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.
