Official estimates point to 2018-19 flu shot being most effective in years

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The flu shot is working better in Canada than it has in years, especially for the young children at greatest risk of being made seriously ill by H1N1.

The first official estimates of the vaccine’s performance, released Thursday, found the shot was 68-per-cent effective at preventing all types of influenza, and 72 per cent effective at warding off H1N1, the strain of influenza A that has prevailed this season.

The vaccine protected 91 per cent of children between the ages of 1 and 8 from H1N1, according to the BC Centre for Disease Control (BCCDC), whose figures are the first to officially reveal how well the shot is working in the Northern Hemisphere.

“This is substantial protection, especially for those at risk for serious outcomes – and we have seen more than usual amongst younger individuals this year,” said Danuta Skowronski, the epidemiology leader for influenza and emerging respiratory pathogens at the BCCDC. “This is good reason to be vaccinated.”

Over all, the 2018-2019 vaccine is a big improvement over the previous four seasons, when the shot provided scant defence.

Last year’s version was only 17-per-cent effective against the dominant H3N2 strain of the virus at midseason. The last time the shot’s overall effectiveness approached 70 per cent was in the 2013-2014 season.

Why is the 2018-2019 shot so effective? “It’s really simple, actually,” said Allison McGeer, the medical director of infection control at Toronto’s Sinai Health System. “It’s because it’s H1N1.”

The flu vaccine generally works better against H1N1 strains, Dr. McGeer said, because they mutate slower than other types of influenza A, making it easier for vaccine designers to keep up.

As well, the vaccine seems able to piggyback on the immunity that adults – especially seniors – have built over time against H1N1.

“In an H1N1 season, you still see outbreaks in nursing homes and old people getting sick, but you see relatively much less of it because all of us old folk have an accumulation of history that gives us a substantial degree of protection,” Dr. McGeer said.

The downside is that H1N1 tends to take a higher toll on children, as it has this season.

Influenza has sent twice as many children to hospital and three times as many to the intensive-care unit compared with this time last season, according to the 12 pediatric hospitals that report flu statistics to the Public Health Agency of Canada (PHAC).

Those same hospitals reported seven pediatric flu deaths to PHAC as of Jan. 12, the most recent figures available. It is not known how many of those children were vaccinated.

However, in the handful of flu deaths covered by local media this season, including three preschoolers in Saskatchewan, the children had not been vaccinated.

Bryna Warshawsky, the medical director for communicable diseases, emergency preparedness and response at Public Health Ontario, said it is not too late to get vaccinated, even though influenza A appears to have peaked in parts of the country.

“We haven’t begun to see the influenza B really pick up," she said. "Getting vaccinated now should offer protection against that as well.”

The BCCDC’s midseason estimates of vaccine effectiveness were published Thursday in the journal Eurosurveillance.

The BCCDC gathers its data from a network of hundreds of primary-care physicians in British Columbia, Alberta, Ontario and Quebec, who offer nasal swabs and questionnaires about the flu vaccine to patients with symptoms of respiratory illness. The results are then sent to the BCCDC for analysis.

“By comparing the proportion that said they were vaccinated in those who test positive [for influenza], compared to those who test negative, we’re able to derive the vaccine protection in the vaccinated compared to the unvaccinated,” Dr. Skowronski explained.