This year’s flu vaccine 40 per cent effective
B.C. expert rates shield against H3N2 strain as ‘decent’
TORONTO — This season’s influenza vaccine is estimated to have been more than 40 per cent effective in preventing illness with the dominant H3N2 viral strain in Canadians who got their shots, a national network of infectious diseases experts says.
The Canadian Sentinel Practitioner Surveillance Network determines vaccine effectiveness by analyzing how many inoculated people tested positive for the flu virus compared to those who were unvaccinated.
“A vaccine effectiveness of 40 per cent against the H3N2 virus is decent,” lead researcher Dr. Danuta Skowronski of the B.C. Centre for Disease Control said Thursday from Vancouver.
“It’s lower than we would like to see, particularly because H3N2 epidemics tend to be associated with more hospitalizations and deaths. But reducing the risk by 40 per cent is really important, not only at the individual level, but also at the population level when we tally the serious outcomes at the end of the season.”
Young children, the elderly and people with underlying health conditions such as heart disease are vulnerable to complications from flu; H3N2 flu is particularly hard on the elderly.
An effectiveness level of about 40 per cent means the risk of getting sick enough to require medical attention is almost cut in half, and Skowronski said that’s important for those at risk of complications.
This season’s vaccine, which contained components aimed at preventing illness from two A strains — H3N2 and H1N1 — and a B strain, has been much better at protecting people than the 2014-15 shot, which was mismatched to the H3N2 genetic variant that ended up circulating that season.
The decision on which influenza strains — and which genetic variations of those strains — to target in an upcoming flu season is decided by the World Health Organization each February, giving pharmaceutical companies time to produce the vaccine.
But in the intervening months, the viruses can genetically mutate, sometimes leading to a mismatch, as occurred in 2014-15.
Skowronski said vaccine effectiveness levels from Canada and other countries, as well as what genetic mutations might have occurred as the flu bug was passed from person to person over the season, help the World Health Organization decide what strains next season’s vaccine should contain.
“There’s a lot of complexity to the influenza virus. We like to think of it as just a uniform beast, but it’s not. It’s constantly changing shape, it’s constantly evolving.”
And from what the surveillance network is seeing, next season’s vaccine could very well be altered.
“There are quite a few emerging variants of H3N2 virus, by province,” she said, noting that genetic decoding of virus samples in Alberta showed a lot of consistency, but B.C.’s cases presented a “rainbow” on a coloured bar graph. “Ontario and Quebec are also showing a smorgasbord of viruses, compared to what Alberta was showing.
“This virus is dynamically evolving, real time, during the epidemic. So there’s variation ... that’s showing up in this virus as it’s trying to evade the immunity in the population.”