Open this photo in gallery Pharmacist Ash Faltaous prepares to administer the Fluzone vaccine at the Danforth Drug Mart IDA, in Toronto, on Oct. 22, 2018. Christopher Katsarov

Canadian pediatric hospitals are seeing a surge in flu-related admissions as H1N1 re-emerges as the dominant strain of influenza this season.

The number of children admitted to hospital for the viral illness so far this season is more than twice as high as it was at this time last year, and more than three times as high as it was at this point in the 2016-2017 flu season, according to the Public Health Agency of Canada (PHAC.)

More children have also been sent to the intensive-care unit to fight severe incarnations of the flu than at this time in recent seasons, the figures show.

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The reason influenza is hitting children and adolescents harder than usual is the return of H1N1 after two seasons in which another type of influenza A, known as H3N2, prevailed.

There is, however, a silver lining to that change: While H1N1 is tough on kids, it’s easier on the elderly, the group most prone to serious complications and deaths from influenza.

Public-health experts also say the flu shot tends to work better against H1N1 than it does against H3N2, which means that influenza could wind up taking a gentler toll overall in the 2018-2019 season than in the past couple of seasons.

“Generally, H1N1 seasons can be milder. The reason they can be milder is that seniors who lived through H1N1 when they were younger have better immunity against H1N1," said Michelle Murti, a public-health physician with Public Health Ontario. “What we do see is a shift to a younger age group in H1N1 years where children and young adults tend to be more susceptible."

NUMBER OF ADMISSIONS TO HOSPITAL (16 YEARS OF AGE OR YOUNGER) Average number of admissions to hospital 2010-11 to 2017-18 Number of admissions to hospital 2018-19 80 60 40 20 0 Sept. 1 Dec. 29 CUMULATIVE NUMBER OF ADMISSIONS TO HOSPITAL (16 YEARS OF AGE OR YOUNGER) REPORTED BY PEDIATRIC HOSPITALS, SEPTEMBER TO DECEMBER, 2018. 0 - 5 months 37 6 - 23 months 105 2 - 4 years 139 5 - 9 years 98 10 - 16 years 35 PEDIATRIC ADMISSIONS TO HOSPITAL/DEATHS SO FAR THIS SEASON (UNDER AGE 16 AT 12 CHILDREN’S HOSPITALS.) Admissions to hospital have been well above the seven-year average every week since the season began. 2016/ 2017 2017/ 2018 2018/ 2019 Admissions to hospital 117 195 414 ICU admissions 18 35 71 Deaths 0 <5 <5 TRISH McALASTER / THE GLOBE AND MAIL SOURCE: CANADA.CA NUMBER OF ADMISSIONS TO HOSPITAL (16 YEARS OF AGE OR YOUNGER) Average number of admissions to hospital 2010-11 to 2017-18 Number of admissions to hospital 2018-19 80 60 40 20 0 Sept. 1 Dec. 29 CUMULATIVE NUMBER OF ADMISSIONS TO HOSPITAL (16 YEARS OF AGE OR YOUNGER) REPORTED BY PEDIATRIC HOSPITALS, SEPTEMBER TO DECEMBER, 2018. 0 - 5 months 37 6 - 23 months 105 2 - 4 years 139 5 - 9 years 98 10 - 16 years 35 PEDIATRIC ADMISSIONS TO HOSPITAL/DEATHS SO FAR THIS SEASON (UNDER AGE 16 AT 12 CHILDREN’S HOSPITALS.) Admissions to hospital have been well above the seven-year average every week since the season began. 2016/ 2017 2017/ 2018 2018/ 2019 Admissions to hospital 117 195 414 ICU admissions 18 35 71 Deaths 0 <5 <5 TRISH McALASTER / THE GLOBE AND MAIL SOURCE: CANADA.CA NUMBER OF ADMISSIONS TO HOSPITAL (16 YEARS OF AGE OR YOUNGER) CUMULATIVE NUMBER OF ADMISSIONS TO HOSPITAL (16 YEARS OF AGE OR YOUNGER) REPORTED BY PEDIATRIC HOSPITALS, SEPTEMBER TO DECEMBER, 2018. Average number of admissions to hospital 2010-11 to 2017-18 Number of admissions to hospital 2018-19 0 - 5 months 80 37 60 6 - 23 months 40 105 20 2 - 4 years 0 139 Sept. 1 Dec. 29 5 - 9 years 98 10 - 16 years 35 PEDIATRIC ADMISSIONS TO HOSPITAL/DEATHS SO FAR THIS SEASON (UNDER AGE 16 AT 12 CHILDREN’S HOSPITALS.) Admissions to hospital have been well above the seven-year average every week since the season began. 2016/ 2017 2017/ 2018 2018/ 2019 Admissions to hospital 117 195 414 ICU admissions 18 35 71 Deaths 0 <5 <5 TRISH McALASTER / THE GLOBE AND MAIL; SOURCE: CANADA.CA

H1N1 is best-known as the culprit behind the 2009-2010 influenza pandemic, when a never-before-seen variant of the virus emerged to sicken and kill many more people than regular seasonal influenza. Today’s H1N1 is very similar to the pandemic strain, but it has much less of an impact now that patients have had the chance to build immunity against it.

Very young children, however, have not had as much opportunity to erect defences against H1N1.

As of Dec. 29, the 12 pediatric hospitals that report flu statistics to PHAC had logged 414 flu-related hospital admissions since September, up from 195 in the fall of 2017 and 117 in the fall of 2016.

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Flu-related admissions to children’s intensive-care units are up, too: 71 so far this season compared with 35 and 18, respectively, at this point in the last two seasons.

The pediatric hospital network has documented at least one flu death so far this season but fewer than five; for privacy reasons, PHAC does not release exact figures when there have been fewer than five deaths.

By contrast, the number of flu outbreaks in long-term care homes is way down.

In British Columbia, for example, there have only been six confirmed influenza outbreaks in long-term care facilities, a considerable drop from 36 such outbreaks by this time last season and 63 the season before that, according to Danuta Skowronski, the epidemiology leader for the influenza and emergency respiratory pathogens program at the BC Centre for Disease Control.

“Three years ago was the last time we had an H1N1-dominant season and we saw far fewer long-term care facility outbreaks than we saw during the 2016-2017 or 2017-2018 seasons,” she said.

Theresa Tam, Canada’s chief public health officer, said another anomaly this season is that influenza began spreading earlier than usual in some parts of the country, especially on the Prairies.

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That helps to explain why the number of laboratory-confirmed cases of the flu is 22 per cent higher than it was at this time last season, with 13,796 cases logged as of Dec. 29, the most recent date for which figures are available. That is up from 11,275 at the same time last season and 6,180 at this point in the season before.

“It may appear like there’s more activity at this point, but it’s really just an earlier start," Dr. Tam said. "What we don’t know is how it’s going to progress. We haven’t seen that peak yet.”

Dr. Tam emphasized that it’s not too late to get vaccinated against the flu, especially in places like Ontario where the influenza season is just picking up steam.

A midseason evaluation of efficacy of this year’s vaccine is not expected until late January or early February, but Dr. Tam said early signs point to the shot working better this year than last year.