Influenza is surging in Canada, with a test positivity rate of 24.3% in the first week of February, according to data from the Canadian respiratory virus surveillance report .

There were 10,449 detections of influenza during the week ending February 8. About 94% of cases resulted from influenza A (H1N1), and about 6% of cases resulted from influenza B (H3N2). Older individuals (age ≥ 65 years) had the highest number of influenza detections, and the rate of hospitalizations per week was 3.4 per 100,000 population.

The province with the highest influenza prevalence is Quebec. Jesse Papenburg, MD, associate professor of pediatrics at McGill University in Montreal, Quebec, Canada, told Medscape Medical News that he is seeing a spike in influenza-like illness in children. Papenburg also is a pediatrician at Montreal Children’s Hospital, Montreal, Quebec, Canada.

“In Canada, we prioritize therapy for outpatients who are at risk for complications or who are sick enough to require hospitalization,” said Papenburg. “Most healthy children, even if they are diagnosed with influenza, do not require antiviral therapy. Many don’t require medical attention at all. But we are seeing a real increase, week over week over week, in the number of hospitalizations due to influenza, and some of these children have even required intensive care. The numbers are quite high.”

During the early years of the COVID-19 pandemic, the prevalence of influenza was low because people stayed home, said Christopher Labos, MD, a lecturer at McGill University. “Especially early on during COVID, people were wearing masks, people weren’t gathering in large groups, so yes, relative to those years, it does seem to be higher. We will have to see what the final numbers look like. We only really know how bad a flu season was in retrospect, but this does seem to be a relatively bad year, all things considered,” Labos told Medscape Medical News .

“This is the highest number of cases we have detected since the 2014-2015 flu season, but it’s a little early to say how this season compares to others in terms of hospitalizations, because one of the things we know about influenza A, which is the predominant strain, is that it affects different populations differently,” said Papenburg.

For example, H3N2 disproportionately affects the older population. For this reason, the 2014-2015 season was a bad year for adult hospitalizations and influenza-associated mortality. H3N2 predominated, and there was a mismatch between the circulating strain and the vaccine strain, said Papenburg.

“Our most vulnerable adults and elderly with chronic medical illness or just clearly older were hit quite hard. To make matters even worse, the H3N2 strain was a poor match for what was in the vaccine, so the vaccine did not offer any protection that year. In contrast, in pediatrics, we had a very early and intense 2022-2023 season, and children in Canada were disproportionately affected by that strain, even though the vaccine match was acceptable,” Papenburg said.

“What is interesting about this year is that we have concurrent high levels of circulation of H1N1 and H3N2. It’s an influenza A outbreak primarily, but within that, both subtypes are present in high numbers. This may be contributing to why we’re seeing so many cases. It’s almost like we are having two outbreaks at the same time because two strains are contributing to the burden of disease,” he said.

Vaccine effectiveness varies from year to year, Papenburg added. “Unfortunately, we need to choose which strains go into next year’s vaccine based on what was circulating in the southern hemisphere. It’s our best guess, based on genomic surveillance and the characteristics of the viruses, to give the vaccine manufacturers at least 6 months to produce next year’s influenza vaccines. Sometimes we get it right and can have a vaccine effectiveness of 90% against the circulating strain. Other years, unfortunately, the virus changes substantially in the interim, and you can have vaccine effectiveness of next to nothing. We know that H3N2 changes genetically at a faster rate than H1N1, so it is more difficult to predict the circulating strain,” he said.

Early estimates indicate that the current vaccine’s effectiveness is 50% against both H1N1 and H3N2 strains.

“The vaccine is performing decently well,” Danuta M. Skowronski, MD, epidemiology lead for influenza and emerging respiratory pathogens at the BC Centre for Disease Control and a clinical professor in the University of British Columbia School of Population and Public Health in Vancouver, British Columbia.

“It cut the risk of influenza severe enough that someone might need to see a doctor by about half, which is decent protection. It’s not perfect, but cutting your risk in half of illness so severe that you needed to see a doctor is pretty good,” Skowronski told Medscape Medical News .

But she emphasized that researchers are starting to see concerning new variants emerging within the H1N1 and H3N2 viruses. “We want to keep monitoring this. It’s especially important because the last week of February is when the World Health Organization has to decide whether it needs to change the vaccine components for next season,” she said.

The most important way to protect against severe illness from influenza is to get vaccinated. “Vaccine makes the biggest difference between whether you get sick or severely sick. Other things you can do that make a difference are to wear a mask if you are sick and have to go out, stay home, and keep your distance from other people,” Labos said.

Improving the air quality of office buildings and other indoor spaces should be considered, said Labos. “If you have an air filter in place and can filter indoor air, you make it less likely that people are going to spread the virus when they come to work or go to school. Air filtration is something we really need to start thinking about,” he said.

Papenburg reported grants from Merck and personal fees from Enanta. Labos and Skowronski reported having no relevant financial relationships.