. Among the cases, 110 are among children up to age 4, and 140 are among young people ages 5 to 17.

In Texas, 211 cases are in Gaines County, where the outbreak was first identified. In New Mexico, most of the cases are in Lea County, which borders Gaines County.

The Oklahoma State Department of Health said that as of March 18, there were four probable measles cases in people with “exposure associated with the Texas and New Mexico outbreak.” All were unvaccinated or had unclear vaccination status.

Although most of the cases have been in people who had not gotten the measles-mumps-rubella (MMR) vaccine or whose vaccination status was unknown, six cases were found in people who said they have gotten at least one dose: two in Texas and four in New Mexico.

Testing labs have been set up in Lubbock, close to the epicenter of the outbreak. This means specimens no longer need to be sent via plane to Austin, reducing the time to get results from 72 hours to the same day.

Local health officials in Texas say there's ample supply of MMR vaccines available. (Photo Credit: Mary Conlon/AP via CNN Newsource)

Friday’s increase in case numbers is not only a result of increased testing capacity, said Katherine Wells, director of Lubbock Public Health, but also of increased spread.

“This is going to be a large outbreak, and we are still on the side where we are increasing the number of cases,” she said at a briefing this week. “I’m really thinking this is going to be a year long.”

Experts say the outbreak is driven by undervaccination. MMR coverage is particularly low in Gaines County, where nearly 1 in 5 incoming kindergartners in the 2023-24 school year did not get the vaccine. Other affected counties also fall below a goal of 95% set by the US Department of Health and Human Services to help prevent spread of infection. Two doses of the MMR vaccine are 97% effective against measles.

Dr. Carlos del Rio, an infectious disease expert and distinguished professor of medicine at Emory University, said the answer to ending the outbreak is to “vaccinate, vaccinate, vaccinate, vaccinate.”

“This requires working with the community,” he said. “This requires a lot of on-the-ground effort to to vaccinate kids and to convince the community that it’s OK to vaccinate kids. … There’s no other way.”

There is ample vaccine supply, Texas officials said.

“Some people are changing their minds [about vaccination], but I’d like to see more,” Wells said.

Last month, Texas announced the outbreak’s first death, a school-age child who was not vaccinated and had no underlying conditions. Health officials in New Mexico are continuing to investigate the cause of death of an unvaccinated person who tested positive for measles.

Less than a quarter of the way into 2025, the US has already seen more cases than in all of 2024. Last year, there were 285 total measles cases reported, according to data from the US Centers for Disease Control and Prevention. CNN’s tally shows that there have been at least 404 cases reported this year, as of Friday, the most since 2019.

Measles outbreak could last a year, health official in Texas says