Region of Waterloo Public Health officials say shutting down schools entirely is a "possibility" as students return to the classroom amid the COVID-19 pandemic.

However, that would be an extreme measure if there was evidence of uncontrolled transmission at multiple schools in the region.

"That ultimately may be necessary," Medical Officer of Health Dr. Hsiu Li Wang said at the region's weekly COVID-19 update on Tuesday.

"It would have to mean that very significant efforts that we would make to try to address outbreaks as they occur, as we've been doing since the time the wave that the first wave has been going down and throughout the summer, it would have to be that such measures would no longer be sufficient."

Last week, the provincial government outlined how schools should handle COVID-19 cases and outbreaks.

In order for a school to declare an outbreak of the novel coronavirus, there would need to be two or more lab-confirmed cases of COVID-19 in students or staff with an epidemiological link between them.

"We are looking for a connection between the two or more cases where at least one case could have reasonably acquired their infection in the school setting," Dr. Wang said. "An outbreak will be declared over if at least 14 days have passed with no evidence of ongoing transmission."

Although schools won't declare an outbreak until there are two cases of COVID-19, Dr. Wang said public health's involvement begins at the first sign of a case.

"As soon as there's a case, there's public health action," she said. "Right away we do case and contact follow up."

Dr. Wang also said there's a strong possibility that a class would be dismissed if there was a positive case.

She said public health officials are already working closely with school boards in the region and will continue to do so as schools prepare to welcome students back. All case and outbreak information will be provided to the public on the school boards' websites, with links accessible on the region's COVID-19 dashboard.

Dr. Wang stressed the importance of continuing to follow public health guidelines in regards to physical distancing, hand hygiene and face coverings, especially as schools and the economy open up.

She added that case numbers will increase as students go back to school.

"We will see increasing numbers of cases," Dr. Wang said. "That said, what we are striving for is to minimize the number of cases as much as we can. And, of course, minimize severe outcomes from cases."

In other settings, like long-term care homes and child care settings, public health may declare an outbreak after only one case.

However, Dr. Wang said the outbreak policy for schools makes sense from an epidemiological perspective.

"This outbreak definition is very reasonable to me," she said. "Again, it is not the threshold to action. The threshold for action is one case."

"Overall I find that the guidance for case contact and outbreak management is pretty conservative, and that's appropriate because we want to help this precautionary approach."

Dr. Wang encouraged caregivers to work on back-up plans for children in the event of a case or an outbreak. She also said people should continue to follow Ontario's social circle guidelines of 10 people or fewer.