Friday, February 18, 2022

CDC FluView Week 6: Sporadic Flu Activity Continues - 1 Novel Flu Detection (H1N2) in California



According to this week's FluView report, while influenza activity has declined sharply the past few weeks (see FluView chart below), a few regions are reporting a slight uptick in activity. 

The Key points from this week's FluView report are summarized below, after which we'll look at a report on the 3rd novel flu detection of the 2021-2022 flu season. 

Key Points

  • Sporadic influenza activity continues across the country. In some areas, influenza activity is increasing.
  • The majority of influenza viruses detected are A(H3N2). H3N2 viruses identified so far this season are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve.
  • The percentage of outpatient visits due to respiratory illness decreased nationally again this week and is below baseline. Influenza is contributing to levels of respiratory illness, but other respiratory viruses are also circulating. The relative contribution of influenza varies by location.
  • The number of hospital admissions reported to HHS Protect increased slightly this week.
  • The cumulative hospitalization rate in the FluSurv-NET system is higher than the rate for the entire 2020-2021 season, but lower than the rate seen at this time during the four seasons preceding the COVID-19 pandemic.
  • CDC estimates that so far this season there have been at least 2.3 million flu illnesses, 22,000 hospitalizations, and 1,300 deaths from flu.
  • An annual flu vaccine is the best way to protect against flu and its potentially serious complications. CDC continues to recommend that everyone ages 6 months and older get a flu vaccine as long as flu activity continues.
  • Flu vaccination coverage remains lower this season compared to last.
  • Flu vaccines are available at many different locations, including pharmacies and health departments. Visit to find a flu vaccine near you.
  • There are also flu antiviral drugs that can be used to treat flu illness.

Most years we see a handful of novel swine variant flu detections in humans - usually among those who have direct contact with swine.  Most are mild, and many undoubtedly go undetected. Only rarely have we seen evidence of human-to-human transmission of theses viruses. 

On a few occasions, we've seen larger outbreaks of these swine variant viruses - usually connected to agricultural exhibits at state and county fairs. Over the 2011-2012 flu season more than 300 cases were recorded, while over the 2016-2017 season 68 cases were detected. 

The CDC's risk assessment for Swine Variant 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 

 Details on the 3rd detection of this flu season follow.

Novel Influenza A Virus

One human infection with a novel influenza A (H1N2) variant virus was reported by California. The infection occurred in an adult ≥ 18 years of age. The patient was not hospitalized and has recovered from their illness. The patient had direct contact with swine prior to specimen collection. Respiratory illness was reported among contacts of the patient, however this occurred during a period of high respiratory illness activity and no specimens from the contacts were received for testing. No ongoing person-to-person transmission has been identified associated with this case.

This is the third human infection with novel influenza A virus that has occurred during the 2021-22 influenza season in the United States. One influenza A(H3N2) variant virus infection was reported by Ohio and one influenza A(H1) variant virus (neuraminidase not determined) infection was reported by Oklahoma.

When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant influenza virus”. Most human infections with variant influenza viruses occur following close proximity to swine, but person-to-person transmission can occur. In most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person. 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 virus 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

The risk of one of these swine variant viruses sparking a pandemic is relatively low, but it isn't zero. 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 
And the CDC currently ranks a Chinese Swine-variant EA H1N1 `G4' as having the highest pandemic potential of any flu virus on their list.

For now, sporadic jumps of swine variant influenza from pigs to humans pose only a minor public health risk. 

But as we saw in 2009, every once in a great while a swine virus finds the right combination of mutations to enable it to jump to humans, transmit efficiently, and spark a pandemic.