Influenza virus detections are rising in the U.S., according to health officials.
In a weekly influenza surveillance report ending on Dec. 4, the Centers for Disease Control and Prevention (CDC) said that while influenza activity remains low nationally, it continues to increase.
“The number of influenza viruses detected by clinical and public health labs has increased in recent weeks. The majority of viruses detected are A(H3N2). Most influenza A(H3N2) infections have occurred among children and young adults ages 5-24 years; however, the proportion of infections occurring among adults age 25 years and older has increased in recent weeks,” it reported.
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The percentage of outpatient visits due to respiratory illness has also trended upward and is “now at the national baseline,” according to the agency.
In a map of “Influenza-like illness” activity determined by data reported to ILINet, only New Mexico was listed as “high” for influenza-like illness during the same week.
The CDC said that Influenza A(H3N2) viruses were reported by public health laboratories in all 10 Health and Human Services (HHS) regions this week.
In November, the CDC had issued an advisory warning about “increased influenza A(H3N2) activity that could mark the beginning of the 2021-2022 influenza season.”
“While influenza activity is still low overall nationally, an increase of influenza A(H3N2) viruses has been detected in recent weeks, with most of these infections occurring in young adults. CDC also is aware of influenza outbreaks in colleges and universities in several states. Influenza vaccination coverage is still low and there is still time this season to benefit from getting an annual influenza vaccine,” it wrote then.
An outbreak at the University of Michigan-Ann Arbor campus is being investigated by federal and state health experts.
About 528 cases of influenza were diagnosed at the University Health Service, with 77% of cases in unvaccinated individuals, according to a university statement.
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“Through prompt detection and collaboration with county and state health officials, as well as School of Public Health and Michigan Medicine researchers, we quickly identified these cases as influenza A(H3N2) virus infections,” Lindsey Mortenson, UHS medical director and acting executive director, said. “Partnering with the CDC will accelerate our understanding of how this flu season may unfold regionally and nationally in the setting of the COVID-19 pandemic.”
Officials have called for communities to get vaccinated against influenza as soon as possible, in order to reduce transmission, protect against severe illness, hospitalization or even death.
Available seasonal influenza vaccines provide protection against the A(H1N1)pdm09, A(H3N2), B/Victoria lineage, and B/Yamagata lineage influenza viruses.
However, A(H3N2) viruses evolve more rapidly to escape human immunity and influenza vaccine effectiveness, in general, has been lower against A(H3N2) viruses than against the other three influenza viruses that could circulate.
While influenza activity was low throughout the U.S. during the 2020-2021 flu season, the CDC said it anticipated an increase of influenza illness and a lower level of community protection.
“A growing body of scientific studies suggest that even when vaccination does not prevent infection it can reduce the severity of influenza illness, helping to avert serious outcomes including hospitalization and death,” the agency stated.
And yet, although the CDC recommends everyone 6 months and older get a flu vaccine, there are early signs that flu vaccination uptake is down this season compared to last.
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An annual flu vaccine is the best way to protect against the flu and vaccines are available at many locations. There are also flu antiviral drugs that can be used to treat flu illness – though they are not a substitute for the vaccine.
Notably, people can get their flu shots at the same time as COVID-19 shots.
Flu season typically runs from October to as late as May, with activity usually peaking from December to February, according to the CDC.