Drug-resistant "dual mutant" flu strains now being tracked in U.S., CDC says

At least two human cases of the new so-called "dual mutant" strains of H1N1 influenza have been detected in U.S. patients, the Centers for Disease Control and Prevention said Wednesday, with genetic changes that could cut the effectiveness of the main flu antiviral that hospitals rely on.

An analysis of the new H1N1 flu viruses with these two concerning mutations – which scientists call I223V and S247N, describing changes to key surface proteins of the virus – was published this week in the agency's Emerging Infectious Diseases journal.

It follows a report by scientists from Hong Kong who first tested the mutations. Their lab experiments, published in March, found the two mutations appeared to raise H1N1's resistance to the flu treatment oseltamivir, commonly sold under the brand Tamiflu by drugmaker Roche.

It is unclear how much the mutations could cut the real-world effectiveness of oseltamivir. The laboratory tests found the mutated viruses were up to 16 times less sensitive to the antiviral, a smaller dropoff than in some previous worrying mutations.

"These mutated viruses retained sensitivity to other anti-influenza medications, including a newer one, baloxavir marboxil. There are no immediate implications to change decisions for clinical care," a CDC spokesperson said in an emailed statement. The spokesperson said that flu vaccination can still offer protection against viruses with these mutations.

Cases on multiple continents

Despite the "rapid spread of dual mutants to countries on different continents," the CDC's report on the new dual mutant flu strains found the mutations appear to still be rare for now. 

Since the mutations first showed up in a case sampled from the Canadian province of British Columbia in May 2023, a total of 101 sequences have been submitted to the global virus database GISAID from Africa, Asia, Europe, North America and Oceania. These make up less than 1% of flu virus sequences during that time.

"However, those data may not necessarily represent the actual proportion of what was in circulation because of differences in surveillance and sequencing strategies in each country," the authors said.

The two U.S. cases were detected by labs at the Connecticut Department of Health and University of Michigan this past fall and winter.

"It is unknown how widely these mutated viruses will circulate in the upcoming season. It is important to continue monitoring the spread of these viruses and the evolution of these viruses," the CDC spokesperson said.

The most commonly prescribed antiviral

The CDC has long been closely tracking potential mutations to the flu virus that could lead to resistance to oseltamivir. 

Oseltamivir ranks as the most commonly prescribed treatment for flu, the CDC says. A study published last year found the drug made up 99.8% of flu antivirals prescribed to kids.

The CDC urges doctors to give flu antivirals as soon as possible to all flu patients who are hospitalized or are at risk of severe disease. Doctors have also turned to oseltamivir to treat humans infected by the ongoing outbreak of H5N1 bird flu in dairy farms this year.

This is not the first time that health authorities have tracked the emergence of a potential threat to the effectiveness of flu treatments.

Before it was wiped off the map by a competing H1N1 strain behind the swine flu pandemic that swept the world in 2009, health authorities from Europe to Japan had reported seeing surges of a H1N1 strain that was resistant to oseltamivir in 2007 and 2008.

That strain had a mutation called H275Y, which at the time were up to 1,000 times less sensitive to oseltamivir in laboratory tests.

In:
• Influenza
• CDC

Alexander Tin

Alexander Tin is a digital reporter for CBS News based in the Washington, D.C. bureau. He covers the Biden administration's public health agencies, including the federal response to infectious disease outbreaks like COVID-19.

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