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| Credit CDC - LINK |
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Seasonal influenza remained elevated across the nation during Epi Week 8, with an increasing shift from H1N1 to H3N2 in across several regions, suggesting this winter's flu season has several more weeks to go.
As the graphic above indicates, while not as horrific as the 2017-18 H3N2 flu season - which is blamed for More Than 900,000 U.S. Hospitalizations & 80,000 Deaths - this year's epidemic continues to exact a significant toll.First some excerpts from this week's FluView report from the CDC, then a quick look at the rapidly shifting flu subtypes and clades across the country.
2018-2019 Influenza Season Week 8 ending February 23, 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 February 23, 2019:
Viral Surveillance: The percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories increased slightly. Nationally, during week 8, 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 and 7.
- 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:The 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) remained at 5.0%, 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: New York City and 33 states experienced high ILI activity; the District of Columbia and eight states experienced moderate ILI activity; Puerto Rico and eight states experienced low ILI activity; one state 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 49 states was reported as widespread; the District of Columbia and one state reported local activity; the U.S. Virgin Islands reported sporadic activity; and Guam did not report.
Influenza-associated Hospitalizations A cumulative rate of 32.1 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. The highest hospitalization rate is among adults 65 years and older (91.5 hospitalizations per 100,000 population).
Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below the system-specific epidemic threshold in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
Influenza-associated Pediatric Deaths: 15 influenza-associated pediatric deaths were reported to CDC during week 8.
In a blog nine days ago (see WHO: (Partial) Recommended Composition Of 2019-2020 Northern Hemisphere Flu Vaccine) we looked at the World Health Organization's unusual decision to delay selection of the H3N2 component of next fall's flu vaccine for 30 days due to the recent rise of a new, antigenically distinct strain (clade 3C.3a).
First, excerpts from today's FluView Influenza Virus Characterization:
This week the CDC reports they had antigenically or genetically characterized 1,258 influenza viruses collected September 30, 2018 – February 23, 2019, and submitted by U.S. laboratories, including 690 influenza A(H1N1)pdm09 viruses, 425 influenza A(H3N2) viruses, and 143 influenza B viruses.
A (H3N2): Phylogenetic analysis of the HA genes from 425 A(H3N2) viruses revealed extensive genetic diversity with multiple clades/subclades co-circulating.
The HA genes of circulating viruses belonged to clade 3C.2a (n=64), subclade 3C.2a1 (n=126) or clade 3C.3a (n=235).
Two hundred and nine A(H3N2) viruses were antigenically characterized by FRA with ferret antisera, and 132 (63.2%) A(H3N2) viruses tested were well-inhibited (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera raised against A/Singapore/INFIMH-16-0019/2016 (3C.2a1), a cell-propagated reference virus representing the A(H3N2) component of 2018-19 Northern Hemisphere influenza vaccines.
Seventy-seven (36.8%) viruses reacted poorly (at titers that were 8-fold or greater reduced compared with that of the homologous virus A/Singapore/INFIMH-16-0019/2016) and of those, 76 (98.7%) belonged to clade 3C.3a.
While just over half the Influenza A viruses (54.1%) identified by the the CDC lab in week 8 were H3N2, among the 64 H3N2 viruses fully characterized since week 7's update, 58 (90%) belonged to the emerging 3C.3a clade.
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| Comparison of 3 recent CDC FluView Graphics |
Clade 3C.3a was barely on the radar 4 months ago (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.
A reminder of that the only thing predictable about influenza is its unpredictability, and of how quickly influenza can change, even in the middle of a flu season.
