Thursday, November 25, 2021

CDC HAN # 00458 : Increasing Seasonal Influenza A (H3N2) Activity



Although the return of seasonal influenza - which has been greatly suppressed the past 18 months due to the pandemic - was always inevitable, there have been hopes we might get through one more winter respiratory season without the dual impacts of both COVID and Flu. 

PHE Study: Co-Infection With COVID-19 & Seasonal Influenza

The Realities Of Crisis Standards Of Care

Those hopes began to wane over the summer when we saw the global rise of RSV (see here, here, and here) as the use of pandemic NPIs (social distancing, face masks, etc.) relaxed, and over the past month as we've seen influenza H3N2 outbreaks in colleges and universities around the nation. 

University of Michigan: CDC & County Health Department Investigating Flu Outbreak


Adding to these concerns has been the 99% drop in influenza samples submitted for sequencing over the past year (see ECDC: Influenza Virus Characterisation - July 2021), raising concerns over our ability to detect changes in the virus which might affect this year's vaccine effectiveness. 

Although influenza activity remains low across the United States (see CDC FluView Week 45 map above), last Friday the CDC issued a warning (see CDC Statement: Tracking Flu in Young Adults) that flu outbreaks - particularly in colleges and universities - were on the rise. 

Late yesterday the CDC released the following HAN Advisory, for clinicians, public health officials, and the general public on early indications that we could be facing a formidable H3N2 flu season, alongside our ongoing COVID pandemic. 

Included in this advisory is the first characterization of the H3N2 viruses that have been detected this fall, which describes them as similar to - but genetically distinct from - this year's H3N2 vaccine component.  

How, or even if, these antigenic differences will impact the performance of this year's vaccine is not yet clear. 

That said, H3N2 dominant flu seasons tend to be more severe than H1N1 seasons - particularly among the elderly - and given the amount of viral diversity across this subtype (see The Enigmatic, Problematic H3N2 Influenza Virus), vaccine effectiveness is often diminished as well. 

The HAN Advisory is lengthy and detailed, and clinicians in particular will want to follow the link to read it in its entirety.  I've included some excerpts below, and I'll have a brief postscript after the break. 

Increasing Seasonal Influenza A (H3N2) Activity, Especially Among Young Adults and in College and University Settings, During SARS-CoV-2 Co-Circulation

Distributed via the CDC Health Alert Network
November 24, 2021, 11:00 AM ET


The Centers for Disease Control and Prevention (CDC) is issuing this Health Alert Network (HAN) Health Advisory about increased influenza A(H3N2) activity that could mark the beginning of the 2021-2022 influenza season. The purpose of this HAN Health Advisory is to
  • Remind public health practitioners and clinicians to recommend and offer the current seasonal influenza vaccine to all eligible persons aged six months and older (Flu vaccine and COVID-19 vaccine can be given at the same visit).
  • Remind clinicians to consider testing for both influenza virus and SARS-CoV-2 in patients with influenza-like illness (ILI).
  • Advise clinicians that antiviral treatment is recommended as early as possible for any patient with confirmed or suspected influenza who is: a) hospitalized; b) at higher risk for influenza complications; or c) developing progressive illness. In patients with suspected influenza, decisions about starting antiviral treatment should not wait for laboratory confirmation of influenza, however COVID-19 should be excluded if a rapid assay is available.
  • Remind public health practitioners and clinicians to consider mitigation measures including antiviral post-exposure prophylaxis during influenza outbreaks in institutions (e.g., long-term care facilities, university dormitories) in the setting of co-circulation of SARS-CoV-2.
  • Remind the public to use non-pharmaceutical interventions (NPI) or everyday preventive actions, in addition to getting a flu vaccine. Everyday preventive actions include staying home when sick, covering coughs and sneezes, and washing hands often.

Recent increases in influenza activity in many places in the United States could mark the beginning of the 2021-2022 influenza season in the United States. 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.

Available seasonal influenza vaccines in the United States provide protection against four different influenza viruses: A(H1N1)pdm09, A(H3N2), B/Victoria lineage, and B/Yamagata lineage viruses. In the past, influenza A(H3N2) virus-predominant seasons were associated with more hospitalizations and deaths in persons aged 65 years and older than other age groups than other influenza viruses. Influenza A(H3N2) viruses evolve more rapidly to escape human immunity.
The influenza A(H3N2) component of this season’s vaccines was recently updated in response to the evolution of a new group of viruses called 2a (i.e., 3C.2a1b.2a) that did not circulate widely last year and were not included in last season’s H3N2 vaccine component.1

Most H3N2 viruses that have been analyzed in the United States so far are genetically closely related to the current vaccine’s H3N2 component. However, this emerging group has continued to evolve, and there are now two subgroups (2a.1 and 2a.2) that are genetically closely related to each other, but do have some antigenic differences from each other (i.e., post-infection ferret antibodies from one virus might not efficiently bind the other virus).2 CDC virus surveillance data shows that most of the A(H3N2) viruses recently identified in the United States (October–November 2021) are in the 2a.2 group that is related to but distinguishable from the vaccine component (i.e., 2a.1).
It is not known what impact the differences in the circulating viruses and the vaccine viruses may have on vaccine effectiveness.

However, influenza vaccine effectiveness in general has been lower against A(H3N2) viruses than against the other three influenza viruses that could circulate [influenza A(H1N1)pdm09 or influenza B viruses].3 Influenza activity during the 2020–2021 season was low throughout the United States and the timing and intensity of the upcoming 2021–2022 influenza season is uncertain.
Because influenza activity was low last season, we are anticipating a lower level of community protection that we rely on year after year to reduce the risk of a severe influenza season. Thus, CDC is anticipating an increase of influenza illness this winter, and both A(H3N2) and B-Victoria viruses are already co-circulating. Moreover, as SARS-CoV-2 continues to circulate in the United States, illnesses associated with both viruses might stress healthcare systems. 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.

CDC recommends that healthcare providers continue to recommend and offer influenza vaccination to persons aged six months and older because influenza activity is ongoing. Vaccination protects against four different viruses and is likely to reduce hospitalization and death associated with currently circulating influenza viruses and other influenza viruses that might circulate later in the season. Influenza antiviral medications are an important adjunct that should be used in addition to influenza vaccination.
While vaccination is the primary means for preventing influenza, antiviral medications are a second line of defense used to treat influenza after infection has occurred. Early treatment with influenza antiviral medications is the most effective way to treat influenza and reduce complications.4-8

Influenza antivirals also can be used for post-exposure prophylaxis (PEP) to prevent infection.4,5,9 This can reduce the risk of influenza among persons who are exposed to someone who has influenza. Influenza antivirals have historically been used for PEP among residents in institutional settings, such as long-term care facilities, to help control influenza outbreaks. In the context of SARS-CoV-2 co-circulation, influenza antiviral treatment and PEP could also be considered in other communal settings (e.g., shelters, university dormitories, prisons) to reduce strain on healthcare services in these institutions during influenza outbreaks. In general, CDC recommends initiating influenza antiviral PEP within 48 hours of contact with someone who has influenza.

         (Continue . . . )

Even without the added complication of an ongoing COVID pandemic - and the uncertainty over the impact of COVID/Flu coinfections  - the early detection of a `drifted' H3N2 virus would raise concerns. 

We went through similar scenarios in 2014 (see CDC HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus) and again in 2017 (see MMWR: Reviewing Our High Severity 2017-2018 Flu Season), both resulting in overwhelmed hospitals and high morbidity and mortality numbers.

Even if the flu vaccine proves to be less effective in preventing infection this year, it may still reduce the severity of one's illness, and that could be a big advantage this year.  I've got mine, and I'm glad I did.  But this year, if I develop flu-like symptoms, I'll also be quick to get tested and talk to my doctor about antivirals.

Despite the recent decline in COVID cases around the nation, there are still some regions where hospitals are overwhelmed and `crisis standards of care' are invoked.

If H3N2 returns in a big way this year, or COVID resurges - or both - the ability of some healthcare delivery systems to function could be imperiled this winter.  Some recent blogs on this possibility include:

NHS England: Record 999 Calls & Most A&E Visits For Month Of October

Denmark: Health Authority Warns On Potential Overwhelmed Hospitals This Winter Due To COVID/Flu

Montana Governor Sends National Guard To Aid COVID-Stressed Hospitals

Idaho DOH Expands Crisis Standards of Care Statewide Due to Surge in COVID-19 Patients Requiring Hospitalization

ECDC Warns Of Potentially Severe Flu Season Ahead