The effectiveness of this season's flu shot has been extremely low, with the vaccine preventing only 10 to 20 per cent of infections caused by the dominant H3N2 strain affecting Canadians, researchers have found.
The mid-season estimate for the vaccine's ability to provide protection from this particular influenza strain is in line with what Australia experienced during the southern hemisphere's flu season, which ended in late August as Canada's was about to begin.
"That means people who were vaccinated should not consider themselves invincible for this season," said lead researcher Dr. Danuta Skowronski, an infectious diseases expert at the BC Centre for Disease Control.
Both Canada and Australia used the same vaccine components, which are meant to offer protection against two A-type influenza viruses – H3N2 and H1N1 – and a B strain.
The identical vaccine was used last season in Canada and was found to be 42 per cent effective in preventing cases of H3N2. But because the virus appears to have genetically mutated since then, that's likely made the shot less effective this year.
Vaccine efficacy was determined by comparing positive influenza tests in vaccinated and unvaccinated patients who sought care at community-based practitioners in British Columbia, Alberta, Ontario and Quebec, representing 80 per cent of the population. These providers make up the Canadian Sentinel Practitioner Surveillance Network that monitors how well the influenza vaccine protects people from influenza.
Network researchers analyzed the data and found that overall, the flu shot was 17 per cent effective in preventing the respiratory illness from the H3N2 virus, but only 10 per cent effective for working-age adults 20 to 64.
Referring to the Canadian and Australian findings, Skowronski said "neither estimate was significantly different from zero, so we can't rule out the possibility of no protection.
"The bottom line is the protection for the H3N2 virus is low this season."
To put the findings in context, a 10 per cent vaccine effectiveness for H3N2 means that if 10 in 100 people who had not been vaccinated came down with that strain of the flu, then nine in 100 people who did get a jab in the arm would still get sick, she explained.
"So it's not a big reduction in risk for the H3N2 component this season."
However, Skowronski said the vaccine was 55 per cent effective overall in preventing cases of the respiratory illness caused by the dominant influenza B strain also affecting Canadians this season. In working-age adults, it was 40 per cent effective.
That strain is known as B/Yamagata. But interestingly, the component in the vaccine is B/Victoria – which has a completely distinct viral lineage from B/Yamagata.
A 55 per cent vaccine effectiveness suggests there is good cross-protection between the two B strains, said Skowronski.
"It's not 90 per cent, but it's not 10 per cent either," she said from Vancouver.
"What this is saying to us is that vaccine effectiveness and immunity to influenza viruses is more than about the antigenic match that we have previously understood to predict vaccine protection. There are other things that are driving it."
Dr. Allison McGeer, director of infection control at Mount Sinai Hospital in Toronto, said there have been a lot of uncertainties among virologists about how potent the vaccine would be against flu A and flu B this year.
"So I think the bad news is that we were kind of hoping that H3N2 might be a little better," McGeer, who was not involved in the network's analysis, said Thursday. "But the good news is that the influenza B efficacy has held nicely."
She said people have a tendency to believe that if a seasonal flu vaccine is only 20 to 30 per cent effective, then it's not worth getting.
"That's just not true," McGeer said. "If I offered you, for instance, a $15 intervention that would reduce your chance of getting in a car accident by 20 per cent, you'd take it in a heartbeat.
"And that's what flu vaccine is like. It would be lovely to have a flu vaccine that was 100 per cent effective.... But in the face of the fact that we don't, a flu vaccine that is 20 per cent effective is still better than nothing."
Still, Skowronski says the findings suggest that inoculated Canadians can't rely solely on the vaccine to protect them from coming down with the flu this season.
"Other protective measures need to be considered, the tried-and-true public health measures – so washing your hands often with soap and water to minimize contamination ... (and) if you do become sick, you should stay home from work to minimize the spread to others, and avoid close contact with higher-risk individuals, so social distancing measures."
Visits to long-term care homes should also be avoided because the elderly have the greatest risk of getting sick from the H3N2 virus and developing complications that can lead to hospitalization or even death.
The mid-season estimates, published Thursday in the online journal Euro Surveillance, are important for helping to inform the World Health Organization, which will meet later this month to decide which influenza strains should be included in next year's vaccine.
The WHO will look at vaccine effectiveness rates from Canada and Australia, as well as those from Europe and the United States, which have not yet been reported.