The first data on how well the flu vaccine is working this season in North America has just been published — and it helps explain why everyone appears to be sick right now.
The study, from the journal Eurosurveillance, found that the flu vaccine was only 10 percent effective against H3N2 (the main flu subtype going around in the US this season) among adults in Canada.
That means the shot only prevents 10 percent of H3N2 flu cases. So if 100 in 1,000 unvaccinated people develop flu, the number would drop to 90 in 1,000 among vaccinated people — a very small difference in flu risk between vaccinated and unvaccinated individuals. And more than 80 percent of confirmed US flu cases have involved H3N2.
"The evidence is mounting, from Australia and now from Canada, that the vaccine protection is low [this year]," said lead study author Danuta Skowronski, an influenza epidemiologist at the British Columbia Center for Disease Control. "Given the same H3N2 subtype [is circulating in the US], our estimate of low vaccine protection should also apply to the US."
For the study, Skowronski and her co-authors compared the vaccine coverage rates in Canadians who went to the doctor and tested positive for the flu with the rates of those who tested negative. In a good year, the flu shot's effectiveness hovers between 50 and 70 percent. But in years when the H3N2 type of flu virus circulates — as it is this year — the vaccine tends to be less protective.
So Skowronski wasn't surprised by the dismal data. Plus, in Australia, where the flu season peaks in August and H3N2 struck as well, early estimates suggested the shot was only 10 percent effective there too.
"Our findings reinforce the concerns that were expressed from Australia using the same vaccine [formulation] for their 2017 epidemic," she said. Flu vaccine effectiveness can vary by country, and the US data has yet to be published, but the Canadian study is telling.
But to understand why the shot appears to be underperforming this year, you need to understand how the flu vaccine is made — a production process that involves eggs.