The 2022-2023 flu season continues its early start as respiratory illness activity climbed to levels typically seen in January or February, according to a new report from the CDC.
One important measure of the season's severity, the proportion of outpatient visits involving influenza-like illness (ILI), rose to 5.8% for the week of Nov. 6-12. The last flu season to have such high activity so early was 2009-10, when visits for flu-like illness hit 7.7% in mid-October, the CDC's data shows. That same season, visit levels quickly dropped and were back to normal by the beginning of January.
As with so many other things, however, the emergence of COVID-19 has brought about changes in how flu activity is measured.
About a year ago, the CDC changed the definition of flu-like illness to exclude specific mention of flu itself, which has brought COVID-19 and RSV (respiratory syncytial virus) into play, since with both patients often have fever plus cough or sore throat. All three viruses are being monitored by the CDC's Outpatient Influenza-like Illness Surveillance Network, which monitors outpatient visits for respiratory illness.
As the CDC monitors flu-like illness, a potential wild card is emerging research showing these viruses don't play well together.
Researchers in Canada reported in February that the flu virus interferes with the coronavirus that causes COVID-19 and helps prevent the coronavirus from replicating itself. The opposite is true, as well, where the coronavirus can interfere with the flu virus, the researchers reported in a study published in the journal Viruses.
Likewise, there is evidence that rhinoviruses, which cause the common cold, may interfere with the coronavirus.
What this means, some experts believe, is that it's unlikely all three viruses would peak at the same time, overwhelming the health care system.
Despite the change in the CDC's tracking definitions, activity during the 2021-22 flu season was below average: The national flu-like illness rate never reached 5% and fell below the current national baseline (an average of the last three non-COVID flu seasons plus 2021-22 and selected weeks of 2019-20) by the end of January.
That does not seem to be the case in 2022-23.
"So far this season, there have been at least 4.4 million illnesses, 38,000 hospitalizations, and 2,100 deaths from flu," the CDC said in the weekly report from its Influenza Division, while also noting that the rate of hospital admissions "is higher than the rate observed [at the same point] during every previous season since 2010-2011."
Among those 2,100 influenza-associated deaths this season are seven children. That is more than occurred through 6 weeks of the 2021-22 season, when the first of 44 total deaths didn't occur until week 8, and through the entire 2020-21 season, when there was only one. In the three flu seasons before that, there were 199 (2019-20), 144 (2018-19), and 188 (2017-18) children who died from the flu, the CDC said.
CDC: "Weekly U.S. Influenza Surveillance Report."
Viruses: "Influenza A(H1N1)pdm09 Virus but Not Respiratory Syncytial Virus Interferes with SARS-CoV-2 Replication during Sequential Infections in Human Nasal Epithelial Cells."
The Lancet: "Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection study."
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Image 1: Centers for Disease Control and Prevention
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Cite this: Is It a Surge? Flu Season Gains Strength Before Holidays - Medscape - Nov 21, 2022.