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For most of us, flu season begins each fall. For flu experts in the United States, however, it might as well begin in February.
That's when scientists and researchers from around the world gather for a meeting hosted by the World Health Organization (WHO), where they spend days reviewing which strains of the flu virus have been making people sick and decide which strains the next season's vaccine should cover.
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But when experts touched down in Beijing this year, their goal — to recommend a vaccine formula for the Northern Hemisphere's 2019-2020 flu season — hit a roadblock.
A new variety of the flu virus known as H3N2 had begun to spread rapidly in several countries. In the United States, it caused a second wave of illnesses late in a flu season that had already seen the rise and fall of another strain entirely.
"That was something that we were concerned about and wanted to incorporate into the Northern Hemisphere vaccine … so we delayed the H3N2 decision by a month,” says virologist and physician Kanta Subbarao, director of the WHO Collaborating Centre for Influenza in Melbourne, Australia, one of six WHO centers that help track flu worldwide.
The extra month gave experts more time to analyze the newly circulating virus. By March, they had updated their recommendation to include a suitable H3N2 strain for this season's flu shot, which is now available.
More importantly, the delay proved an old truth about an ancient virus: The flu is a moving target, and for the network of laboratories, agencies and manufacturers who help make the flu vaccine each year, tracking it is only the start.
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A needle in a haystack
The challenge starts with the virus itself.
An expert shape-shifter, the flu is constantly changing — mutating as it replicates itself — in ways that allow its strains to get past our body's immune defenses even if we've had the flu before, or if we roll up our sleeves for the shot each fall.
The result? “It's a bit of a war between us and the virus,” says David Wentworth, director of the WHO Collaborating Centre for Influenza in the U.S., which is run out of the Centers for Disease Control and Prevention (CDC) in Atlanta.
This battle plays out not only within the bodies of people who come down with the flu's signature fever, chills and muscle aches, but also in laboratories around the world, where researchers must work quickly to analyze how the flu virus is changing in order to predict what it might do next.
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