Explainer: The H5N1 bird flu outbreak

STORY: [What you need to know about the bird flu outbreak]

Avian influenza, commonly called bird flu, has killed more than 200 million birds in the past year.

It’s also infected other animals.

Human cases, however, remain very rare and global health officials have said the risk of transmission between humans is still low.

Here’s what you need to know.

The strain that's causing concern is clade 2.3.4.4b, a subtype of the H5N1 virus that spread quickly among wild birds and poultry.

Here's Jennifer Rigby, Reuters’ global health correspondent.

“The fear among scientists is that this strain of bird flu could jump to humans and spread between humans and causing another pandemic. That's a fear for lots of different pathogens. This one's come into the news because of the record number of infections in birds. And then this jump to mammals like seals, bears, a few others around the world.”

It emerged in 2020 and has since spread around the world.

Unlike earlier outbreaks, this H5N1 subtype 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.

Most of those cases have been mild... but in February an 11-year-old girl in Cambodia died after she was infected.

The way the virus enters and infects cells is one reason for muted concern, health experts told Reuters in February.

Human flu viruses typically bind to receptors found in the upper respiratory tract.

The H5N1 virus still prefers to bind to avian-type receptors to enter and infect cells.

Those receptors are scarce in humans and located deep in the lungs.

But the virus could have an impact in other ways, said Rigby.

"When I speak to scientists, and I talk to mainly scientists who are looking at the threat for humans, they actually all warn about the threat for food security because it's affecting so many domestic birds and farmed poultry. And then there's also a risk of biodiversity even, because there's been such big die-offs, among birds and other species worldwide.”

Experts see the spread to other mammals as a warning sign to step up virus surveillance, rather than a signal of a new pandemic.

Still, they have not ruled out the possibility that H5N1 or another avian flu virus could mutate and spark a health crisis.

Vaccine makers have already been preparing.

“A lot of them [vaccine makers] have been working for years on pandemic potential flu vaccines. So, if a flu strain jumps to humans, they have already got kind of prototype vaccines that could be used to protect humans.”

Production could be slow to ramp up, said Rigby.

And coming off the heels of the COVID-19 pandemic, Rigby said global health experts are wondering if we’ll be ready for the next one, whether it's sparked by avian flu or another pathogen.

“I think there is genuine concern among global health experts that we haven't learned the lesson of COVID, around vaccine inequality and around inequality in all sorts of... tests, treatments and even PPE.”