Friday, March 26, 2021

CDC FluView Week 11: Novel Influenza Variant A/H1N1v virus (2020)


While seasonal influenza remains at historic lows, in late January in CDC FluView Week 3: 1st Novel Flu (H3N2v) of 2021 - Wisconsin, we looked at the first novel flu report in the United States for 2021.

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.  Today however, this week's FluView carries a report of an A/H1N1v infection in an  adult > 18 years of age in North Carolina.

The illness onset, however, occurred in 2020, and has only recently been reported to the CDC. 

Novel Influenza A Virus

One human infection with a novel influenza A virus was reported by North Carolina. This person was infected with an influenza A(H1N1) variant (A(H1N1)v) virus in 2020. The patient is an adult > 18 years of age, was not hospitalized, and has recovered from their illness. An investigation into the source of the infection revealed that the patient worked with and had daily contact with swine.
No human-to-human transmission has been identified in association with this patient. While this variant virus infection was not identified until 2021, it represents the first influenza A(H1N1)v virus identified from a specimen collected in the United States in 2020 (during the 2020-21 season). No influenza A(H1N1)v virus infections have been identified in specimens collected in 2021 thus far.

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

Additional information regarding human infections with novel influenza A viruses can be found at
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 

Recently China's EA H1N1 `G4' swine flu 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).

Six weeks ago the CDC added this 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].

Although it only happens rarely, swine influenza viruses can sometimes adapt well enough to humans to spark a pandemic, and so we monitor these viruses carefully.