A new variant of COVID-19 appears to spreading through a northern Ontario city with previously low levels of infection, prompting health officials to test all residents of an apartment building and shutter a public school.

The North Bay Parry Sound District Health Unit's top doctor called on Tuesday for people to strictly adhere to a stay-at-home order in light of the “serious threat we're now facing.”

“We have community spread as a variant of concern, and it is spreading quickly,” Dr. Jim Chirico, medical officer of health for the health unit, said at a Tuesday news conference.

An outbreak was declared this week at a North Bay, Ont., apartment building where at least three people have likely been infected with a variant of the virus, based on preliminary screening results.

The health unit said Tuesday that 19 residents at the building and two visitors had tested positive for COVID-19.

More than 100 residents at Skyline - Lancelot Apartments were tested over the weekend after two people with no connection other than living in the same building tested positive for COVID-19.

Some of the infected residents live in the same unit. Public health was still investigating the links on Tuesday, and Chirico urged any residents who hadn't been tested for the virus yet to do so.

Chirico also announced his decision to close Sunset Park Public School in North Bay after contact tracing linked a person infected with COVID-19 to the apartment outbreak.

“This was done out of an abundance of caution as we have the opportunity to stop the spread of COVID-19 variant of concern before it's too late,” he said.

Three people in the outbreak have screened as positive for a variant, with further testing to determine the exact strain. The health unit said every positive sample has been sent to a lab for additional screening.

Public Health Ontario is currently screening for three COVID-19 variants that are believed to be more infectious and potentially cause more severe illness.

There were 24 active cases of COVID-19 in the North Bay region's health unit as of Tuesday.

But Chirico said relatively low infection numbers in the area until last Friday present an opportunity to contain the outbreak.

“In order to do that, we must all act now, we must all follow public health measures to keep one another safe, save lives and prevent our health-care system being overrun,” he said.

This report by The Canadian Press was first published Feb. 9, 2021.