Why is Calgary ground zero for the flu in Alberta again?

The flu season is just getting started but nearly 1,000 cases have already been confirmed in the Calgary health zone, which is themost in the province by far.

The Calgary zone accounts for 65 per cent of the total flu cases in Alberta, despite having only 38 per cent of the population.

It was a similar story around this time last year, when most of Alberta's flu cases were also recorded in the Calgary area.

• From December 2017:Calgary hit by more than half of all flu cases in province, AHS says

But the rest of the province later caught up and, by the end of last season, the influenza rate inCalgary was right on par with the provincial average.

Flu, by its nature, is a complicated and unpredictable virus or, more precisely, a family of viruses. So it's not clear exactly why Calgary is once again being hit so hard, so early.

But Dr. Eddy Lang has a hypothesis.

"The most likely explanation I've heard is that, because Calgary is seeing more international airway traffic than it has in previous years, perhaps, we're seeing more people arriving into the city already carrying the flu bug," said Lang, who's in charge of adult emergency medicine for the Calgary zone.

Calgary saw more than 1.7 million international travellers pass through its airport last year, compared with fewer than half a million in Edmonton, according to data from each airport authority.

"That's the only possible thing that, I think, would distinguish an earlier peak in Calgary than in Edmonton, which have similar populations, similar demographics," Lang said. "I can't see, unless it's a coincidence, why the flu would be more rampant here than in Edmonton."

He said emergency rooms in Calgary have seen a recentsurge in cases of influenza, with its "unmistakable" symptoms that differentiate it from other illnesses like the common cold.

The flu's symptoms include high fever and severe muscle aches, Langsaid, and patients often find themselves so fatigued that they can barely get out of bed for several days.

So far this season, 200 people have been admitted to hospital in Calgary with lab-confirmed cases of influenza.That's quadruple the number in Edmonton.

The lone flu-related death so far this year also came in the Calgary zone.

But Dr. Jia Hu, medical officer of health for the Calgary zone, isn't convinced air travel is to blame.

Flu numbers over the years

"I think that's potentially a hypothesis explaining why this is the case," Hu said. "But, again, I think it's more likely the case that flu, itself, is relatively unpredictable."

The early spike in flu numbers in the past two years could simply be a coincidence, Hu said, noting Calgary saw far fewer flu cases in previous years.

In 2015/16, for instance, the Calgary zone had the lowest rate of influenza in the province.

Click or tap on the interactive graph below to see flu rates, by health zone, in recent years.

Can't see the graph? Click here for a version that should work in your browser.

Regardless of the the exact reasons, Hu said one thing is clear.

"The bottom line is, we have seen a rapid increase in the number of flu cases in the Calgary zone, which is why it's so important for people to get immunized," he said.

Free flu shots are now availableat clinics across the city.

You can search for the clinic nearest you at this Alberta Health Services website.

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