South Carolina measles outbreak is triggering dangerous brain swelling in some children

Brain swelling is one of the nastiest side effects of measles, and it’s happening in South Carolina

By Meghan Bartels edited by Tanya Lewis

A simple case of measles is unpleasant, but the complications of the disease are even worse. One of these complications has been confirmed during the ongoing and unprecedented measles outbreak in South Carolina: encephalitis, or swelling of the brain.

Linda Bell, South Carolina’s state epidemiologist, confirmed the complication was occurring there during a press briefing on February 4, according to Wired. Bell did not specify the number of people affected or the severity of their cases.

Encephalitis occurs in about one in 1,000 cases of measles and kills about one in five people who develop it. The complication is not well understood but appears to be the result of the virus triggering the immune system to attack a certain protein produced by certain brain cells.

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As of February 3, South Carolina had reported 876 cases of measles linked to the outbreak in the northern region of the state. Most of these cases involve children who have not been vaccinated against the disease. Since January 2025, nearly 3,000 people have been infected with measles in the United States, according to independent tracking by Johns Hopkins University. The 2,267 cases in the United States last year represent the country’s highest annual total since 1991. The virus’s continued spread will likely end the 25-year streak in which measles has been officially considered eliminated in the United States, experts say.

Measles is a notoriously contagious virus, with each contagious person transmitting it to an average of 15 people. The combined measles-mumps-rubella vaccine is 93 percent effective after one dose and 97 percent effective after the recommended two doses. But the extreme contagiousness of the virus means cases can rise quickly where vaccination rates fall below 95 percent.

Even when a person appears completely cured of measles, the virus can cause serious complications years after infection. It can trigger “immune amnesia” toward other pathogens that the body has detected, apparently by attacking the cells that the immune system uses to remember those germs.

Infected people in South Carolina who have not yet developed encephalitis may do so. In rare cases, the virus can lie dormant in the brain for 10 or 15 years, picking up genetic mutations until it is able to destroy neurons, causing a disease called subacute sclerosing panencephalitis, or SSPE, which is almost always fatal. A Los Angeles County child died from SSPE in September 2025.

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