Not one person in the NWT was diagnosed with flu this year. Why?

Public health measures designed to stop Covid-19 may have all but extinguished the less-transmissible flu virus – at least for the time being.

Damien Healy, a spokesperson for the NWT’s Department of Health and Social Services, confirmed by email on Monday that the Northwest Territories has recorded zero cases of influenza this year.

The absence of seasonal influenza isn’t unique to the NWT. It’s happening all over the world.

“It’s not surprising to me, actually,” said Philippe Lagacé-Wiens, a professor of medical microbiology at the University of Manitoba. “All the cases we detected across Canada were oddballs. They weren’t even proper cases of influenza. To hear the Northwest Territories, with its small population, has no confirmed cases is not surprising to me at all.”

Lagacé-Wiens is the lead author of a March study examining the impact of Covid-19 public health measures on the number of people in Canada testing positive for the flu.

The study found the number of positive influenza tests began to drop after provincial and territorial governments released voluntary public health messaging in early March of last year. As governments learned of the severity of Covid-19 and responded with stricter public health measures, influenza rates plummeted.

“Three or four weeks after these interventions were put in place, we saw the virtual elimination of influenza from the population,” said Lagacé-Wiens.

The same is true around the world.

The World Health Organization reported lower-than-normal global influenza activity last year. In some regions, like southern Asia and tropical South America, the WHO’s Global Influenza Programme detected zero cases. The organization hasn’t yet released data for 2021.

Positive influenza tests in the NWT last year dropped to zero by mid-April and stayed there. It is not uncommon for the territory to record flu cases into May.

‘Something new’ for the next flu?

The same public health measures that are struggling to contain Covid-19 have stopped the flu almost entirely. Lagacé-Wiens says the main difference is transmissibility.

“Nobody had ever been infected with Covid. Nobody had ever been vaccinated for Covid. Everybody was just a ripe source of infection,” he said. “Transmission happens very easily in that kind of setting. There’s no herd immunity. With influenza, very similar influenzas have been going around year after year.”

Scientists use the value R0 (pronounced R nought) to mean the average number of infections a single infectious person will cause in a population with no immunity. If the R0 value of a virus drops below one, the number of cases of the virus will decline.

Research tells us Sars-Cov-2, the virus that causes Covid-19, has an R0 value of between two and three. That means the average person with Covid-19 will infect between two and three other people.

Influenza has an R0 value of just over one, making it easier to tamp out.

“When we intervened by having social distancing and closures and curtailing social activities, we basically crashed that influenza R0 value down well below one,” said Lagacé-Wiens. “If a disease has an R0 value less than one, it will just disappear. And that’s exactly what happened.”

He said although it’s difficult to determine which public health measures were most impactful, pausing non-essential services likely had the largest influence.

We’ve beaten back the flu before. Flu strains go extinct all the time, like the virus behind the 1918 flu pandemic that has drawn so many comparisons to Covid-19.

With attention on Covid-19 vaccines, Lagacé-Wiens said now isn’t the time to let up on flu vaccinations either.

“Flu viruses go through these decades-long cycles and eventually something replaces them, and they disappear,” said Lagacé-Wiens. “It’s certainly possible that the two main strains of influenza virus that were circulating in humans before will get replaced with something new.”