Canada has entered a flu epidemic, the national public health agency said Tuesday, after the test positivity rate nearly doubled in one week.

The Public Health Agency of Canada said in its FluWatch report that the rate jumped to 11.7 per cent the week of Oct. 30 to Nov. 5, compared to 6.3 per cent the previous week.

The agency declared an epidemic, which happens most years after the threshold of a five per cent positivity rate is surpassed, though it said influenza levels are higher than would have been expected when compared to pre-pandemic years.

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Dr. Timothy Sly, an epidemiology professor at Toronto Metropolitan University, said the flu season started earlier than usual and cases have surged sharply because natural immunity against common cold viruses -- such as rhinovirus, parainfluenza virus and respiratory syncytial virus -- has dropped due to COVID-19 restrictions the last two winters.

“This is because the masks have worked. We have been protecting against COVID and (that) has managed to make us more vulnerable now to these seasonal viruses,” he said in an email.

Sly said the flu cases are expected to increase.

“This could be one of the worst flu seasons that we have seen in years,” he said.

The agency said there were 13 confirmed influenza outbreaks in the first week of the month, including nine in long-term care homes and one in a school or daycare.

Canada's health-care system is dealing with a triple threat of respiratory viruses -- COVID-19, influenza and RSV.

Sly said if hospitals continue to remain overcrowded with patients, it will lead to cancellations of surgeries and delays in diagnosis of other health conditions such as cancer and heart diseases. Already, pediatrics hospitals in Ontario are overwhelmed by a surge in patients with respiratory illnesses, leading to the cancellation of some non-urgent surgeries.

“We need to slow the spread of all respiratory conditions” to decrease pressure on hospitals, he said.

Sly said everyone should get vaccinated against COVID-19 and influenza, and wear a mask in indoor public spaces such as malls, subways, buses and classrooms.

This report by The Canadian Press was first published Nov. 15, 2022.