California has already reported more measles cases this year than in all of 2024, a worrisome development that comes as the nation is suffering its largest outbreak of the super-infectious disease in decades.

The extent of the national outbreak has rocketed measles from a back-of-mind issue — one rarely, if ever, encountered by a whole generation of Americans — to a pressing public health concern.

“Measles is completely preventable,” Dr. Elizabeth Hudson, regional chief of infectious diseases at Kaiser Permanente Southern California, told The Times.

The virus is spreading almost universally among people who either haven’t been vaccinated, or whose vaccination status is not known, authorities note. But the MMR shots, so named because they also afford protection against mumps and rubella, has long been in the crosshairs of anti-vaccine activists and skeptics — some of whom are now in charge of shaping U.S. policy regarding childhood immunizations.

There have been 17 cases of measles reported so far this year among California residents, up from the 15 reported all of last year, according to the state Department of Public Health. Doctors have been increasingly on alert for measles, which for decades has been so rare that many physicians haven’t encountered a single case.

Because of that recent rarity, many may be unfamiliar with how readily measles can spread — or the serious symptoms it can cause.

“This is not just a mild childhood disease,” Dr. Erica Pan, director of the California Department of Public Health and state public health officer, said in a recent briefing to doctors.

Measles has a death rate of 1 for every 500 to 1,000 cases, Pan said. Nationwide, more than 145 people have been hospitalized with measles so far this year, federal data show, and many of those hospitalized have needed intensive care.

“This can cause very severe disease — often causes a viral pneumonia that is severe enough to need intensive care,” Pan said.

Measles is one of the most contagious viruses known to science. It is so infectious you can catch it just by being in a room where an infected person has been — even up to two hours after they left, according to the U.S. Centers for Disease Control and Prevention.

The uptick in confirmed measles cases in California, while still somewhat concerning for local public health officials, pales compared with the significant outbreak that began in Texas earlier this year and has since spread to surrounding states.

Texas has reported at least 750 measles cases; New Mexico, 81; Kansas, 80; North Dakota, 28; and Montana, 23, according to the respective states’ health departments. Michigan has reported 15 cases; Ohio and Illinois, 10, and Arkansas and Indiana, eight.

The overwhelming majority of the measles cases are associated with outbreaks tied to close-knit communities with low rates of vaccination, according to the CDC. Recent outbreaks have occurred in areas with a notable population of certain Mennonite Christian communities, starting last fall in Ontario, Canada, which was linked to a large gathering in New Brunswick; then in Texas and New Mexico in late January; and more recently in the Mexican state of Chihuahua, according to the Associated Press.

The AP reported that officials in Mexico and the U.S. say the outbreaks in their countries match the strain found in Canada.

The CDC says that 95% of this year’s measles cases in the U.S. occurred among people who were either unvaccinated or had an unknown vaccination status. About 29% of cases occurred in children younger than 5, with an additional 37% in those age 5 to 19.

In all, the U.S. has reported 1,227 confirmed measles cases so far this year. That’s already rivaling the highest single-year total seen this century — 1,274 were reported in 2019.

You would have to look back a generation to find a higher total than that: 2,126 cases were reported nationwide in 1992.

“And those are probably an underreporting,” Pan said of this year’s case count. Doctors, she said, have learned that most people affected by the measles outbreaks are reluctant to get themselves or their children tested.

Three measles deaths have been reported so far this year, according to the CDC. Two were Texas school-aged children — both of whom were unvaccinated and had no known underlying health conditions, according to state health officials. The other was an unvaccinated adult in New Mexico who didn’t seek medical care before dying, according to health authorities there.

One death has been reported in Mexico, according to the World Health Organization. That person was also unvaccinated. More than 3,300 confirmed and suspected measles cases have also been reported in Canada so far this year.

One infant, who was infected with measles before being born premature, died in Canada earlier this month, according to Dr. Kieran Moore, the chief medical officer for the province of Ontario. The infant’s mother was not vaccinated.

The infant faced other serious medical complications unrelated to the virus, but measles “may have been a contributing factor in both the premature birth and death,” Moore said in a statement.

This year marks the first time a child has died from measles in the United States since 2003 — and the child in that case had a rare genetic condition that impaired the immune system. It’s also the first time since the 1990s that more than two people have died from measles in the U.S. in a single year.

While most associated with its telltale rash, health officials warn measles can cause symptoms that cross the boundary from discomfort to danger. Measles can cause encephalitis, or inflammation of the brain. And years after contracting measles, people can later develop subacute sclerosing panencephalitis, or SSPE, a rare but fatal complication resulting from a weakened form of the virus remaining in the body, and later infecting the brain.

“This is a really, really unusual time to see this many deaths — and seeing previously healthy children with a vaccine-preventable death in the United States,” Pan said.

Measles can also cause something called “immune amnesia.” “It can actually suppress people’s immune systems for a few years after having that measles infection,” Pan said.

The measles outbreaks in Texas, Kansas and New Mexico are so severe that state health officials are recommending early vaccination in babies as young as 6 months in the hardest-hit counties. Those officials are also recommending a second dose for many children earlier than the typical schedule.

The standard CDC recommendation is that children receive the first of two measles vaccine shots when they turn 1, unless they are traveling internationally, in which case they can get vaccinated at 6 months old.

In California, there is no active measles outbreak, so there’s no recommendation for additional or accelerated vaccine doses, Pan said. Most of California’s measles cases this year have involved people who traveled internationally.

Helping matters is California’s relatively high vaccination rate among kindergartners. For the 2023-24 school year, 96.2% of California’s kindergartners were vaccinated against measles, according to the CDC — one of the higher rates of any state.

The latest figure is down slightly from the 96.5% seen the year earlier, but it remains above the levels seen prior to 2014-15, when a severe measles outbreak linked to Disneyland prompted state legislators to strengthen vaccination requirements for children who attend school.

Experts aim for a 95% vaccination rate for measles to guard against outbreaks.

Today, California’s measles case rate remains much lower than the national rate. But that doesn’t mean the state is invulnerable.

There are settings in which unvaccinated people can cluster together, such as group outings of children who are homeschooled.

Another vulnerable setting can be doctor’s offices. In 2008, an unvaccinated 7-year-old boy returned home to San Diego from a trip to Switzerland, became sick, then went to school and was taken to the pediatrician. Investigators found that measles then spread to 11 other children who were either unvaccinated or too young to be vaccinated — five in his school, four who were in the pediatrician’s office at the same time he was, and both of his siblings.

Among those infected were three infants younger than 1, one of whom was hospitalized for two days, according to a report published by the CDC. Another infant traveled by plane to Hawaii while infectious.

Some California counties also have measles vaccination rates for kindergartners that are below the 95% goal. For the 2023-24 school year, San Diego County’s rate is 94.8%; San Bernardino County’s is 93.5%; Santa Cruz County’s is 91.8%; and Kern County’s is 90.7%.

In 2017, a measles outbreak in Los Angeles County was centered in an Orthodox Jewish community, with a number of patients living in Westside L.A., the Santa Monica Mountains and the San Fernando Valley. In early 2019, California’s biggest measles outbreak was in Butte County, and started after a man visited the Philippines, a country that was then in the midst of a massive outbreak.

The last time measles caused scores of deaths nationwide was in the late 1980s and early 1990s. Between 1989 to 1991, there were about 55,000 cases and more than 130 suspected measles-associated deaths, according to the CDC.

In California, that epidemic was the worst between 1988 and 1991 — with 18,000 cases, 3,500 hospital admissions and 70 deaths reported, according to state health officials. A big problem officials identified at that time was that poor parents could not afford vaccinations for their children. One of the nation’s largest outbreaks in 1989 involved mostly preschool-aged children in Los Angeles.

In light of the outbreak, the CDC’s Advisory Committee on Immunization Practices recommended a second dose of the measles vaccine be given between the ages of 4 and 6. And in the early 1990s, the federal government created the Vaccines for Children program, which funds a number of vaccinations for children with low-income parents. In 2000, officials declared that measles was no longer being continuously transmitted locally in the U.S., and new cases were only found when someone was infected abroad.

L.A. County has recorded four cases of measles so far this year among residents, up from one all of last year. Most of the cases this year were associated with someone who either traveled internationally or to domestic areas suffering community transmission of measles, according to the local Department of Public Health.

There were also two reported cases among non-county residents this year, including an infant traveling through Los Angeles International Airport on a flight from South Korea who was returning home to Orange County.

In 2024, there were six non-L.A. County residents who had measles while traveling through L.A. County; four had traveled through Los Angeles International Airport while infectious.

“With measles cases rising across the country and globally, it’s important for unprotected individuals to consider preventive measures. Immunization provides the best protection. Additionally, anyone who was exposed should monitor for symptoms,” the Orange County Health Care Agency said in a statement to The Times.

Symptoms include fever, rash, cough and red, watery eyes. People can be contagious from about four days before the rash begins through four days afterward.

Those suspecting they have measles should call their medical provider before they go to the doctor’s office to avoid potentially exposing other patients, the Health Care Agency said.

The rash traditionally starts at the hairline and moves its way down the body, according to Pan.

Kaiser has seen one case of measles this past month, which was in the Santa Clarita area, Hudson said. Health officials said people were possibly exposed to the contagious individual at a Costco, Trader Joe’s and Walmart on the afternoon of May 29.

“The MMR vaccine, which protects against measles, mumps, and rubella, has an excellent safety record. Hundreds of millions of children have safely received the vaccine worldwide,” she said in a statement to The Times.