What will flu be like in the Bay Area this fall? Here’s what experts say

And because the flu's symptoms — muscle aches and fever — are so similar to another dreaded disease, "people will go crazy," he said. "Every time your kid has the sniffles, you'll be worried about COVID."

Kaiser Permanente, a health system so big that it can forecast off its own flu data each year, estimates that influenza is likely to rebound and could hit the Bay Area by late November.

"I'm predicting it will be higher because last year it was pretty much a zero," said Dr. Darvin Scott Smith, an infectious disease specialist at Kaiser Medical Center in Redwood City. "It's tough to know, but the changes with respect to what people are doing as things open up would indicate that."

Flu cases in the Southern Hemisphere, where it's winter, are already higher this month than at the same time last year, the World Health Organization reports, though still far below 2019 levels.

If the flu does surge, Chin-Hong and other experts warn that the vaccines in the U.S. could be less well-matched to the strains this year than usual, because there was so little flu around the world last year.

Scientists depend on data from influenza outbreaks, usually in the Southern Hemisphere, where flu season hits earlier than in the U.S., to create each year's new flu vaccine in a way they believe will best be able to combat whatever the current strains are.

Because the vaccine's benefits are believed to last about six months, it's necessary to get a shot every year. But the influenza virus mutates frequently, so vaccine scientists are always trying to stay on top of it.

"It's kind of a guessing game," Chin-Hong said, noting that scientists had even fewer clues to go on this year because of the reduced case numbers globally.

The World Health Organization credits not only pandemic protections for the drop in flu cases, but "viral interference," a phenomenon where one virus can weaken another in the same host — in this case, COVID-19 inhibiting or even preventing flu infections. Even so, experts caution that people can have both illnesses at the same time.

In a typical year, scientists often create vaccines with components to fight off three flu viruses, hedging their bets. For 2021, the Centers for Disease Control and Prevention says it added a fourth.

The new flu vaccine will fight two influenza A viruses and two B viruses, the CDC reports. Many pharmacies are already offering the vaccine, while Bay Area medical centers are accepting appointments for rollouts this month.

One new development is that people can get a COVID vaccine at the same time as the flu vaccine, rather than having to wait as originally advised.

It's unclear whether the overall pandemic-era push for vaccinations will lead to more people getting a flu shot. By the end of February, manufacturers had handed out 194 million doses of flu vaccine in the U.S. for last winter — more than ever, although the CDC says some of it probably went unused.

This year, manufacturers expect to distribute about the same amount.

Unlike COVID vaccinations, which are increasingly required for participating in public life, flu shots are rarely mandated. In an unusual move, the University of California required flu shots last year for those on campus —but but UC officials say they haven't decided whether to do that again this year.

Despite this year's extra uncertainty about whether the flu vaccine will match the virus that actually shows up, public health experts say getting the shot is important for everyone older than 6 months, including pregnant people — especially because COVID-19 is surging among those unvaccinated for the coronavirus.

Protecting against the flu will help California again avoid the double viral hit it escaped last year, experts say. Hospitals and intensive care units were near capacity last year because of COVID — add a flu surge, and it could happen again.

"That is a very real risk," said Dr. Jeffrey Silvers, medical director of infectious disease at Sutter Health. "If you aren't vaccinated, you could be in trouble."

He added: "You could end up in the hospital. You could die. You could give it to a loved one. Or you could be the source of spreading the disease."

Elizabeth Worthington is taking no chances. She showed up at a CVS pharmacy in San Francisco on Thursday for her flu shot.

At 20, she's at an age where people often feel invulnerable to the viruses that exist, unseen, on surfaces, in water, in the air and in our bodies. But Worthington knows all too well what it feels like to have the flu — and how easy it is to pick it up at college, where she will be a junior this year at the University of Oregon.

"You just feel really weak, really achy," she recalled. "My throat always feels sore, and I feel like I can't swallow."

She said she's gotten the flu a handful of times even though she's been getting vaccinated yearly since babyhood.

Recent studies indicate that the vaccine reduces flu risk by 40% to 60% when it's well matched to the ever-evolving virus, the CDC says. Yet in 2019, before the pandemic, flu shots prevented 7.5 million cases and saved 6,300 people from dying of influenza, the health agency reports.

"I figure it probably doesn't hurt to keep trying," Worthington said, moments before baring her left arm unflinchingly for nurse practitioner Diana Huang to inject the fluid of hope. Worthington said she understands that some people can't be vaccinated for medical reasons, but that others who avoid it "are kind of selfish" because they can infect vulnerable people.

"It's a life-and-death situation," she said.

The popularity of the internet search term "flu 2021" suggests that many people are aware of this. The term hit a peak in popularity on Google Trends in early September, while "flu 2020" peaked last October.

Other internet searches reveal that the infamous scare tactics intended to misinform people about COVID-19 are bleeding into the influenza world, as well. One such myth is that the CDC is revoking authorization for COVID tests, claiming they can't distinguish between the coronavirus and influenza.

Chin-Hong slapped that one down quickly, noting that the viruses look nothing alike.

"It's not even comparing apples to oranges," he said. "It's more like saying: Is this a flu or is this a steak?"

Nanette Asimov is a San Francisco Chronicle staff writer. Email: [email protected] Twitter: @NanetteAsimov