A nurse prepares a dose of the measles, mumps and rubella virus vaccine March 7 at a pop-up clinic at the Carrollton-Farmers Branch ISD Administration Building in Carrollton. (Liz Rymarev/The Dallas Morning News/TNS)

DALLAS — The Texas measles outbreak has grown to 309 confirmed cases, not counting dozens of additional cases in neighboring states.

Cases have been identified in three additional counties, according to the Texas Department of State Health Services’ Friday update.

Those three counties are all in West Texas, near the original outbreak area. The majority of cases — 211 of the 309 confirmed cases — are in Gaines County, near the New Mexico border, where the outbreak originated.

Since the outbreak began, a total of 40 people in Texas have been hospitalized, according to the Department of State Health Services. One school-aged child in Texas has died.

In addition to the Texas cases, the states of New Mexico and Oklahoma have both reported measles cases that are believed to be linked to the Texas outbreak.

Oklahoma has reported four probable cases. New Mexico reported 38 cases as of Tuesday, along with two hospitalizations. One adult in New Mexico has also died.

The vast majority of the Texas measles cases have been among people who are either unvaccinated or whose vaccination status is not known, according to the Department of State Health Services.

Measles is a highly contagious virus that spreads mostly among unvaccinated people. The virus can live in the air and on surfaces for up to two hours after an infected person has left.

The two-dose measles-mumps-rubella vaccine is 97% effective at preventing measles infection. The vaccination is typically recommended for children beginning at around 12 months of age, with possibility for an earlier dose beginning at around six months in cases of international travel or active outbreaks.

The vaccine is recommended for the vast majority of people above the age of 12 months old, with the exception of people who are pregnant or immunocompromised.

The current measles outbreak is the largest that the state of Texas has reported in decades.

Already in 2025, the state has reported more cases than in any full year since 1992.

Measles has been considered eliminated in the U.S. since 2000, but there have been outbreaks across the country. In 2018 and 2019, for instance, there were two large and “closely related” measles outbreaks in New York, according to the U.S. Centers for Disease Control and Prevention. Those two outbreaks, with a total of more than 1,100 cases, threatened the country’s elimination status.

Local health authorities across Texas offer the measles shots at vaccination clinics. People can also contact their doctor or pharmacy to ask about vaccination.