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The flu shots given out free in Ontario are even less effective this year than anyone thought, says the province’s public-health agency.

In fact, the strains of influenza many were immunized against are almost a complete mismatch to the dominant strain of the virus that is circulating out there this season.

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That doesn’t mean the vaccine is useless — just much, much less effective than would be ideal.

Statistics released in the United States on Thursday point to the problem.

The U.S. Centers for Disease Control released data showing that the flu vaccine, which is substantially the same in the U.S. as here, has been 23% effective.

That means that people who got the flu shot have been 23% less likely to have to see a doctor about influenza than people who didn’t get it. In a good year, that number is about 60%. In a year when the vaccine is a bad match, it’s typically about 40%.

A flu shot is actually a small collection of vaccines all given together. Each of them are designed to fight off a particular family of influenza viruses. What goes into each year’s vaccine is based on experts’ educated guesses about which viruses are going to be the most common each year.