#13,920
Influenza remains elevated across most of the United States, and while the level of activity has declined slightly, we are still nowhere near the end of this year's flu.
Influenza viruses are constantly changing, and it isn't all that unusual to see a shift from one dominant influenza subtype to another as our flu season progresses. Most often, it is a shift from Influenza A (either H1N1 or H3N2) to influenza B (Yamagata or Victoria) in the spring.
This year, however, we are witnessing a mid-season shift from one dominant influenza A virus (H1N1) to another (H3N2).In just two months, H3N2 has gone from making up just 17% of the viruses submitted to the CDC, to over 62%. You can see this shift from H1N1 (orange) to H3N2 (red) in the FluView chart below.
Even more remarkably, we're seeing a major shift in the dominant clade of H3N2 circulating in the United States.
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| Recent FluView H3N2 Clade Charts |
At the start of the flu season, Clade 3C.3a was barely on our radar (comprising only 4% of H3N2 viruses in Week 44), but it has now overtaken - and nearly supplanted - the two dominant H3N2 clades (3C.2a1 & 3C.2a) of the past couple of years (see chart above).
Complicating matters, this 3C.3a clade appears less well inhibited by this year's flu vaccine. According to the CDC, just under 62% of H3N2 viruses tested this winter were well-inhibited by ferret antisera raised against A/Singapore/INFIMH-16-0019/2016 (3C.2a1), which represents the A(H3N2) component of 2018-19 Northern Hemisphere influenza vaccines.
Nearly all of the viruses that reacted poorly belonged to clade 3C.3a.This unexpected rise of clade 3C.3a in the United States, and elsewhere in the world, convinced the World Health Organization last month to wait an extra 30 days before deciding on next fall's H3N2 vaccine component (see WHO: (Partial) Recommended Composition Of 2019-2020 Northern Hemisphere Flu Vaccine).
A final decision is expected in a couple of weeks.Below you'll find some excerpts from today's (week 9) FluView report.
2018-2019 Influenza Season Week 9 ending March 2, 2019
All data are preliminary and may change as more reports are received.An overview of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component, is available at http://www.cdc.gov/flu/weekly/overview.htm.Synopsis:
Influenza activity remains elevated in the United States. Influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B viruses continue to co-circulate. Below is a summary of the key influenza indicators for the week ending March 2, 2019:
- Viral Surveillance:The percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories increased slightly. Nationally, during week 9, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses. During the most recent three weeks, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses in HHS Regions 2, 4, 6, 7 and 8.
- Virus Characterization:The majority of influenza viruses characterized antigenically are similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.
- Antiviral Resistance:TThe vast majority of influenza viruses tested (>99%) show susceptibility to oseltamivir and peramivir. All influenza viruses tested showed susceptibility to zanamivir.
- Influenza-like Illness Surveillance:The proportion of outpatient visits for influenza-like illness (ILI) decreased slightly to 4.7%, which is above the national baseline of 2.2%. All 10 regions reported ILI at or above their region-specific baseline level.
- ILI State Activity Indictor Map: 32 states experienced high ILI activity; Puerto Rico and seven states experienced moderate ILI activity; New York City, the District of Columbia and eight states experienced low ILI activity; three states experienced minimal ILI activity; and the U.S. Virgin Islands had insufficient data.
- Geographic Spread of Influenza: The geographic spread of influenza in Puerto Rico and 48 states was reported as widespread; the District of Columbia and two states reported local activity; the U.S. Virgin Islands reported sporadic activity; and Guam did not report.
- Influenza-associated Hospitalizations A cumulative rate of 36.6 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. The highest hospitalization rate is among adults 65 years and older (107.7 hospitalizations per 100,000 population).
- Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was above the system-specific epidemic threshold in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
- Influenza-associated Pediatric Deaths: Nine influenza-associated pediatric deaths were reported to CDC during week 9. Eight deaths occurred during the 2018-2019 season and one death occurred during the 2015-2016 season.
While this year's flu season is nowhere near as severe as last year's, the CDC estimates it has sickened more than 22 million people and claimed between 19,000 and 31,000 lives. And those numbers will most certainly go higher.
If you haven't gotten a flu shot, you should seriously consider doing so, even if it is less effective against this new clade. You should continue to practice good flu hygiene (covering coughs, washing/ sanitizing hands, and staying home if you are sick), as well.
If you get sick, call your doctor. Early treatment with antivirals can shorten your illness, and for some patients, be life saving.
