California is battling its worst measles outbreak in seven years, as cases of the world’s most infectious disease surge nationwide.
Over the last seven days, the state has recorded four new infections, bringing its total to 39 patients this year, the most since 2019.
The uptick is being driven by an escalating outbreak in Sacramento, while a new case was also recorded in an infant in San Francisco, marking the city’s first infection in seven years.
It’s just the latest state to record an uptick after 40 new infections were registered in Utah over the last two weeks. Three were also recorded in Michigan, while two each were registered in Colorado, Oregon and Washington and one each in Arizona and Texas.
Just this year, the US has recorded 1,714 infections, more than double the 800 infections reported nationwide at this time last year.
In 2025, the country recorded a total of 2,287 measles infections, the most since 1991.
The surging cases put the US at risk of losing its measles elimination status, which is lost if a country records 12 months of ongoing transmission of the virus.
Dr Eric Sergienko, California’s infectious diseases chief, said at a press conference this week: ‘With measles, this has been a significant year for us in that we are only a quarter of the way through… and we already have 39 cases.
‘With four new cases coming up over the last week, we anticipate that this outbreak will be going on for at least another… 21 days or so.’
California has recorded 39 measles cases so far this year, the seventh-highest tally by state in the US
In Sacramento County, a total of 11 infections have been recorded so far this year. Officials said only one of these patients was vaccinated against measles.
The county reported its first measles cases in February, when health officials revealed that an unvaccinated toddler was infected with the disease. The child had recently returned from South Carolina, which is recording a major measles outbreak of more than 1,000 cases.
In San Francisco, the infant was reported to be too young to have received a measles vaccine. Officials said the child became infected during an international trip. All the infant’s family is vaccinated.
Across California, out of the 39 patients, two have been hospitalized. No deaths have been reported this year.
Ninety-five percent of the state’s infections have been recorded in individuals who were not vaccinated against measles or had no known vaccination, while eighty percent of patients are younger than 20 years old.
Health officials have urged anyone who is not vaccinated against measles to get the vaccine to slash their risk of infection.
Measles is an extremely infectious disease, easily spread via coughs and sneezes, that infects nine in 10 unvaccinated people who are exposed.
Just one dose of the measles vaccine slashes the risk of infection by 93 percent, while two doses reduce it by 97 percent.
In the US, children are offered their first dose of the measles vaccine at 12 to 15 months old, and their second at four to six years old. It triggers protection against the virus for life.
Children under five years, pregnant women and those with weakened immune systems are most at risk from a measles infection.
Symptoms of the infection begin as a high fever, cough or runny nose, before a flat red rash emerges on the face and gradually spreads over the entire body.
About one in five unvaccinated people who are infected are hospitalized, reports the CDC, while one in 20 unvaccinated children develop pneumonia and about one in 1,000 suffer from encephalitis, or swelling of the brain.
Nearly one to three out of every 1,000 unvaccinated children who are infected with measles die from the disease.
Nationwide, California has the sixth-highest measles tally in the country so far this year.
South Carolina has recorded the most infections, 667 in total, linked to a major outbreak upstate, while Utah has recorded more than 400 and Texas and Florida have each recorded more than 100.
Arizona, which has also recorded more cases than California, has registered 59 to date.
South Carolina’s outbreak now appears to be winding down, with no new infections recorded for at least two weeks. Officials say they may be able to declare it over by the end of April.
Warning signs of a measles infection include a high fever, followed by a red rash that starts on the head before spreading over the body (stock image)
The state’s outbreak started in late October and accelerated over the winter, with most infections being recorded in its upstate region.
It comes amid concerns that the US may lose its measles elimination status, which the country has held since 2000.
Countries lose their status if an ongoing measles outbreak is reported within their borders for at least 12 months.
So far this year, several countries have been stripped of their measles elimination status, including the United Kingdom and Spain. Canada lost its status in late 2025.
Officials say the shift is being driven by declining measles vaccination rates, which have been linked to the Covid vaccine rollout.
In the US, about 92.5 percent of kindergarteners were vaccinated against measles last year.
This is below the 95 percent level that experts say is needed to prompt ‘herd immunity’ and stop the virus from spreading.
