The number of cases of influenza, as well as hospitalizations and deaths due to the viral respiratory illness, continue to escalate in Canada, and infectious diseases experts say there's no telling how soon the seasonal scourge will reach its peak and begin to taper off.
Figures from the Public Health Agency of Canada's FluWatch report show there were 1,948 laboratory-confirmed cases of influenza in the final week of December — the latest period for which statistics are available — a jump from 1,229 a week earlier and 692 the week before.
By the end of 2016, a total of almost 6,200 lab-confirmed cases of flu had been detected since the current season started in the early fall, says PHAC, which reported outbreaks in 206 institutions, almost 70 per cent of them long-term care facilities that house the elderly.
But those numbers provide only a snapshot of the 2016-17 influenza season thus far, as most people who come down with flu don't seek medical treatment and aren't tested to confirm the diagnosis.
"It's definitely the tip of the iceberg," said Dr. Danuta Skowronski, an influenza expert at the B.C. Centre for Disease Control in Vancouver. "None of these (figures) should be interpreted as absolute."
The predominant influenza strain circulating in Canada this year is A-H3N2, which can cause a particularly nasty form of flu that tends to disproportionately target the elderly.
As of Dec. 31, almost 900 Canadians had been hospitalized due to influenza and 20 had died, according to PHAC, quoting figures reported by the provinces and territories. Those aged 65 and older accounted for two-thirds of hospitalizations and 80 per cent of the deaths, the vast majority related to H3N2 infections.
Figures compiled by the pediatric hospital-based Immunization Monitoring Program Active (IMPACT) network show that so far no children have died as a result of the flu, but 117 were admitted to hospital, with kids under two accounting for more than a third of those admissions.
Skowronski said the influenza season began earlier in the western provinces than those farther east, and she described subsequent flu activity as being like a wave rolling out across the country.
"What's consistently true is where influenza has been occurring. It seems to have started earlier in the west, so we're further along in the evolution of the curve," she said of her home province.
"In central and eastern Canada, there's been some delay in the start of the season relative to the west — B.C., Alberta and Saskatchewan ... So it's pretty clear that there's variation in the start and the stage of the season across the country."
Still, if spikes in hospital emergency department visits are any gauge, parts of central and eastern Canada are also starting to feel influenza's bite.
Dr. Anil Chopra, head of emergency medicine at the University Health Network (UHN) in Toronto, said many hospitals in Canada's largest metropolis are seeing a significant increase in the volume of patients presenting with flu-like symptoms.
"It seems to be impacting a lot of the emergency departments in hospitals in the GTA," he said, explaining that there's been an influx of patients showing up with symptoms of the flu such as fever, headache, cough, and aches and pains.
Most are discharged to ride out the illness at home and given the age-old advice to take acetaminophen for pain and fever and to drink plenty of fluids. But "anticipate that you'll have days to weeks not feeling well, (with a) persistent cough," he advised those who come down with the flu.
Some patients with a severe bout of influenza, which often include the elderly and those with underlying chronic health conditions that put them at risk for complications, are admitted to hospital and may receive an antiviral medication such as oseltamivir (Tamiflu), he said.
While UHN has not yet tallied how many people with flu its four hospitals have admitted, Chopra said his perception is that more people are being hospitalized this season compared to 2015-16, when the predominant circulating strain was A-H1N1, the bug that caused the 2009 pandemic.
And winter's nemesis is not only affecting patients, he said. "Since just before Christmas, we've been having a lot of staff members calling in sick.
"So unfortunately we seem to be all experiencing overcrowding, lack of capacity. We're calling in extra staff to help, but we're really having a hard time finding extra staff because so many of them have been ill in the last three weeks."
Skowronski said it's difficult to predict when this season's infection numbers will hit their zenith and begin to decline. Typically, the flu season starts in late fall, gathering momentum until it peaks sometime between December and March, although cases can occur as late as May.
"There are some pockets of fairly intense activity and some other areas have yet to be affected," she said. "When west meets east or east meets west, we can't predict ... It's still evolving.
"But it's looking like a typical H3-dominant season — and, as always, those seasons tend to be more severe."
Some facts about the flu
Here are answers to some commonly asked questions about influenza.
WHAT IS FLU?
The flu is a contagious respiratory disease caused by the influenza virus, which infects the nose, throat and lungs. Infection can cause mild to severe illness, and in some cases lead to hospitalization and even death.
WHAT ARE SYMPTOMS OF FLU?
Fever, runny nose, sore throat, cough, muscle or body aches, headache and fatigue. Some people may have vomiting and diarrhea, although those symptoms are more common in children.
HOW DOES FLU SPREAD?
Influenza viruses are believed to spread from one person to another mainly by air-borne droplets when infected people cough, sneeze or talk. Touching a surface or object contaminated by the virus and then touching one's mouth, eyes or nose can also lead to infection.
HOW SOON DO SYMPTOMS BEGIN?
Symptoms begin about one to four days after exposure — on average, in about two days.
HOW LONG IS A PERSON CONTAGIOUS?
Flu can be passed to someone else even before symptoms appear. Most adults may be able to infect others beginning one day before symptoms develop and up to five to seven days after getting sick. Some people, especially young children and those with weakened immune systems, may be infectious for even longer.
HOW DO I KNOW IF IT'S FLU OR A COLD?
Influenza and the common cold are caused by different viruses, but both share some common symptoms. However, fever is rare with a cold and body aches, if they occur, are generally mild, while those associated with flu can be intense. Extreme fatigue is common with the flu but unusual with a cold.
HOW CAN I PROTECT MYSELF FROM GETTING FLU?
Infectious diseases experts say the best way to prevent infection is to get a seasonal flu vaccination each year. Other preventive measures — staying away from people who are sick, covering the mouth and nose when coughing and sneezing, and frequent hand-washing — also help slow the spread of influenza.
Sources: Public Health Agency of Canada; U.S. Centers for Disease Control.