Flu activity increasing in US, but remains ‘very low’
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Although influenza activity continues to increase in the United States, it remains low enough that the CDC does not consider the flu season to have started yet, a CDC expert said.
“While the proportion of respiratory specimens testing positive for influenza is increasing, it is still very low,” Lynnette Brammer, MPH, who leads the domestic surveillance team in the CDC’s Influenza Division, told Healio. “Determining when the flu season starts each year involves a careful review of existing data. This season may be more challenging than usual given ongoing spread of COVID-19.”
Despite fears of a so-called COVID-19 and influenza “twindemic,” there was an “unusually low” level of influenza activity in the U.S. last season — likely a result of the various measures taken to lessen the spread SARS-CoV-2. Activity was so low, in fact, that the CDC was unable to estimate the effectiveness of seasonal influenza vaccines.
There was a lot of uncertainty entering this season amid a loosening of COVID-19 restrictions and the possibility that immunity to some influenza viruses has waned due to less exposure. So far, activity has remained below the level at which the CDC would consider the U.S. to be in an influenza epidemic.
During the week ending Dec. 4, 2.5% of outpatient visits in the U.S. were for respiratory illness, according to the latest CDC FluView data, which is updated every Friday morning. That matches the U.S. baseline for influenza-like illness (ILI), which the CDC develops “by calculating the mean percentage of patient visits for influenza-like illness during non-influenza weeks for the most recent three seasons,” Brammer said.
The CDC is currently excluding a chunk of the COVID-19 pandemic — from March 2020 through September 2021 — and adding two standard deviations to develop the baseline for circulating ILI, Brammer explained.
“The flu season is said to start when indicators used to monitor influenza activity are above baseline consistently for a number of consecutive weeks,” she said. “While ILI activity is one of those factors that CDC looks at, it is not the only factor. ILI can reflect other respiratory virus activity.”
Because of this, the CDC also follows the proportion of respiratory specimens that have tested positive for influenza in clinical and public health laboratories “to ensure that ILI activity is actually a result of influenza virus and not some other respiratory virus.” For the week ending Dec 4, 2.6% of respiratory specimens tested at clinical labs were positive for influenza.
Per FluView, the majority of detected infections in recent weeks were caused by influenza A(H3N2), with about 80% of cases occurring in children and young adults between the ages of 5 and 24 years.
References:
CDC. FluView. https://www.cdc.gov/flu/weekly/index.htm. Updated Dec. 10, 2021. Accessed Dec. 15, 2021.
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