Bird flu vaccine candidates already exist. But if H5N1 sparks a pandemic, making enough doses won't be easy

This story is part of CBC Health's Second Opinion, a weekly analysis of health and medical science news emailed to subscribers on Saturday mornings. If you haven't subscribed yet, you can do that by clicking here.

While there's no sign a dangerous form of bird flu has gained the ability to transmit between humans, the steady spread of the virus to new species of mammals — most recently, dairy cattle throughout the U.S. — suggests H5N1 is closer to us than ever before.

Officials are quick to note that without a few key evolutionary leaps, this pathogen won't spark a human health crisis. Dr. Michael Ryan, executive director of the World Health Organization's (WHO) health emergencies program, said as much during remarks on Wednesday.

"Nobody is suggesting that H5N1 is the new, next pandemic. I don't believe anybody can predict that," he said. 

"But it's certainly concerning when a virus like this begins to infect multiple mammalian species, which means the virus that is adapting to [animals] that are more like us than birds, and therefore there's a higher level of alert."

Given the risks, WHO officials say that behind-the-scenes processes are in place, including agreements with drugmakers, to produce "billions" of H5N1 vaccine doses within the first year of a pandemic, should this virus gain the ability to spread between humans.

Other scientists warn that's easier said than done, given the complexities involved in manufacturing and distributing a new set of vaccines.

"Even though we have a massive global infrastructure capable of producing flu vaccines… we will never be ready for a pandemic," McMaster University immunologist and vaccine researcher Matthew Miller said.

The spread of H5N1 throughout cattle herds across at least nine U.S. states is the latest curve ball from a virus that's proven adept at latching onto new hosts, raising questions over whether the world is prepared for more surprises.

U.S. farm workers in multiple states may now be in regular contact with infected cattle, as officials are monitoring at least 260 people for symptoms — amid growing concerns that limited testing could be masking the true scale of the outbreak, both in cows and in humans. (On Friday, the U.S. federal government did announce major funding to expand testing efforts, which includes paying up to $75 to farm workers who take part in studies.)

Early research, which hasn't yet been peer reviewed, also suggests the cells in cows' mammary glands can be infected by both avian and human influenza viruses. Scientists warn that scenario may provide a viral mixing vessel, fuelling the development of adaptations that hike the risk to human hosts.

None of these signals are a smoking gun that H5N1 will evolve to sustain transmission in a human population. Yet the possibility is always present, even if the virus's next moves remain somewhat unpredictable.

In response to questions from CBC News, WHO officials stressed that there are two H5N1 vaccine candidates already available that could be used in a pandemic scenario, along with options targeting other forms of avian influenza. 

There's no need to begin manufacturing these shots now, the WHO's Ryan said. Scaling up to produce pandemic vaccines would also mean a trade-off where manufacturers are forced to hit pause on annual shots for other strains.