Fertilized chicken eggs are used to manufacture flu vaccines. Credit: Jason Alden/Bloomberg via Getty
During the summer of 2023, fur farms in Finland that raise mink, foxes and raccoon dogs were hit by an outbreak of H5N1 avian influenza. Nearly half a million animals were culled during the outbreak, and Finnish health officials were on high alert. “We worried that under circumstances where many animals are confined in small places, the pathogen might rearrange genetically into a new pandemic virus,” says Hanna Nohynek, a vaccinologist and chief physician at the Finnish Institute for Health and Welfare in Helsinki.
Finnish authorities responded by offering to vaccinate groups at high risk of H5N1 exposure, including fur-farm workers, laboratory technicians and veterinarians — the first and only country to do so.
Nature Spotlight: Influenza
This was a precautionary measure: although H5N1 is spreading among birds and other animals all over the world, human cases are still rare. But this state of affairs could change. An H5N1 pandemic “might be the big one that many of us have worried about”, says Ashish Jha, a physician and dean of public health at Brown University in Providence, Rhode Island.
The bird-flu shot used in Finland was developed by CSL Seqirus in Holly Springs, North Carolina — one of a handful of companies developing vaccines against H5N1. Amassing a global arsenal of vaccines for the virus is a tricky proposition. Like most other influenza vaccines, the shots are manufactured in chicken eggs or cell lines. New batches take up to six months to produce, which is a long wait if a pandemic starts to rage.
And there is no guarantee that H5N1 will ever be a deadly threat. By the time Finland implemented its vaccine programme, having waited until the European Medicines Agency (EMA) in Amsterdam licensed the Seqirus shot in July 2024, the outbreak in its fur industry had ended without a single human infection. “You don’t want to stockpile millions of vaccine doses that you may never need,” says Marco Cavaleri, head of the EMA’s Office of Biological Health Threats and Vaccine Strategy.
Bird flu in brief
There are four basic types of influenza virus: A, B, C and D. Influenza A and B both cause seasonal flu in people, but only the A type has a history of causing human pandemics. The virus is further broken out into subtypes based on two surface proteins. One of the proteins, haemagglutinin, binds to receptors on host cells and has 18 subtypes (H1–18). The other protein, neuraminidase, has roles in the spread of the virus and has 11 N subtypes.
Influenza A viruses circulate in various animal species. And when the virus jumps from birds to humans, the consequences can be deadly. H7N9 bird flu, which emerged in China in 2013 and has so far infected nearly 1,600 people, has a fatality rate in humans of around 40%. Nearly 1,000 people have been infected with H5N1 since 2003, mostly through direct contact with infected animals. Roughly half of the people affected died. Human-to-human bird flu transmission has been documented only in rare instances involving prolonged exposure to a seriously ill person.
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