CDC FluView Week 7 - Increased Activity & Rising H3N2 Clade 3C.3a In U.S.

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With influenza still increasing across the nation, and a noticeable shifting from H1N1 to H3N2 in several regions, this winter's flu season is likely to extend well into spring.  

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 has been far from mild.

And with a growing number of H3N2 infections being reported over the past few weeks - particularly in the Southeast and Midwest - the impact on older patients will likely increase.

First some excerpts from this week's FluView report from the CDC, then a quickly look at the rapidly shifting flu subtypes and clades across the country.

2018-2019 Influenza Season Week 7 ending February 16, 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 continues to increase 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 16, 2019:

• Viral Surveillance:The percentage of respiratory specimens testing positive for influenza viruses in clinical laboratories increased. While influenza A(H1N1)pdm09 viruses predominated in most areas of the country, influenza A(H3) viruses have predominated in HHS Region 4 and accounted for 47% of subtyped influenza A viruses detected nationally during week 7. During the most recent three weeks, influenza A(H3) viruses were reported more frequently than influenza A(H1N1)pdm09 viruses in HHS Regions 6 and 7 and influenza A(H1N1)pdm09 and influenza A(H3) viruses were reported in approximately equal numbers in HHS Region 2.
• 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) increased to 5.1%, 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 30 states experienced high ILI activity; the District of Columbia and 11 states experienced moderate ILI activity; six states experienced low ILI activity; the U.S. Virgin Islands and three states experienced minimal ILI activity; and Puerto Rico had insufficient data.
• Geographic Spread of Influenza: The geographic spread of influenza in Puerto Rico and 48 states was reported as widespread; one state reported regional activity; the District of Columbia reported local activity; the U.S. Virgin Islands and one state reported sporadic activity; and Guam did not report.
• Influenza-associated Hospitalizations A cumulative rate of 27.4 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. The highest hospitalization rate is among adults 65 years and older (75.6 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: Seven influenza-associated pediatric deaths were reported to CDC during week 7.

(Continue . . . )

In a blog two 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).

They wrote:

The majority of A(H3N2) viruses collected from September 2018 to January 2019 belonged to the phylogenetic subclade 3C.2a1b; however, the number of clade 3C.3a viruses has increased substantially since November 2018 in several geographic regions.

There has continued to be considerable genetic diversification of the HA and NA genes, but viruses in subclade 3C.2a2 were much less prevalent than in the previous reporting period.

In this week's FluView (week 7) Antigenic characterization of circulating H3N2 viruses, we can see a similar rise in the number of clade 3C.3a viruses detected this winter (bolding & underlining mine).

A (H3N2): Phylogenetic analysis of the HA genes from 381 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=62), subclade 3C.2a1 (n=122) or clade 3C.3a (n=197).

One hundred ninety-four A(H3N2) viruses were antigenically characterized by FRA with ferret antisera, and 128 (66%) 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.

Sixty-six (34%) 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, 65 (98.5%) belonged to clade 3C.3a.

While more than half (51%) of all H3N2 viruses tested by the CDC this winter are now clade 3C.3a - just 7 weeks ago (week 1 FluView) just 18% belonged to clade 3C.3.

That increase is even more apparent when you look at the 254 H3N2 viruses examined by the CDC since the first week of January, 68% (174 of 254) now belong to this emerging clade.

As clade 3C.3a viruses are less well inhibited by this year's vaccine, its recent gains could make the remainder of this flu season more challenging, particularly for the elderly, who are often the most harshly affected by H3 flu viruses.

As always, it isn't too late to get the flu shot. But now is also the time to rigorously practice good flu hygiene. Stay home if you are sick, avoid crowds, wash your hands frequently, and cover your coughs and sneezes

If you think you have the flu, contact your doctor right away. Antivirals, when taken early by those at higher risk of complications, can be lifesaving.