California could see an early start to the annual flu season, as a combination of low vaccination rates and late mutations to the virus may leave the state particularly exposed to transmission, health experts say.
Already, there are warning signs. Los Angeles County recently reported its first flu death of the season, and other nations are reporting record-breaking or powerful, earlier-than-expected flu seasons.
Typically, flu picks up right after Christmas and into the New Year, but Dr. Elizabeth Hudson, regional physician chief of infectious diseases at Kaiser Permanente Southern California, said she expects increases in viral activity perhaps over the next two to three weeks.
“We’re expecting an early and likely sharp start to the flu season,” Hudson said.
Last year’s flu season was the worst California had seen in years, and it’s not usual for there to be back-to-back bad flu seasons. But a combination of a decline in flu vaccination rates and a “souped-up mutant” is particularly concerning this year, according to Dr. Peter Chin-Hong, an infectious diseases expert at UC San Francisco.
“That may translate into more people getting infected. And as more people get infected, a proportion of them will go to the hospital,” Chin-Hong said.
The timing of this new flu subvariant — called H3N2 subclade K — is particularly problematic. It emerged toward the end of the summer, long after health officials had already determined how to formulate this fall’s flu vaccine, a decision that had to be made in February.
H3N2 subclade K seems to be starting to dominate in Japan and Britain, Hudson said.
“It looks like a bit of a mismatch between the seasonal flu vaccine strains” and the new subvariant, Hudson said.
It remains unclear whether subclade K will reduce the effectiveness of this year’s flu shot.
In California and the rest of the U.S., “things are quiet, but I think it’s just a calm before the storm,” Chin-Hong said. “From what we see in the U.K. and Japan, a lot more people are getting flu earlier.”
Chin-Hong noted that subclade K is not that much different than the strains this year’s flu vaccines were designed against. And he noted data recently released in Britain that showed this season’s vaccines were still effective against hospitalization.
According to the British government, vaccinated children were 70% to 75% less likely to need hospital care, and adults were 30% to 40% less likely. Flu vaccine effectiveness is typically between 30% to 60%, and tends to be more effective in younger people, the British government said.
Even if there is some degree of mismatch between the vaccine and circulating strains, “the flu vaccine still provides protection against severe illness, including hospitalizations,” according to the Los Angeles County Department of Public Health.
“Public Health strongly encourages everyone who has not received the flu vaccine yet this year to receive it now, especially before gathering with loved ones during the holidays,” the department said in a statement.
But “while mismatched vaccines may still provide protection, enhanced genetic, antigenic and epidemiological … monitoring are warranted to inform risk assessment and response,” according to scientists writing in the Journal of the Assn. of Medical Microbiology and Infectious Diseases Canada.
Because the vaccine is not a perfect match for the latest mutated flu strain, Chin-Hong said getting antiviral medication like Tamiflu to infected patients may be especially important this year, even for those who are vaccinated. That’s especially true for the most vulnerable, which include the very young and very old.
“But that means you need to get diagnosed earlier,” Chin-Hong said. Drugs like Tamiflu work best when started within one to two days after flu symptoms begin, the U.S. Centers for Disease Control and Prevention says.
There are now at-home flu testing kits that are widely available for sale for people who are showing signs of illness.
Also worrying is how the flu has surged in other countries.
Australia’s flu season came earlier this year and was more severe than usual. The Royal Australian College of General Practitioners said that nation saw a record flu season, with more than 410,000 lab-confirmed cases, up from the prior all-time high of 365,000 that were reported last year.
“This is not a record we want to be breaking,” Dr. Michael Wright, president of the physician’s group, said.
Hudson noted Australia’s flu season was “particularly hard on children” this year.
L.A. County health officials cautioned that Australia’s experience isn’t a solid predictor of what happens locally.
“It is difficult to predict what will happen in the United States and Los Angeles, as the severity of the flu season depends on multiple factors including circulating strains, pre-existing immunity, vaccine uptake, and the overall health of the population,” the L.A. County Department of Public Health said.
The new strain has also thrown a wrench in things. As Australia’s flu season was ending, “this new mutation came up, which kind of ignited flu in Japan and the U.K., and other parts of Europe and Asia,” Chin-Hong said.
On Friday, Japan reportedly issued a national alert with flu cases surging and hospitalizations increasing, especially among children and the elderly, accompanied by a sharp rise in school and class closures. The Japanese newspaper Asahi Shimbun said children ages 1 through 9 and adults 80 and up were among the hardest-hit groups.
Taiwanese health officials warned of the possibility of a second peak in flu this year, according to the Central News Agency. There was already a peak in late September and early October — a month earlier than normal — and officials are warning about an uptick in flu cases starting in December and then peaking around the Lunar New Year on Feb. 17.
Taiwanese officials said 95% of patients with severe flu symptoms had not been recently vaccinated.
British health officials this month issued a “flu jab SOS,” as an early wave struck the nation. Flu cases are “already triple what they were this time last year,” Public Health Minister Ashley Dalton said in a statement.
In England, outside of pandemic years, this fall marked the earliest start to the flu season since 2003-04, scientists said in the journal Eurosurveillance.
“We have to brace ourselves for another year of more cases of flu,” Chin-Hong said.
One major concern has been declining flu vaccination rates — a trend seen in both Australia and the United States.
In Australia, only 25.7% of children age 6 months to 5 years were vaccinated against flu in 2025, the lowest rate since 2021. Among seniors age 65 and up, 60.5% were vaccinated, the lowest rate since 2020.
Australian health officials are promoting free flu vaccinations for children that don’t require an injection, but are administered by nasal spray.
“We must boost vaccination rates,” Wright said.
In the U.S., officials recommend the annual flu vaccine for everyone age 6 months and up. Those age 65 and up are eligible for a higher-dose version, and kids and adults between age 2 and age 49 are eligible to get vaccinated via the FluMist nasal spray, rather than a needle injection.
Officials this year began allowing people to order FluMist to be mailed to them at home.
Besides getting vaccinated, other ways to protect yourself against the flu include washing your hands frequently, avoiding sick people and wearing a mask in higher-risk indoor settings, such as while in the airport and on a plane.
Healthy high-risk people, such as older individuals, can be prescribed antiviral drugs like Tamiflu if another household member has the flu, Chin-Hong said.
Doctors are especially concerned about babies, toddlers and young children up to age 5.
“Those are the kids that are the most vulnerable if they get any kind of a respiratory illness. It can really go badly for them, and they can end up extraordinarily ill,” Hudson said.
In the United States, just 49.2% of children had gotten a flu shot as of late April, lower than the 53.4% who had done so at the same point the previous season, according to preliminary national survey results. Both figures are well below the final flu vaccination rate for eligible children during the 2019-20 season, which was 63.7%.
Among adults, 46.7% had gotten their flu shot as of late April, slightly down from the 47.4% at the same point last season, according to the preliminary survey results, which are the most recent data available.
“Before the COVID-19 pandemic, flu vaccination coverage had been slowly increasing; downturns in coverage occurred during and after the pandemic. Flu vaccination levels have not rebounded to pre-pandemic levels,” according to the CDC.
The disparaging of vaccinations by federal health officials, led by the vaccine-skeptic secretary of the U.S. Department of Health and Human Services, Robert F. Kennedy Jr., has not helped improve immunization rates, health experts say. Kennedy told the New York Times on Thursday that he personally directed the CDC to change its website to abandon its position that vaccines do not cause autism.
Mainstream health experts and former CDC officials denounced the change. “Extensive scientific evidence shows vaccines do not cause autism,” wrote Daniel Jernigan, Demetre Daskalakis and Debra Houry, all former top officials at the CDC, in an op-ed to MS NOW.
“CDC has been updated to cause chaos without scientific basis. Do not trust this agency,” Daskalakis, former director of the CDC’s National Center for Immunization and Respiratory Diseases, added on social media. “This is a national embarrassment.”
State health officials from California, Washington, Oregon and Hawaii on Friday called the new claims on the CDC website inaccurate and said there are decades of “high quality evidence that vaccines are not linked to autism.”
“Over 40 high-quality studies involving more than 5.6 million children have found no link between any routine childhood vaccine and autism,” the L.A. County Department of Public Health said Friday. “The increase in autism diagnoses reflects improved screening, broader diagnostic criteria, and greater awareness, not a link to vaccines.”
Hudson said it’s important to get evidence-based information on the flu vaccines.
“Vaccines save lives. The flu vaccine in particular saves lives,” Hudson said.
Rong-Gong Lin II
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