CDC FluView Week 14: Novel H1N1v Reported In Wisconsin

CREDIT CDC 

#15,914

Between social distancing, facemasks, hand hygiene and a potential suppressive impact from the coronavirus itself (aka `viral interference'), influenza and ILIs in the United States (and globally) have been, and remain, at historical low levels for the entire 2020-2021 flu season (see chart below).

While the feared `twindemic' of flu & COVID did not materialize, influenza will return at some point, likely in concert with COVID-19.  We looked at the WHO's Recommended composition of influenza virus vaccines for use in the 2021 - 2022 northern hemisphere influenza season in late February, and the challenges experts are facing in choosing vaccine strains with limited data.

Complicating matters even further, we continue to see scattered reports of novel flu viruses jumping from pigs to humans - both in the United States - and around the world. The assumption is that while relatively rare - given very limited testing and surveillance - only a small percentage are ever detected. 

In late January, in CDC FluView Week 3: 1st Novel Flu (H3N2v) of 2021 - Wisconsin, we looked at the first such report of 2021 in the United States.  Today the CDC is reporting a second case, also from Wisconsin, but this time of the H1N1v (swine variant) subtype. 

From today's FluView Report.

Key Points

• Flu activity is unusually low at this time.

• A case of human infection with a novel influenza A virus was reported this week. The virus was an influenza A(H1N1) variant (A(H1N1)v) virus. This season there have been three novel influenza A viruses reported – one A(H3N2)v virus and two A(H1N1)v viruses.

• An annual flu vaccine is the best way to protect against flu and its potentially serious complications.

• There are also flu antiviral drugs that can be used to treat flu illness.

(SNIP)

Novel Influenza A Virus

One human infection with a novel influenza A virus was reported by Wisconsin. This person was infected with an influenza A(H1N1) variant (A(H1N1)v) virus. The patient is a child < 18 years of age, was not hospitalized, and has completely recovered from their illness. Investigation into the source of the infection revealed that the patient had direct contact with swine. No human-to-human transmission has been identified in association with this patient. This is the first influenza A(H1N1)v virus infection detected in the United States occurring in 2021 and the third novel influenza A virus infection (one A(H3N2)v and two A(H1N1)v viruses) occurring during the 2020-2021 season.

Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be more fully understood and appropriate public health measures can be taken. Additional information on influenza in swine, variant influenza infection in humans, and strategies to interact safely with swine can be found at http://www.cdc.gov/flu/swineflu/index.htm.

Additional information regarding human infections with novel influenza A viruses can be found at http://gis.cdc.gov/grasp/fluview/Novel_Influenza.html.

While most swine variant infections don't appear to transmit well in humans, more than 470 cases have been reported in the United States since 2005. The CDC's IRAT (Influenza Risk Assessment Tool) lists 3 North American swine viruses as having at least some pandemic potential (2 added in 2019). 

H1N2 variant [A/California/62/2018]  Jul   2019   5.8  5.7 Moderate

H3N2 variant [A/Ohio/13/2017]          Jul   2019   6.6  5.8 Moderate

H3N2 variant [A/Indiana/08/2011]      Dec 2012   6.0  4.5 Moderate 

The CDC's Risk Assessment for these viruses reads:

Sporadic infections and even localized outbreaks among people with variant influenza viruses may occur. All influenza viruses have the capacity to change and it’s possible that variant viruses may change such that they infect people easily and spread easily from person-to-person. The Centers for Disease Control and Prevention (CDC) continues to monitor closely for variant influenza virus infections and will report cases of H3N2v and other variant influenza viruses weekly in FluView and on the case count tables on this website 

Recently China's EA H1N1 `G4' virus has garnered a lot of attention (see ECDC Risk Assessment: Eurasian avian-like A(H1N1) swine influenza viruses), as have other swine variant viruses round the globe (Brazil: Paraná Health Reports Novel H1N2 Flu Case) 

Nine weeks ago the CDC added the EA H1N1 `G4' virus to their IRAT list (see CDC Selects New Swine-Variant EA H1N1 Virus For The Top Of Their IRAT List), giving it the highest risk assessment of any of the 20 novel viruses on their list.

Summary: A risk assessment of Eurasian avian-like swine influenza A(H1N1) [A/swine/Shandong/1207/2016] virus, clade 1C.2.3 and genotype 4, was conducted in July 2020. With point scores ranging from 1 to 10, the overall IRAT risk assessment score for this virus falls into the moderate risk category, which ranges from 4.0 to 7.9.

The average risk score for potential emergence of the virus to achieve sustained human-to-human transmission was 7.5, within the upper moderate range. The average risk score for the virus to impact public health if it were to achieve sustained human-to-human transmission was 6.9, also in the upper moderate range. Full report here pdf icon[PDF – 272 KB]. 

While novel H5 & H7 avian flu viruses tend to produce greater mortality in humans - and therefore get a lot of media attention - novel swine viruses cannot be dismissed as a trivial threat.  

After a year without appreciable seasonal influenza - and community immunity to seasonal influenza presumably waning with each passing month - we are entering uncharted territory. 

No one really knows how, or when, flu will return. But we should be prepared for surprises. 

Which makes it important that we remain vigilant when novel viruses emerge and test the waters. Most will fail to make much of an impact, but as COVID-19 has shown.

It only takes one.