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A new strain of bird flu that transmits easily among wild birds has triggered an explosive spread into new corners of the globe, infecting and killing a variety of mammalian species and raising fears of a pandemic more lethal than COVID-19.

But the very changes that have allowed the virus to infect wild birds so efficiently likely made it less prone to infecting human cells, leading disease experts have said. Their views underpin global health officials’ assessments that the current outbreak of H5N1 poses a low risk to people.

The new strain, called H5N1 clade, emerged in 2020 and has spread to many parts of Africa, Asia and Europe as well as North and South America, causing unprecedented numbers of deaths among wild birds and domestic poultry.

The virus has also infected mammals ranging from foxes and grizzly bears to seals and sea lions, likely from feeding on diseased birds.

Unlike earlier outbreaks, this subtype of H5N1 is not causing significant disease in people. So far, only about a half dozen cases have been reported to the World Health Organization (WHO) in people who had close contact with infected birds, and most of those have been mild.

“We think the risk to the public is low,” said Dr. Timothy Uyeki, chief medical officer of the Influenza Division at the U.S. Center for Disease Control and Prevention (CDC), in an interview. The WHO expressed a similar view in an assessment earlier this month.

The way this virus enters and infects cells is one reason for the muted concern, flu experts told Reuters. They say the attributes that have made this virus thrive in wild birds likely make it less infectious to people.

“It’s clear that this is a very, very successful virus for birds, and that almost excludes it from being a very, very successful virus in mammals,” said Richard Webby, director of the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds at St. Jude Children’s Hospital.

Experts see the spillover into mammals as an early warning sign to step up virus surveillance rather than a signal of a new pandemic.

“Everybody take a breath,” said Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota who has tracked H5N1 since it first emerged in 1997, of those sounding alarm bells.

<a class=”fresco” href=”; data-fresco-group=”inline-images” data-fresco-group-options=”ui:’inside'” data-fresco-caption=”Chickens in cages at a farm in Buenos Aires, Argentina, on Wednesday. Argentina’s government is adopting new measures to prevent the spread of bird flu and limit potential damage to exports as cases rise in the region. REUTERS

What raised concern among virologists was a study published in January in the medical journal Eurosurveillance showing potential mammal-to-mammal transmission of the virus on a mink farm in Spain.

“It is highly plausible that a virus capable of mink-to-mink transmission is capable of human-to-human transmission,” Michelle Wille, an expert in the dynamics of wild bird viruses at the University of Sydney, wrote in an email.

That is a scenario that disease experts have been warning about for decades. Mink share many attributes with ferrets, an animal often used in flu experiments because of their similarity to humans.

Although the exact changes required for a bird flu virus to become easily transmissible in people are not known, a pair of landmark studies done a decade ago offer some clues.

Using so-called gain of function experiments, scientists intentionally altered the H5N1 virus to make it transmissible in ferrets and found that as few as five highly specific mutations were required.

Most of the mammalian cases so far have had only one of these mutations — in a gene called PB2 — which was present in the mink. Webby said the virus can make that change easily.

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The Japan Times

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