The “twindemic” of influenza and coronavirus feared by public health officials has so far failed to materialize.
The flu typically kills tens of thousands of Americans and can account for hundreds of thousands of hospitalizations during the winter season. But this year, it’s fizzled almost entirely in the Bay Area — a stroke of fortune needed as coronavirus cases, hospitalizations and deaths surge uncontrolled statewide.
“The flu is essentially not present in Northern California at this point,” said Dr. Randy Bergen, a pediatric infectious disease specialist at Kaiser who is also the clinical lead for Northern California’s flu vaccine program. “Our hospitals are always filled during the winter, and in normal years, it’s because of influenza. This is a very abnormal year, but it’s because of a different respiratory virus.”
A typical year sees about a 20%-40% positive rate for influenza tests in the hospital, Bergen said, but this year, doctors are not really seeing any such cases. About one in four patients who come to the hospital for respiratory symptoms test positive for the coronavirus, but the three others are in for something else: not the coronavirus, not the flu, and not respiratory syncytial virus, or RSV, another common respiratory virus that accounts for thousands of deaths each year in the U.S. but that has also fallen off the map entirely.
Dr. Gary Green, an infectious disease specialist with Sutter Medical Group of the Redwoods in Santa Rosa, says physicians there have seen only sporadic cases of influenza A and influenza B — with consistently more influenza A in the past 3-4 weeks.
The minimal flu season in the Bay Area reflects what was already demonstrated in the Southern Hemisphere, in places like Australia, New Zealand and South Africa, where flu season passed with no influenza to speak of. That same trend is now being reflected in the U.S. and in California, which are seeing much lower rates of influenza activity this season than usual.
According to Centers for Disease Control and Prevention data tracking doctor visits for influenza-like symptoms as a percentage of total visits, California has hovered between 1% and 2% since the season began Oct. 1. In the past four years, that share has ranged anywhere from 3.5% to 6.5% by this time.
Reports from the California Department of Public Health’s Influenza Surveillance Program, which tracks the flu in weekly reports, paint an even more startling picture of a flu season that has gone off the radar.
Take the first week of January, for example, which marks the halfway point of flu season, when cases and hospitalizations typically reach a peak. In the 2019-20 season, that week saw widespread flu activity around the state, with more than 26.9% testing positive for flu in the laboratory, 19 outbreaks and 70 deaths for the season to that point. Hospitalizations were above expected levels.
The 2020-21 report couldn’t look more different, with 0% flu admissions to the hospital, a 0.3% positive test rate for flu in the lab, and zero outbreaks since the fall. Seven deaths were reported, but a map of California shows almost no flu activity around the state.
So what happened?
So far, some experts have tied this year’s quiet season to a robust flu vaccine rollout, which saw people getting their shots in droves, some for the first time and many earlier than usual. But that still doesn’t explain the quieting, said Dr. Lawrence Drew, a retired virologist who headed UCSF’s clinical virology laboratory.
“The best we ever see in the U.S. for vaccine cooperation is 50-60% and that would not be enough to explain it, and it wouldn’t explain RSV,” he said.
Some people, Drew said, have also ascribed the quiet season to an idea called viral interference, when an organism that has been infected with one virus somehow resists infection with a second virus. But that too seems unlikely, he said, because it typically involves similar viruses — and influenza, SARS COV-2 and RSV are not similar.
What is far and away the likeliest explanation, Drew and other experts say, is that the pandemic public health measures of masking and social distancing are extremely effective for preventing the spread of influenza and RSV — even more than for the spread of the coronavirus.
There’s a scientific basis for that: Studies have shown that SARS COV-2 is far more transmissible than the major seasonal respiratory viruses, which include influenza and RSV. A new variant of the coronavirus, which has made its way to the U.S. and California, is even more contagious, with some figures pointing to a 70% increase in transmissibility.
According to the CDC, flu activity is unusually low across the country this season. Though California has now surpassed the nation’s rate for flu-like illness, such spikes are not uncommon in flu seasons. They could also be tied to the state’s mounting surges of coronavirus cases, mostly in Southern California, which officials have attributed in part to more lax adherence to public health guidelines. (At this time last year and at the start of the current flu season, California’s average was lower than the nation’s.)
At the end of December, doctor’s visits for influenza-like illness in California accounted for 2.3% of total visits, while the nation’s rate was 1.6%. Drew suspects the Bay Area may be doing better than other parts of the state — and country — that may not be adhering to the public health guidelines with as much rigor.
There could be another “wild card” factor for the slow flu season, according to Bergen, the Kaiser pediatric infectious disease specialist: Influenza seasons almost always start in schools, and most schools in the Bay Area are still remote or hybrid, with mask-wearing and social distancing.
The coronavirus vaccine rollout in California and the U.S. has gotten off to a slow start, and the long COVID-19 year is likely months away from getting under control. And flu season doesn’t always peak in December or January, Bergen said. As a result, if people stop wearing masks, social distancing and following hand hygiene, a twindemic could still be on the horizon.
Drew’s long view has even more urgency. If people come out of this flu season and approach the next as if this one never happened — avoiding masking, social distancing and so forth — all that safety will reverse. The guidelines that have helped us avoid the flu this year, he says, are crucial to continue.
“A couple years from now — five or 10 or maybe only one — we are going to have an influenza pandemic,” Drew said, adding that variants are likely to crop up that scientists will not have vaccines for. “Everybody knows it. … There is no question about it. We’re way overdue.”