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An influenza strain that hits children hard is circulating widely in Canada, leading to increased pediatric hospitalizations, which experts say could worsen as we approach the peak of flu season.

Infectious disease experts say a type of influenza B is circulating at about the same rate as influenza A, which is not typical. In most years, influenza B doesn’t circulate until late in the season. This has experts on alert because this type of influenza is linked to a higher risk of hospitalizations and deaths among children.

The Public Health Agency of Canada’s most recent FluWatch report, released Friday, says the increased presence of influenza B likely explains why pediatric illnesses and hospitalizations are higher this year. According to PHAC, the number of pediatric hospitalizations linked to influenza A this season is similar to previous years.

From Dec. 15 to Jan. 4, there were 258 pediatric hospitalizations linked to the flu reported by a national surveillance network that consists of 12 pediatric hospitals. PHAC said the number of cases is above the average seen in the previous five years.

Over all, there have been 370 pediatric hospitalizations linked to the flu in Canada so far this season. Of those, 54 per cent were tied to influenza B. About two-thirds of hospitalizations were in children under five years old. No flu-related deaths have been reported in children this season.

The particular type of influenza B that is circulating belongs to a family called the Victoria lineage, which is known to cause a higher rate of hospitalizations and deaths among children.

U.S. health officials say they are seeing a surge in flu cases and a much higher than usual number of pediatric flu hospitalizations and deaths. So far this flu season, 27 children have died in the U.S. as a result of the infection, with 18 of those cases being linked to the influenza B strain, according to the U.S. Centers for Disease Control and Prevention. Nine deaths were linked to H1N1, which is an influenza A virus.

"Any time we see more influenza B, especially [the Victoria lineage], we will see more children be affected,” said Michelle Murti, a physician at Public Health Ontario. “Some proportion of those will either be hospitalized or die.”

Alyson Kelvin, a virologist at Dalhousie University and a member of the Canadian Centre for Vaccinology, said it’s quite possible the number of pediatric cases will continue to climb here, as they have in the United States. She said people who haven’t had their flu shot should still get one.

“I think it’s a good idea to be vaccinated, due to this evidence,” she said.

This year’s flu shot is not an exact match to the influenza B strain in circulation and experts don’t yet know how well the vaccine protects against the B strain, according to Dr. Murti. The good news, she said, is that the vaccine should still offer fairly good protection from influenza B, even if the shot isn’t matched to the virus.

Data showing how well this year’s flu shot match up to the circulating viruses won’t be available until next month, Dr. Murti said.

So far this year, 12,500 laboratory-confirmed cases of the flu have been detected in Canada. But these cases only represent a portion of the total, as most are typically not verified by a lab.

Just more than half of the flu cases detected in Canada this year are linked to the influenza A virus. Of those, H1N1 and H3N2 are both circulating, with the amount of H1N1 increasing in recent weeks. The flu shot is well matched to the H1N1 virus, Dr. Murti said.

According to PHAC, there have been 230 adult hospitalizations, seven intensive care unit admissions and fewer than five deaths linked to the flu. Of those, 90 per cent have been linked to influenza A. The FluWatch report says that 87 per cent of influenza A hospital admissions were linked to H1N1.

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