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Drug-resistant 'dual mutant' flu viruses now being tracked in US
A 'dual mutant' flu virus has been detected in the US that could make medications less effective, officials are warning.At least two Americans were infected with a new strain of H1N1, which contains two new major mutations, last flu season, a study found, in a warning sign it may be gaining a foothold.
Across the world, 101 cases were detected out of 15,000 infections analyzed — or 0.67 percent — in five continents, in another sign it is spreading.
Scientists at the CDC are calling the strain a 'dual mutant' because it carries two major mutations that make it up to 16-fold more resistant to the main antiviral for flu, oseltamivir, which is prescribed to almost every patient with the infection. There are no indications at this stage the strain is more infectious or more deadly than other currently circulating viruses.
H1N1 is the strain of flu behind the 2009 flu pandemic, which killed 12,000 people, that spread from pigs to humans. It differs from the H5N1 strain currently sparking alarm in official circles, which is causing an outbreak among US cattle. Although the strain was found to be more resistant to oseltamivir, further research showed it had no effect on other antivirals for treating flu like baloxavir.
Scientists first detected an H1N1 strain that was more resistant to flu, but had different mutations, in the US during 2007 and 2008. No more cases were detected after the 2009 flu pandemic, but the new 'dual mutant' strain shows possible re-emerging resistance to oseltamivir. It comes amid concern among CDC officials over H5N1, which they warn that — if it spreads to humans — could cause a major outbreak.
The H1N1 strain carries two mutations — I223V and S247N — which alter key proteins used by the virus to make copies of itself and which are targeted by oseltamivir. This means the drug becomes less effective, but it is less able to stop the virus from making copies of itself. It was not clear where the two American patients were based or if they survived their infections, although both were likely tested for the virus while in hospital.
For the study, published as an early release in the CDC's own journal Emerging Infectious Diseases, agency scientists analyzed data on genetic sequences of flu viruses.
During the flu season from 2023 to 2024, there were at least two Americans infected with the mutant strain the study found. The above graph shows how cases surged over the season
Only a handful of the millions of flu infections are tested for strains every year, meaning many more people may be infected than data suggests.
The first case of a 'dual mutant' infection was detected in Canada in May 2023, with the last detected in Europe in February 2024. The Netherlands recorded the most cases, 40 infections, followed by France, 24, Bangladesh, 11, and Oman and the UK, nine each.
Alongside the US, Australia and Spain also recorded two infections with the virus. The two US cases were detected by labs at the Connecticut Department of Health and University of Michigan this past fall and winter. A number of flu infections also had only one of the two mutations, which also conferred resistance to oseltamivir.
Researchers warned that these viruses could pick up the other mutation via a process known as re-assortment. This is when two different flu viruses infect a cell at the same time and then swap genetic material between each other.
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