South Carolina reported nearly 250 new measles cases this week, official figures showed on Friday, bringing the total of confirmed infections to 558 since October as the threat of a wider outbreak increased.
State epidemiologist Linda Bell said on Wednesday that the risk to unprotected individuals, including those traveling to the outbreak area in the northwestern part of the state, was increasing.
At least 531 people are in quarantine after being exposed to the virus, and another 85 people who are symptomatic are in isolation to keep from spreading the disease.
"It is only a matter of time before a very severe or even fatal case occurs," said Dr Amesh Adalja, infectious disease expert at the Johns Hopkins Centre for Health Security, noting that about one in every 1,000 measles cases results in death.
He said the larger the outbreak grows, the more difficult it will be to bring under control, and the more offshoots it will start in other parts of South Carolina and in other states, as is already happening.
North Carolina, Ohio and Washington State have all reported cases linked with travel to South Carolina.
The United States is seeing a resurgence of measles.
The Trump administration and Health Secretary Robert Kennedy Jr, an anti-vaccine activist, have made sweeping changes to the US childhood vaccination schedule – moves that many experts say will increase vaccine hesitancy.
In 2025, the United States reported 2,242 confirmed measles cases, the most in three decades, putting the nation's prized status of having eliminated local transmission of the disease at risk.
South Carolina's outbreak, which began in October, is centred on Greenville and Spartanburg Counties along the state's northern border with North Carolina.
Of those infected, 483 were unvaccinated, six were partially vaccinated with one of the recommended two-dose measles-mumps-rubella vaccines, 13 were fully vaccinated and 56 had unknown vaccination status.
Most cases were reported in children in the 5-17 age group, followed by those below five years of age.
North Carolina officials reported on Thursday two new measles cases related to the South Carolina outbreak, in Buncombe County, bringing that county's total to five.
North Carolina has set up a measles dashboard, which lists a total of seven cases since December, and urged residents to make sure their children are up to date on all childhood immunisations, including the measles vaccine.
Measles is a highly contagious respiratory virus that can hang in the air for as long as two hours.
It causes rash, cough and high fevers and can lead to severe complications including pneumonia, encephalitis and suppression of the immune system.
South Carolina's outbreak follows a large outbreak in West Texas that started in January 2025 and infected more than 800 people in the state.
Utah and Arizona are also battling outbreaks involving 201 cases in Utah and 223 in Arizona.
Non-medical exemptions to school vaccine mandates have risen since the coronavirus (Covid-19) pandemic to an average of more than 3% of schoolchildren in 2023-24, compared with 0.6% in 2010-11.
To prevent measles, 95% of the population needs to be vaccinated.
In some schools in South Carolina's Spartanburg County, vaccination coverage is as low as 20%, Bell said this week.
Stacy Ellis Matheson, public health director in North Carolina's Buncombe County, said in a briefing this week that 87% of children in her county attend schools in which fewer than 95% of students are vaccinated.
"We've got a fire raging in South Carolina, and these pockets of lower vaccination rates are kind of like tinder," she said, adding there was the definite possibility of a wildfire.