Last year’s flu season was the worst California had seen in years — and state health officials warn this year could potentially be just as bad.

While forecasting disease isn’t an exact science, there are some troubling signs. In Asia, the flu has made an early comeback, and quickly swelled to epidemic proportions in Japan and Taiwan.

And stateside, some experts are sounding the alarm about continued lower uptake of the flu vaccine. There’s also the possibility of a seasonal COVID-19 wave — the likes of which didn’t materialize last winter, but had been commonplace since the pandemic — as well as a simultaneous rise in respiratory syncytial virus, or RSV.

California health officials have previously forecast that this fall-and-winter’s respiratory virus season is expected to be similar to last year’s. If that’s the case, flu would again be the dominant virus fueling hospital admissions compared to COVID-19 and RSV. During the winters of 2022-23 and 2023-24, COVID made up the majority of California hospital admissions caused by respiratory viruses.

“Having people get vaccinated is going to be really key for influenza,” said Dr. Peter Chin-Hong, a UC San Francisco infectious diseases expert.

While flu, COVID and RSV are currently at low levels in California, there are signs that respiratory virus season is starting to gear up as temperatures drop and people spend more time indoors.

Compared to rates recorded in the summer, the Los Angeles County Department of Public Health has seen increases in people reporting cough, fever, chills, aches, sore throat and runny nose, officials said, citing a text-based health survey. The test positivity rate for rhinoviruses and enteroviruses, which typically cause the common cold, is 19.87%. That’s higher than that of the virus that causes COVID-19, 4.2%; or the flu, 1.04%.

In San Francisco, doctors have seen the number of colds being reported in the hospital double, Chin-Hong said. Some workplaces in the Bay Area have seen a number of employees call out sick.

With RSV, flu and COVID-19 rates expected to climb by the holidays, “this is a perfect time to get immunized,” the L.A. County Department of Public Health said in an email.

The fall and winter of 2024-25 brought the nation’s worst flu season in many years, according to the U.S. Centers for Disease Control and Prevention.

The number of children who died from flu last season was the highest since the H1N1 swine flu pandemic season of 2009-10, according to a recent report published in the CDC’s Morbidity and Mortality Weekly Report. Officials reported 280 confirmed deaths among children last season, and about 9 in 10 of those kids were not vaccinated.

Health officials became particularly concerned about reports of a rare, severe complication that targets the brain — influenza-associated encephalopathy, or IAE, which was reported in 109 children nationally last season.

Three in four children with IAE needed intensive care treatment. The median age of children with IAE was 5, and 55% of those diagnosed with the condition had no underlying health conditions.

Of those children, 37 had acute necrotizing encephalopathy, or ANE, a particularly severe form of the disease that results in rapid neurologic decline. Two in five children with ANE died.

Only 1 in 6 of the kids with IAE who were eligible to be vaccinated had gotten the flu shot, health officials say.

The CDC recommends that everyone age 6 months and up get a flu vaccine. New this year, officials are allowing people to order FluMist to be mailed to them at home. FluMist is approved for people ages 2 through 49 and administered by nasal spray rather than injection.

Doctors have been concerned about declining flu vaccination levels. As of late April, just 49.2% of children had gotten a flu shot, 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 much lower than the final flu vaccination rate for vaccine-eligible children during the 2019-20 season, which was 63.7%.

Among adults, 46.7% had gotten their flu shot, slightly down from the 47.4% at the same point last season, according to the preliminary survey results.

“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,” the CDC said.

Early data from the Southern Hemisphere indicate that the flu shot’s effectiveness was decent this season — cutting the risk of hospitalization by 50%.

Asia is already reporting plenty of flu cases.

“For influenza right now, it’s raging in Asia,” Chin-Hong said.

Flu is considered to be at epidemic levels in Japan — where news reports say this was the second-earliest start to the season in 20 years — and in Taiwan. Hong Kong health officials have described the flu situation in September and October, before shots became widely available, as “relatively severe.”

The situation there could provide a glimpse of how the flu season will play out in California and beyond.

In the San Francisco Bay Area, officials are also closely monitoring “high” levels in wastewater of a specific kind of cold virus — enterovirus D68, or EV-D68. In rare cases, that virus can cause polio-like paralysis in children called acute flaccid myelitis, or AFM. High levels were reported in a large swath of the Silicon Valley and San Francisco, according to WastewaterSCAN. High levels have also been found in sewage in western San Bernardino County, including Ontario, Chino and Fontana.

The L.A. County Department of Public Health has not yet detected increased signals of EV-D68, and no cases of AFM have been reported this year in Los Angeles or Orange counties.

That virus can transmit through an infected person’s saliva and mucus and likely spreads “when an infected person coughs, sneezes or touches a surface that is then touched by others,” the CDC said.

Parents should call a doctor if their child suffers any symptoms of AFM, which include slurred speech, difficulty swallowing, difficulty moving the eyes, drooping eyelids, pain in the neck, back, arms or legs, weakness in the arm or legs or facial droop.

To protect yourself against respiratory viruses, experts recommend getting vaccinated, washing your hands often, keeping rooms well-ventilated, wearing face masks in crowded indoor public settings, and staying away from sick people.

The California Department of Public Health recommends updated COVID-19 vaccines for everyone age 6 months to 23 months, seniors age 65 and up; as well as older children, teenagers and adults who either have risk factors for severe COVID, or are in close contact with at-risk people.

Children who have never been inoculated against COVID-19 should also get the vaccine, as should pregnant women and anyone else who wants to get the shot, the state says.

RSV immunizations are recommended for everyone age 75 and up, babies younger than 8 months, and pregnant women between 32 weeks to 36 weeks of gestation.

The immunizations are also recommended for adults age 50 to 74 with risk factors, as well as babies with risk factors between 8 months and 19 months. Older adults who have been previously immunized against RSV generally don’t need to get another vaccine, according to current guidelines.