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CDC data suggest children and young adults are increasingly affected, with the circulating subtype being one associated with a high risk of burden.
Influenza activity was on the rise through last week, according to new data from the Centers for Disease Control and Prevention (CDC) that showed children and young adults aged 0 to 24 years old are among the most likely groups to present with influenza-like illness (ILI) at a US outpatient care center.
In the last monitored week for flu prevalence in 2021, the CDC reported that 10.9% and 6.1% of all reported outpatient visits for patients aged 0 to 4 years and 5 to 24 years were regarding an ILI, respectively. The rate of ILI among patients aged 25 to 49 (3.3%), 50 to 64 (2.2%), and ≥65 (1.3%) were lower than the younger populations—but were still each increases from the previous week’s rate (2.0%; 1.4%; 0.9%, respectively).
Cases of influenza A (H3N2) are the most prevalent in the US during the early weeks of the 2021-22 flu season; all 375 tested specimens from the last week of 2021 were confirmed as the subtype, bringing the season’s tested total to 4110 (99.9%) in the week ending December 25.
As Tim Uyeki, MD, MPH, MPP, Chief Medical Officer of the CDC’s Influenza Division, National Center for Immunization and Respiratory Diseases, explained to sister publication Contagion® earlier this year, many of the most severe flu epidemics in recorded history have been associated with type A(H3N2) variant virsues, which have not been the most commonly circulating virus in the US since 2018-19.
“The bottom line is we could be at risk on a population basis of a rather severe influenza epidemic with type A (H3N2) viruses, depending upon how antigenically similar the viruses are compared to what people have been exposed to in the past, and how well the H2N3 vaccine strain has been mapped to it,” Uyeki explained at the time.
The CDC, which at the time of the most recent national report has characterized 208 flu viruses since October 3, 2021, provided relatively assuring context of what’s currently understood about the circulating subtype—with the caveat that more needs to be understood about it.
“While there are little data to date, most of the H3N2 viruses so far are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve,” the report read. “Virus antigenic and antiviral susceptibility data will be reported later this season when a sufficient number of specimens have been tested.”
As influenza A(H3N2) is among the subtypes with significant resistance to adamantanes, the CDC recommends against the use of antivirals for treatment and prevention of influenza A virus infection.
Because Omicron variant cases of SARS-CoV-2 are currently at exponentially greater levels in the US, among other circulating respiratory viruses, the rate of outpatient visits due to respiratory illness is expected to continuing trending upwards in the early weeks of 2022. That said, these new data—as well as reports of increased hospitalization for flu-like illness and decreased vaccination among eligible persons this year—indicate a need to bolster protection against the flu.
“The flu season is just getting started,” the CDC wrote. “There’s still time to get vaccinated. An annual flu vaccine is the best way to protect against flu and its potentially serious complications. CDC recommends everyone 6 months and older get a flu vaccine.”