2 Children Die As 'Super Flu' Sickens CA. How Effective Is The Vaccine?

California health officials confirmed the first two pediatric flu deaths of the season this month as cases climb statewide.

The latest variant, subclade K, has been dubbed the "super flu." Subclade K is a mutation of the H3N2 strain, and it's responsible for the bulk of positive tests in California.

“It seems to be a more aggressive strain, and it seems to be a tougher problem,” Sam Torbati, MD, co-chair and medical director of the Cedars-Sinai Emergency Department, said in a written statement. “Last year, I don't recall this many patients becoming this ill.”

Between the week ending Dec. 27, 2025, and the week ending Jan. 17, more than 30,00 cases of the flu were confirmed in the Golden State, according to the California Department of Public Health. Many of the cases have involved children. Over the same period, roughly a third of confirmed cases involved people younger than 18.

The confirmed cases, which reached high levels in the Bay Area and Central Valley, are likely an undercount, wastewater data shows.

Torbati said it is still early in the West Coast flu season.

“It may get much worse,” he warned.

Influenza A strain H3NS has mutated seven times, making the flu a more serious threat than in years past. It caused a severe flu season around the world, including in the U.K., Canada, Japan and Australia. Recent CDC data show that nearly all virus samples since late September were the subclade K mutation.

More: New Flu Variant Raises Concerns About Bad Flu Season

According to Dr. Monica Gandhi, an infectious disease expert at UC San Francisco, respiratory infections have slammed the Golden State this winter.

“If you think everyone around you is sick … it’s true,” she told SFGATE.

As of Saturday, state health officials confirmed 139 flu fatalities this season, 26 RSV deaths and 917 COVID deaths — mainly linked to the summer surge in cases.

Health officials in San Francisco confirmed two flu deaths this week.

“It is always sad when members of our community pass away because of the flu," San Francisco Health Officer Dr. Susan Philip said in a statement Friday, urging people with symptoms to go to a health care provider. "This unfortunate news is a reminder that the flu should not be taken lightly and that it is not too late to get vaccinated."

The CDC said earlier this month that the current influenza wave is expected to last several weeks. The agency also noted that RSV activity is elevated in many areas of the country, with emergency room visits and hospitalizations increasing in children under 4. COVID-19 activity is low but increasing nationally.

Health officials continue to recommend vaccination to defend against flu infection. Vaccines are updated each year to tackle the most common strains.

How Effective Is The Vaccine?

While this year's flu shot doesn't cover subclade K as it wasn't identified when the vaccine was developed, health experts say that getting a flu shot is still the best way to prevent infection.

“Current seasonal flu vaccines remain effective at reducing severe illness and hospitalization, including the currently circulating viruses,” said Dr. Erica Pan, CDPH Director and State Public Health Officer. “It is important for families to know that flu vaccines, tests, and treatments remain widely available for all Californians and that it is not too late to get a flu vaccine. As a pediatrician, parent, and daughter, I have made sure that my own family is protected with this season’s flu vaccine.”

Though a flu vaccine is typically recommended earlier in the season, officials say it is not too late to get one. While it isn't perfectly formulated against the current strain, it can still reduce symptoms and shorten the length of the infection, according to health experts.

There's also some evidence that the vaccine is more effective than experts thought. A new study from the University of Pennsylvania found that this year's vaccine "will likely be somewhat effective at preventing H3N2 subclade K virus infections," though that study has not been peer-reviewed yet.

The study found that despite the strains in the vaccine not being a perfect match to subclade K, it elicited antibodies against the current popular strain. It was the same for all ages, from 24 to 81.

"Our study highlights the benefits of receiving influenza vaccinations, even in seasons that include circulation of variant viruses," the study concludes.

More people who get the flu are able to recover at home with rest, fluids and over-the-counter medications. If symptoms are severe or worsen, or if you are at high risk for complications, contact your health care provider.

People who are sick should seek emergency care right away if they have trouble breathing, constant chest pain, confusion, severe vomiting, or if they start to feel better but then get worse again with a higher fever or stronger cough. For children, get emergency care if their skin looks bluish, they are not drinking enough fluids, or they are very sleepy and hard to wake.

Patch staffers Fernando Haro, Jack Slocum and Veronica Flesher contributed to this report.