April 27 (UPI) — South Carolina on Monday declared an end to a measles outbreak in its upstate region, which at 997 cases was the largest in the United States in 35 years.

The state’s department of health declared a formal end to the outbreak, which started in October 2025, because it has been 42 days since the last case was confirmed, on March 15.

Measles outbreaks are considered to be ended 42 days after the last confirmed case because it is twice the 21-day incubation period of the rubeola virus, which causes measles, and indicates that the chain of transmission has been broken, the health department said in a press release.

The health department’s interim director, Edward Simmer, said that a key part of ending the outbreak was a 93.6% increase in MMR vaccination — the vaccine against measles, mumps and rubella — due to the “tireless efforts” of healthcare providers in South Carolina’s upstate region.

“The outbreak was predominantly contained to one area of one county and never went statewide, thanks to timely investigations, identification of those exposed and people’s willingness to stay home,” Simmer said in a statement.

“In many ways, this was a textbook response to dealing with an outbreak,” he said.

The outbreak started in October and by mid-December health officials had confirmed 114 cases and had 254 people in quarantine, nearly all upstate and nearly all among people who had not received the two-dose MMR vaccine.

On Jan. 27, the outbreak had grown to 789 cases and was named the worst measles outbreak the United States had seen since 2000, when the infection was declared eradicated in the country.

Of the 997 cases confirmed in the outbreak, 932 were among unvaccinated people, roughly two-thirds of cases — 639 — were among people aged 5 years to 17 years and the vast majority of cases were within Spartanburg County, S.C., the state’s health department said.

The U.S. Centers for Disease Control and Prevention reported 48 measles outbreaks in 2025 with 90% of the 2,288 confirmed cases linked to an outbreak, compared to 16 outbreaks in 2024 and 69% of the 285 cases that year linked to an outbreak.

As of April 24, the CDC has confirmed 1,782 cases of measles across 37 states so far in 2026, roughly 93% of which have been linked to 22 new outbreaks this year.

Ten additional cases in the United States this year have been reported by international visitors to the country.

The best way to prevent measles is with the two-dose MMR vaccine, which has been shown to be 97% effective against the infection when people have received the full inoculation, the CDC said.