Flu Activity is Increasing in the U.S. but remains ‘very low’
The CDC does not consider the influenza season to have started yet.
- By Shereen Hashem
- Dec 16, 2021
Although flu 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. “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 so-called COVID-19 and influenza “twindemic,” there was an “unusually low” level of influenza activity in the U.S. last season. This is likely a result of the various measures taken to lessen the spread of coronavirus. Activity was so low that the CDC was unable to estimate the effectiveness of seasonal influenza vaccines. There was a lot of uncertainty for this season since COVID-19 restrictions became a bit more relaxed in certain areas and the possibility that immunity to some flu viruses has waned due to less exposure. The CDC says, so far, that activity has remained below the level considered for the U.S. can be in an “influenza pandemic.”
According to the latest CDC FluView data, data the week ending on December 4, 2.5 percent of outpatient visits in the U.S. were for respiratory illnesses. This 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.
Brammer explained that 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.
“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.”
According to an article, because of this, the CDC follows the proportion of respiratory specimens that have tested positive for the flu in clinical and public health laboratories “to ensure that ILI activity is actually a result of influenza virus and not some other respiratory virus.”
The majority of detected infections in recent weeks were caused by influenza A(H3N2) with about 80 percent of cases occurring in children and young adults between the ages of five and 24 years old.