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.
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.
THE CANADIAN PRESS/Pawel Dwulit/FilesEven a badly matched vaccine can lessen the symptoms if you catch the flu anyway, medical professionals say.
The catch is that the decision, through the World Health Organization, has to be made in February to allow all the vaccines to be produced.
For eight months, influenza viruses circulate at low levels, evolving and “drifting” into slightly different forms. People start getting shots in about October and flu season takes off in November. That’s when we find out which strains of influenza are actually the most common, and how closely the vaccine matches them.
“The U.S. is saying that two-thirds of their strains are mismatched,” Dr. Bryna Warshawsky, a doctor with Public Health Ontario, said on Thursday.
“We’re seeing much higher … closer to 100% of our strains mismatched.”
Specifically, of 160 finely distinguished types of flu Public Health Ontario has detected this season, 158 are considered bad matches for the vaccine.
That doesn’t mean with total certainty that it’ll be even less effective in Ontario than in the United States, but that’s likely the case, Dr. Warshawsky said. How well it’s worked on actual patients is the research Public Health Ontario is still working on.
The problem is that the most common strain of flu this year was nowhere last February.
“The strain that’s predominantly circulating now did not show up on the radar screen, was not detected until the end of March,” she said. “There was no way to mitigate this. It’s a function of the delay between the time the strains are chosen and the time the season starts.”
The findings don’t mean this season’s vaccine is useless, Dr. Warshawsky emphasized.
“There still is protection,” she said. “You’re a quarter less likely to get the flu if you have been vaccinated than if you haven’t.” Even a badly matched vaccine can lessen the symptoms if you catch the flu anyway, because the viruses are not completely different. And there’s also sometimes a second strain of flu that makes the rounds late in the winter, against which one of the components in this year’s vaccine could provide better protection.
The Centers for Disease Control has also found that the vaccine has been particularly ineffective for older people in the United States: only 14% effective for vaccine recipients older than 65. They’re usually the most vulnerable to influenza, but the strain of flu most common this year is exceptionally hard on them. On the flip side, the U.S. authorities found, the vaccine is a little more effective in small children. But still only about 26%.