The “twindemia” of the flu and coronavirus feared by public health authorities has not materialized until now.
The flu usually kills tens of thousands of Americans and can be responsible for hundreds of thousands of hospitalizations during the winter. But this year, he failed almost entirely in the Bay Area – a necessary stroke of luck as coronavirus cases, hospitalizations and deaths escalate wildly across the state.
“Flu is not essentially present in Northern California at this time,” said Dr. Randy Bergen, a Kaiser pediatric infectious disease specialist who is also the clinical leader of the Northern California flu vaccine program. “Our hospitals are always full during the winter and, in normal years, it is because of the flu. This is a very abnormal year, but it is because of a different respiratory virus. “
A typical year sees about a 20% -40% positive rate for flu tests in the hospital, Bergen said, but this year, doctors are not really seeing any of those cases. About one in four patients who arrive at the hospital due to respiratory symptoms test positive for coronavirus, but the other three are about to do something else: not the coronavirus, not the flu and not the respiratory syncytial virus, or RSV, another This common respiratory virus is responsible for thousands of deaths each year in the United States, but it has also disappeared entirely from the map.
Dr. Gary Green, an infectious disease specialist at the Sutter Medical Group of the Redwoods in Santa Rosa, says doctors there have seen only sporadic cases of influenza A and influenza B – with consistently more influenza A over the past 3-4 weeks .
The minimum flu season in the Bay Area reflects what has already been demonstrated in the southern hemisphere, in places like Australia, New Zealand and South Africa, where the flu season passed without any flu. That same trend is now being reflected in the United States and California, which are seeing much lower flu activity rates this season than usual.
According to data from the Centers for Disease Control and Prevention that track visits to the doctor for flu-like symptoms as a percentage of total visits, California has fluctuated between 1% and 2% since the start of the season on 1 January. October. In the past four years, this proportion has varied from 3.5% to 6.5% at 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 surprising picture of a flu season that came off the radar.
Consider, for example, the first week of January, which marks the middle of the flu season, when cases and hospitalizations usually peak. In the 2019-20 season, that week saw widespread flu activity across the state, with more than 26.9% testing positive for flu in the lab, 19 outbreaks and 70 deaths in the season to that point. Hospitalizations were above expectations.
The 2020-21 report could not be more different, with 0% of flu admissions in the hospital, a positive test rate of 0.3% for flu in the lab and zero outbreaks since the fall. Seven deaths have been reported, but a map of California shows almost no flu activity in the state.
So what happened?
So far, some experts have tied this year’s quiet season to a robust flu vaccine launch, which saw people receiving their vaccines en masse, some for the first time and many earlier than usual. But that still doesn’t explain the silence, said Dr. Lawrence Drew, a retired virologist who headed UCSF’s clinical virology laboratory.
“The best we have seen in the US for vaccine cooperation is 50-60% and that would not be enough to explain, and it would not explain RSV,” he said.
Some people, Drew said, also attributed the quiet season to an idea called viral interference, when an organism that was infected with a virus somehow resists infection with a second virus. But that also seems unlikely, he said, because it usually involves similar viruses – and influenza, SARS COV-2 and RSV are not similar.
What is by far the most likely explanation, Drew and other experts say, is that pandemic public health measures of masking and social detachment are extremely effective in preventing the spread of influenza and RSV – even more so than the spread of HIV. coronavirus.
There is a scientific basis for this: studies have shown that SARS COV-2 is much more transmissible than the main seasonal respiratory viruses, which include influenza and RSV. A new variant of the coronavirus, which reached the United States and California, is even more contagious, with some numbers pointing to a 70% increase in transmissibility.
According to the CDC, flu activity is abnormally low across the country this season. Although California has already surpassed the national flu-like illness rate, these spikes are not uncommon during flu seasons. They may also be linked to the growing outbreaks of coronavirus cases in the state, particularly in southern California, which officials have attributed in part to looser adherence to public health guidelines. (At that time last year and at the beginning of the current flu season, California’s average was lower than that of the country.)
At the end of December, medical appointments for influenza-like illnesses in California accounted for 2.3% of total visits, while the national rate was 1.6%. Drew suspects that the Bay Area may be doing better than other parts of the state – and the country – that may not be adhering to public health guidelines so rigorously.
There may be another “wildcard” factor for the slow flu season, according to Bergen, Kaiser’s 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. social wear and detachment.
The launch of the coronavirus vaccine in California and the US got off to a slow start, and COVID-19’s long year is probably months away from being under control. And the flu season doesn’t always peak in December or January, Bergen said. As a result, if people stop wearing masks, distance themselves socially and follow hand hygiene, a twindemia may still be on the horizon.
Drew’s long-term vision is even more urgent. If people leave this flu season and approach the next one as if it never happened – avoiding masking, social detachment and so on – all security will be reversed. The guidelines that helped us avoid the flu this year, he says, are critical to continuing.
“In a few years’ time – five or ten or maybe just one – we’ll have a flu pandemic,” said Drew, adding that variants are likely to arise for which scientists will not have vaccines. “Everybody knows it. … there is no question about it. We are very late. “
Annie Vainshtein is a writer for the San Francisco Chronicle. Email: [email protected] Twitter: @annievain