MERCIER, QUE. -- A town south of Montreal has partially shut down—again—all because of the recklessness of a few teenagers, its mayor says.

About 50 people have now tested positive for COVID-19 in a new outbreak in the Montéregie region, all of them directly linked to two house parties that happened there recently, health authorities say.

Most of the 50 are young, said Montérégie public health spokesperson Chantal Vallée. But the infection has spread past the partygoers, she said.

“Some cases are parents, friends or colleagues of these young people,” she said.

In the town of Mercier, the owner of a local bakery is now in quarantine, his shop closed since Monday, after one of his young employees caught the virus.

"It was a moment of madness, of carelessness, weakness,” said Frederic Dugas, the co-owner of Fantaisie du Blé bakery.

He said the employee is usually a good kid, but he slipped up and went to a party in the neighbouring town. He probably didn’t envision the potential consequences.

Employees of at least five businesses in Mercier have now tested positive, including people who work at the town’s IGA grocery store and its McDonald’s, says mayor Lise Michaud.

“It’s an enormous impact,” she told CTV News. “We have so many businesses that have had to close their doors.”

A teenaged girl had decided to have one of the two house parties, and one of her guests had tested positive for COVID-19 before coming. The girl later acknowledged on Facebook that “it was stupid.

Michaud said she agreed with the girl on that.

“She didn’t have the right to do that,” she said.

All 30 employees of the bakery now have to get tested. And some businesses are closing, or partially closing, just to be on the safe side while they wait for the outbreak to subside.

"The last thing we wanted is Mercier to be the epicentre of a second wave—nobody wants that,” said Alex Ganivet Boileau, who owns a microbrewery in town.

He decided to close down his terrasse as a precaution.

Locals reported that around nine or 10 Mercier businesses seem to have shut their doors since Monday. Many local people said they were angry.

"It seems no one will be having any consequences for their actions, which, in my opinion, is ridiculous," said Jennifer Kinsella Wedrick, who lives a few minutes from Mercier in nearby Chateauguay and normally works at a hospital in the area.

She said the chilling effects were spreading to her own town, with some confirmed cases at businesses there, too. People are "upset and scared," she said.

"My children haven't been able to hug their grandparents since March, yet a bunch of teens feel they... get to throw house parties, putting us all at risk."

The regional health authority said they’ve been hard at work tracing back through everyone who’s had prolonged contact with the 60 people who attended the two parties, and trying to understand all the ways the virus could have spread from there.

“The public health survey…identifies people who have been in contact within two metres for more than 15 minutes during the infectious period” of the original positive cases, said Vallée.

Those people who have had prolonged contact are then considered to be close contacts.

People become contagious about 48 hours before the onset of symptoms, she explained. 

“Close contacts may or may not have participated in the same social events as the [original] cases,” she said, but if the close contacts eventually test positive then they’re officially linked to the outbreak as well. 

The number of cases has more than doubled since Monday, when 20 cases were reported. Montérégie had 22 new cases diagnosed today, compared to yesterday, Vallée said.

A mobile testing clinic has been set up in Mercier.