San Francisco Chronicle - (Sunday)
Spring flu surge possible after mild winter
For the second consecutive year, California had an unusually mild flu season. But health experts warn that a late spring surge could be on the way.
While outbreaks so far have been far fewer than in a typical flu season, infections are ramping up more than usual for this time of year in some parts of the country, federal data shows, as people emerge from pandemic caution.
California has seen only four influenza outbreaks and 36 flu deaths since the start of the 2021-22 season in early October. In a typical year, the total deaths for the flu season are around 100. As of Monday, positive flu test results and doctor’s office visits have also remained low as the flu season nears its typical close, according to public health data.
That is quite a turnaround from three years ago, before the coronavirus pandemic, when the state saw one of its deadliest flu seasons in the past decade.
“The obvious explanation is the same precautions that work against COVID-19 work against influenza,” said Dr. John Swartzberg, an infectious diseases expert at UC Berkeley. “It’s not surprising since transmission of the viruses is very similar.”
But experts warn that could change now that public health leaders have pulled back requirements to limit the spread of the coronavirus, such as mask wearing, and guidelines including social distancing.
As people become less cautious, are less meticulous about hygiene, and resume pre-pandemic habits of movement and activity, including a return to indoor restaurants and office work, there is a chance that the flu and other respiratory viruses could surge before the flu season’s typical last gasp around May.
“It’s easier to escape flu when you’re protecting yourself against COVID,” said Dr. Peter Chin-Hong, an infectious diseases expert at UCSF. “What we’ve learned in the last few years is that human behavior affects disease impact.”
After last year’s flu season, which was the mildest on record, there was speculation that cases and hospitalizations would return to more normal levels this year.
That has yet to happen, despite reports of fewer people getting vaccinated against the flu compared with last year. Those flu shots were also a flop, with only 16% efficacy against the influenza strain in circulation, data from the U.S. Centers for Disease Control and Prevention indicates.
“The irony here is we had such a terrible winter with omicron,” Swartzberg said, referring to the coronavirus variant that spurred a major winter COVID surge nationwide. “Omicron is so transmissible that close to 50% of the American population got infected in less than 10 weeks. That made people become extremely cautious and hunker down.”
There was also some concern that the Bay Area could face a twindemic, or rise in co-infection of flu and the coronavirus at the same time, dubbed “flurona.”
That didn’t happen, either. “It turned out to be an appropriate concern, but it didn’t turn out to be anything,” Swartzberg said, citing the unpredictable nature of SARS-Cov-2.
Theories have arisen around the idea that perhaps the coronavirus crowded out the flu, causing viral interference. Swartzberg said such theories require more research but may prove unlikely.
Immunity granted by coronavirus infections “does not give us any cross-reactivity with influenza,” he said. “It’s how we behave. I haven’t had a cold since this pandemic began. Just washing your hands more often has been an enormous help in limiting the spread of respiratory viruses.”
In a normal season, the flu remains a serious threat. So far this season, 16 children have died from the flu, including two in the past week. In contrast, 199 children died in the pre-pandemic year of 2019, and 144 the year before that.
But influenza activity is on the rise nationally, according to the CDC, especially in the central and south-central regions of the country.
Hospitalizations for flu this year so far total 8 per 100,000 population by the CDC’s estimates, compared with 0.8 per 100,000 in the 2020-21 season. Both years reflect dramatically smaller rates than the prepandemic 2019-20 season which saw 66 flu hospitalizations per 100,000 people.
The nation so far has recorded at least 3.8 million flu cases, 38,000 hospitalizations, and 2,300 deaths related to the flu in this season, the CDC reports.
In California, the percent of positive tests for flu is 4.6%, compared with 0% for the same time last year, state health officials report.
“Flu season typically goes until the end of April or May, so there’s time for people to catch flu,” Chin-Hong said. “All infections may be out of whack because the seasons got shifted by COVID. The interesting thing we learned in the summer of 2021 is we were seeing more RSV (a common cold-like respiratory virus) in the summer. Maybe the flu season may be atypical this year.”
Either way, a quieter flu season now may also portend a more severe one next year, with less natural immunity coming out of the current season and less opportunity to formulate an effective flu vaccine because of low infection levels. Typically, flu vaccines are tailored each year to the strain that’s expected to dominate.
“Whenever we do get a typical flu season, it’s going to be bad,” Chin-Hong said. “Flu doesn’t give you a lot of chances to make predictions.”