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An Entire Flu Strain Was Seemingly Wiped Out During COVID-19 Pandemic


Researchers are debating whether to still include protection for this strain in vaccines.



A new victim of the COVID-19 pandemic has been revealed, and it’s a strain of the flu.

Every year, multiple strains of the flu make their way through the population. There are four main strains that generally impact the population: two strains of the A variant and two strains of the b variant. Typically, flu vaccines are designed to protect against these four strains.

However, after the pandemic, researchers believe that they made no longer need to protect against one of the B strains.

The specific strain, known as the Yamagata clade, has not been detected since 2020, according to a report from CNN.1 It’s not yet known if the strain has been wiped out or if instances of infection are so low that they haven’t been reported. Due to this, researchers are still uncertain as to whether they should continue to include protection against it in future flu vaccines.

During the pandemic, general infections of the flu saw a significant decrease. According to a study published in the National Library of Medicine,2 this was likely due to precautions taken against COVID infections. For example, people wearing masks in public are believed to have reduced both the number of COVID infections and flu infections at the same time.

Other factors, such as social distancing and increased hand washing, also likely played a role. While influenza didn’t completely disappear during the pandemic, the flu seasons for 2020 and 2021 both saw significant reductions in flu infections. As pandemic precautions were lifted and people returned to pre-pandemic behaviors, flu infections began to return to pre-pandemic levels. The most recent flu season appears to have been more active than normal, with infections hitting higher than average levels.

That is, except for Yamagata clade strain.

Dr. Jodie Guest, senior vice chair of the Department of Epidemiology at Emory University’s Rollins School of Public Health, told CNN, “Anytime these flu vaccines are being produced, they are–depending on which vaccines you are talking about–using live or attenuated virus, and you do have to grow it.”

She continued, “So while it would be an anticipated, incredibly small risk, there is the possibility you could reintroduce it into the population by having it contained in a vaccine.”

CNN also reports that some experts have suggested that instead of including protection against the Yamagata clade strain, they could instead double the dose for one of the other strains. The vaccine’s protection against one of the A strains is reportedly less effective than the others, and some researchers are arguing that it would make most sense to double the dose against that strain in the vaccine.

As of March 1, 2024, Memorial Sloan Kettering3 reports that there have been an estimated 26 million illnesses caused by flu infection in the U.S. during the most recent flu season. Of those illnesses, there are an estimated 290,000 hospitalizations and 18,000 deaths credited to the flu.

Of the reported infections, about two thirds came from A strains, while only a third came from the remaining active B strain.


  1. Goodman, Brenda. The Covid-19 Pandemic Killed Off One Strain of the Flu, and That Will Change the Next Vaccines. CNN. March 5, 2024. Accessed March 7, 2024. https://www.cnn.com/2024/03/05/health/flu-vaccine-yamagata-strains/index.html
  2. Takeuchi, Hikaru; Kawashima, Ryuta. Disappearance and Re-Emergence of Influenza during the COVID-19 Pandemic: Association with Infection Control Measures. National Library of Medicine. January 23, 2023. Accessed March 7, 2024. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9862942/
  3. Weekly U.S. Influenza Surveillance Report. Memorial Sloan Kettering Library. March 1, 2024. Accessed March 7, 2024. https://libguides.mskcc.org/RespiratoryViruses/FluSeason
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