In 2009, Getting a Seasonal Flu Shot Put You At Risk for That Year’s Pandemic Swine Flu

In the spring of 2009, the H1N1 pandemic influenza, commonly referred to at the time as swine flu, had the world in distress as they faced what had the potential to be an incredibly deadly emergent disease. Meanwhile, in Canada, an unnerving trend was unfolding, says Helen Branswell for the Canadian Press.

Canadian researchers noticed in the early weeks of the pandemic that people who got a flu shot for the 2008-2009 winter seemed to be more likely to get infected with the pandemic virus than people who hadn’t received a flu shot.

In a new study, lead by Danuta Skowronski with the British Columbia Centre for Disease Control, researchers have confirmed this initial inkling of a link between the 2008-2009 seasonal flu vaccine and the 2009 H1N1 swine flu pandemic. The researchers gave 16 ferrets, the preferred animal for studying human influenza, the seasonal flu vaccine. Then, they gave these 16 inoculated ferrets and 16 other ferrets a dose of the 2009 pandemic flu. “The ferrets in the vaccine group became significantly sicker than the other animals, though all recovered,” says Branswell.

The flu, or influenza, comes in a whole range of strains, with fancy names like H5N1 or H3N2, scientific nomenclature which often competes with more colorful colloquialisms such as “swine flu” or “avian flu.” The diversity in naming mirrors either the physical make-up of the virus, or the animal host in which it is thought to have evolved. But for any given year, influenza viruses also break down into one of two broad categories: seasonal influenza, or pandemic influenza.

The viral cause of seasonal influenza varies from year to year, but the consequences are usually the same: a lot of healthy people get sick, or miss work to take care of children. And, between 3,000 and 49,000 people die of flu-related illnesses, predominantly very young children, the elderly, or people with weakened immune systems.

Pandemic influenza, on the other hand, crops up when a new strain of influenza-causing virus evolves, one against which our bodies’ immune systems have only limited protection, and is able to spread widely around the world. Death rates can vary widely, peaking in the hundreds of thousands as was the case with the 1918 “Spanish Flu,” or the 2009 H1N1 swine flu.

According to Branswell for the Canadian Press, researchers still don’t really know exactly why getting the seasonal flu vaccine increased the risk from the pandemic flu. But, cautions the study’s lead researcher Skowronski, this shouldn’t be seen as a knock against getting your seasonal flu shot.

“Pandemics are infrequent occurrences, but seasonal influenza recurs on an annual basis. It’s a substantial cause of morbidity and mortality,” — science’s term for illness and death — “and the seasonal vaccine substantially protects against that severe outcome due to seasonal influenza.”

More from Smithsonian.com:
How to Give a Ferret a Deadly Flu
Curating Pandemic at the American History Museum

Colin Schultz | | READ MORE

Colin Schultz is a freelance science writer and editor based in Toronto, Canada. He blogs for Smart News and contributes to the American Geophysical Union. He has a B.Sc. in physical science and philosophy, and a M.A. in journalism.