Main Influenza Strain Has Mutated, Current Vaccines May Not Prevent Infection: Researchers

A nurse prepares a vaccine against the flu in a file photo. Valery Hache/AFP/Getty Images

Several researchers said that the main circulating influenza strain in the United States has mutated, noting that vaccines may not prevent infection.

According to a pre-print study (pdf) led by Scott Hensley, a professor of microbiology at the University of Pennsylvania, flu shots do not protect against a mutated variant of the H3N2 influenza virus, or the most common one, as vaccines do not match the strain well anymore.

“From our lab-based studies it looks like a major mismatch,” Hensley told CNN on Thursday. He noted that his study covered only H3N2, adding that the vaccines are supposed to protect against H3N2, H1N1 and two strains, of influenza B.

“We have been monitoring this virus for several months,” he said, noting that influenza viruses mutate constantly.

The new H3N2 strain includes changes that allow it to evade antibodies that that body creates in response to the vaccines, he told the network.

The Centers for Disease Control and Prevention (CDC) previously influenza cases dropped significantly last year with the emergence of COVID-19. But Hensley noted that because of this phenomenon, overall population immunity to the flu is likely low.

“Once COVID-19-related restrictions are eased or lifted, it is possible that influenza viruses will circulate widely due to lack of infection-induced population immunity over the past two years,” they wrote. “In recent weeks, a unique H3N2” variant “has circulated at elevated levels in the United States and other parts of the world,” they continued.

And they further noted the H3N2 “clade emerged early in the COVID-19 pandemic and almost completely displaced other H3N2 clades in Europe, Oceania, South Asia, West Asia, and North America in 2021.”

According to data provided by CDC, influenza kills between 12,000 and 52,000 people per year, depending on the season, and it puts as many as 700,000 people into the hospital annually.

Earlier this month, the CDC said in a surveillance report that influenza cases are rising across the United States.

“The number of influenza viruses detected by clinical and public health labs has increased in recent weeks. The majority of viruses detected are A(H3N2),“ said the federal health agency. ”Most influenza A(H3N2) infections have occurred among children and young adults ages 5-24 years; however, the proportion of infections occurring among adults aged 25 years and older has increased in recent weeks.”

A flu outbreak at the University of Michigan impacted more than 700 people, according to local health officials. The CDC said it is aware of influenza outbreaks in universities and colleges across several states, adding that influenza vaccination coverage is low.