It's the time of year when experts crunch the numbers to see how well the flu shot worked. The result? Better than last year, but still not good enough.

"Overall, just shy of 45 to 50 per cent," said Dr. Danuta Skowronski of the BC Centre for Disease Control, who presented the data to the Global Influenza Vaccine Effectiveness meeting at the World Health Organization last week.

"That's lower than we would like to see, but it's an improvement over the previous year, because it couldn't be worse, frankly"

In 2014-15, the flu shot offered essentially zero protection against the circulating influenza virus of that season. Back then, the prevailing strain was H3N2.

'It's an improvement over the previous year, because it couldn't be worse, frankly.' - Dr. Danuta Skowronski , BC Centre for Disease Control

This year's main circulating virus was H1N1, which flu watchers will remember from the pandemic of 2009-10.

Skowronski said the vaccine was well-matched, but overall, the protection was disappointing.

"We would like to see it higher than 50 per cent," she said. "Previously we have measured H1N1 protection closer to 60 or 70 per cent, so we would like to know why now it's lower."

Made-in-Canada protocol reveals vaccine weakness

Experts used to believe the annual flu shot protection was much higher, around 70 to 90 per cent. But not anymore. Those early estimates were based on industry-funded clinical trials that were extrapolated to apply across all ages and flu seasons.

"It was a blanket assumption that is simply not true," Skowronski said.

That assumption changed dramatically, after Skowronski and colleagues developed a protocol that revealed the true picture of vaccine efficacy. It's called the test negative design (TND) first piloted in Canada in 2004.

"The test negative design has opened our eyes to all kinds of variables that we were blind to for years. We were flying blind," said Skowronski.



The TND system tracks people who show up with flu-like symptoms at several hundred doctors' offices across Canada. Doctors take a sample from every patient for lab testing. They also record whether the person got the flu shot. That simple protocol reveals how often the flu shot fails.

In April, The Lancet published a meta-analysis of test negative design studies, now used globally, and cited Canada as the pioneering group.

"It is no exaggeration to say that the TND has transformed our understanding of the variability in influenza vaccine protection — by season, by subtype, by age group, by vaccine product," said Skowronski.

There's no use promoting a vaccine that isn't working well. - Dr. Danuta Skowronski, BC Centre for Disease Control

The good news is the data are more reliable. But the bad news is that experts now realize the flu vaccine protects only about half of the time.

"Our public health goal is to minimize the serious health problems association with the flu," said Skowronski. "There's no use promoting a vaccine that isn't working well. The only way you can do better is to recognize the problem in the first place."

This year's data also show that only about a third of Canadians got the flu shot, about five per cent less than last year.

"That's not a surprise coming out of a year where effectiveness was so low. That had to have an impact," said Skowronski.

Flu shot paradox

Scientists also once again observed the paradox that previous vaccination against the flu seemed to lower protection.

The so-called "Canadian problem" was first identified by Skowronski's team during the H1N1 pandemic.

So far, scientists have not been able to explain why people who get the shot with no prior vaccine exposure seem to have better protection than people who get the shot year after year.

This year's Canadian data also reveal a trend, observed in the U.S., that the flu mist vaccine is not working.

The mist, made with a live, but weakened virus, was originally believed to be more effective in children and less upsetting than getting a needle.

But last month a U.S. expert panel advised doctors to stop using it, after it performed poorly for three years in a row.

As for a recommendation about whether people should still get the flu shot if it's only working half the time, Dr. Skowronski said it is strongly recommended for elderly people and for those who are immunocompromised.

"For people at high risk of serious complications of influenza, like hospitalization or death, the stakes can be especially high. For them, a 50 per cent reduction in risk compared to if they had not been vaccinated is a very important." she said.

"They are strongly recommended to receive the vaccine."