CDC FluView Week 3: 1st Novel Flu (H3N2v) of 2021 - Wisconsin

CREDIT CDC

#15,759

Between social distancing, face covers, hand hygiene and a potential suppressive impact from the coronavirus itself, seasonal flu in the United States is at historical low levels for this time of year (see chart below).

One of the few opportunities these days to be infected by an influenza virus - particularly a novel swine-origin flu virus - is on a farm.   And today the CDC announces the first such detected infection of 2021.  

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(H3N2) variant (A(H3N2)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 child lives on a farm with swine present. This is the first influenza A(H3N2)v virus infection detected in the United States in 2021.

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.

Since 2005, over 465 human `swine variant' infections (H1N1v, H1N2v or H3N2v) have been documented in the United States, with over 300 of those reported in 2012. H3N2v viruses have been, by far, the most common - followed by H1N2v and then H1N1v.

This past year we've seen a big drop in novel flu reports in the United States, almost certainly due to the shuttering of county and state fairs which have previously been linked to large outbreaks. 

While most swine variant infections don't appear to transmit well in humans, 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 

H1, H2, and H3 swine-origin flu viruses are not considered `reportable animal diseases' to the OIE - much like many avian flu viruses (H9N2, H6N1, H3N1, H10N8) - and even though they may pose some risk of human infection, are poorly tracked.

Which means the next swine-origin or avian influenza pandemic could be brewing unnoticed just about anywhere in the world - and like we saw in 2009 - our only clue will come when large numbers of sick people start showing up at hospitals. 

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).

Our overall lack of flu this winter is a good thing - as it makes things far less complicated during the COVID-19 pandemic and flu is a killer in its own right - but there is a potential downside.

One that is mostly theoretical right now, but is at least plausible. 

The longer we go without seasonal influenza, the lower community immunity to seasonal H1N1 and H3N2 drops.  As long as there is very little flu circulating, that isn't a problem, but presumably influenza will eventually return in the years ahead.

 And that could mean that the next real flu season we see could be a particularly severe one. 

And there are studies that suggest that novel flu viruses may have a better chance of emerging when seasonal influenza activity is low, and community immunity to influenza is reduced (see PLoS Comp. Bio.: Spring & Early Summer Most Likely Time For A Pandemic,).

As I say, its mostly theoretical.  

But this COVID pandemic has changed the viral status quo of our world in ways we are just beginning to fathom, and we should be prepared for surprises going forward.