No, there's not enough evidence to prove two flu strains are now extinct

Flu season is now underway in the U.S., and public health officials are encouraging people to get their flu shot this season. Doctors say they are expecting to see more flu cases this year than they did in the 2020-2021 season when COVID-19 mitigation methods helped limit the spread of the virus amid the pandemic.

In a tweet with more than 135,000 likes as of Nov. 1, a woman, citing a recent study, claims she heard at least two strains of the flu have become extinct during the last 18 months of the pandemic because of mask-wearing and social distancing. 

THE QUESTION

Is there evidence to prove two flu strains are now extinct?

THE SOURCES

• U.S. Centers for Disease Control and Prevention (CDC)
• Dr. Panagis Galiatsatos, assistant professor at the Johns Hopkins School of Medicine and a physician in pulmonary and critical care medicine
• Dr. Robert Y. Goldberg, pulmonary and critical care physician at Providence Mission Hospital
• Study on influenza lineage extinction during the COVID-19 pandemic

THE ANSWER

No, there's not enough evidence to prove two flu strains are now extinct because researchers say they need more data to definitively make that conclusion.

WHAT WE FOUND

The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs, according to the U.S. Centers for Disease Control and Prevention (CDC). Different than the common cold, the flu usually comes on suddenly. It can cause mild to severe illness, and at times, lead to death. 

There are two main types of human flu viruses: types A and B. According to the CDC, flu A and B viruses are responsible for the seasonal flu each year. Flu A viruses can be broken down into subtypes depending on the genes that make up the surface proteins, like H1N1 and H3N2, and are the only influenza viruses known to cause flu pandemics. The CDC says the currently circulating H1N1 viruses are related to the pandemic 2009 H1N1 virus that emerged in spring 2009. Meanwhile, the CDC says H3N2 viruses tend to change more rapidly out of all the influenza viruses that routinely circulate and cause illness in people.

Flu B viruses are not divided into subtypes like flu A viruses. Instead, the CDC says they are further classified into two lineages: B/Yamagata and B/Victoria. Flu B viruses generally change more slowly in terms of their genetic and antigenic properties than flu A viruses, according to the CDC. 

Although the viral tweet claims two strains of the flu are now extinct, in the September 2021 study published in Nature Reviews Microbiology, researchers focus on only one lineage of flu: B/Yamagata. The researchers found the B/Yamagata lineage disappeared from April 2020 to Aug. 2021, suggesting that it may have become extinct due to pandemic protections. In the article, the researchers do say the flu B virus has appeared to alternate in recent years, and lineages like B/Victoria and B/Yamagata have been known to periodically enter a state of “dormancy” for long time intervals, so it is too soon to tell if B/Yamagata is really gone for good. 

“The onset of the COVID-19 pandemic coinciding with a period of low incidence and low antigenic diversity, coupled with prolonged use of a well-matched influenza vaccine, may have enabled massive suppression of B/Yamagata during the pandemic,” the researchers wrote. “However, as sampling and sequencing is not comprehensive, it is difficult to distinguish with certainty between lack of detection and true extinction.”

Dr. Robert Y. Goldberg, a pulmonary and critical care physician at Providence Mission Hospital in Mission Viejo, California, told VERIFY influenza viruses typically have multiple hosts, like bird hosts, that can sometimes pass the virus on to humans. Goldberg says it’s possible that B/Yamagata may be hiding in another host and waiting for the right time to become more prevalent again. 

“It's still pretty early. We are not seeing those strains, but we don't know if they just haven't been prevalent because there's not as much around the world right now because of the fact that we've been isolating and we've been concentrating on hygiene. It's possible that it could reoccur. It could be sitting in the environment somewhere,” said Goldberg. 

Dr. Panagis Galiatsatos, an assistant professor at the Johns Hopkins School of Medicine, told VERIFY that while it’s hard to determine whether B/Yamagata is in hiding or extinct right now, it is possible to keep its numbers low even if it does resurface one day.  

“I never like to say never as a physician and scientist,” said Galiatsatos. “But I do think we can keep those numbers incredibly low. Whether we remove them off of existence? I think it's hard to prove.”

The original tweeter later clarified in a follow-up tweet that it will take time to know for sure if these strains have been completely eradicated. For now, according to CDC data, flu activity in the U.S. remains low this season, at around 0.1%.  

More from VERIFY: Yes, you can get a COVID-19 vaccine booster shot and the flu shot at the same time