Covid-19 clusters in Glasgow and Lanarkshire have been linked to house parties.
A joint statement from NHS Greater Glasgow and Clyde and NHS Lanarkshire said the two boards were working together on the outbreak.
A total of 14 linked Covid cases have been identified in north-east Glasgow in addition to eight North Lanarkshire cases.
Meanwhile, a pupil at a primary school in Paisley has also tested positive.
A contact tracing operation is now under way at Todholm Primary school.
Renfrewshire council said there was no current evidence the virus had been transmitted inside the school and it remained safe for pupils and staff to attend.
NHS Greater Glasgow and Clyde and NHS Lanarkshire confirmed on Monday that the cases in their areas were linked.
They also said that evidence of social gatherings with no social distancing was a factor in their investigation.
A spokesman for both organisations said: “We can confirm there are links to cases across both health boards. We work closely on cross-board issues on a regular basis, including on the current clusters, where investigations are ongoing.
“There is currently no evidence of transmission in the schools.
“There is evidence that mixing socially, particularly around social gatherings in houses, without maintaining physical distancing measures can transmit the virus and is a factor in this investigation.”
Scotland’s national clinical director Jason Leitch warned that “unregulated” house parties were a “real concern”.
He said young people were meeting in numbers beyond what was allowed and with no physical distancing.
“This our first real palpable example of [house parties] giving people positive infections of a virus that can make us very sick,” he said.
Mr Leitch told BBC Radio Scotland’s Drivetime programme communication was a difficulty and said “influencers” and teachers should be used to help get the message across to young people that house parties were helping spread the infection.
It emerged on Sunday that a pupil at Bannerman High in Baillieston, who had attended classes when the school reopened last week, had tested positive for the virus.
Several other pupils at Bannerman High had previously tested positive, but began self-isolating prior to the return to class last week.
Five pupils at nearby schools in North Lanarkshire have tested positive in recent days, including:
- Three pupils from St Ambrose High School in Coatbridge
- One pupil from St Andrew’s High School in Coatbridge
- One pupil from Caldervale High School in Airdrie
There are also three further linked cases who are not pupils or staff at the schools.
Test and Protect is now tracing those who were in contact with the pupil at Bannerman High and Glasgow City Council has written to parents.
Parents of a senior pupil at one of the affected schools in North Lanarkshire told the BBC they were “disappointed” their son had to self-isolate after “the selfish actions of others”.
They did not want to be identified, but said they were informed their son was sitting close enough to one of the positive cases for more than 15 minutes and had to self-isolate.
His mother said: “We’ve done everything we can to protect our children during lockdown. So we are really disappointed that the actions of others has resulted in this.
“We got a phone call on Sunday night and our world turned upside down. Test and trace phoned to advise that our teenage son had been identified as a contact of a positive pupil at Caldervale High.
“Our son had already seen it via social media, he knew who the boy was and had sat next to him in a class. And because he had sat next to him for over 15 minutes he has to get contacted by Test and Protect.”
She added: “He has to isolate until 27 August. He’s now missing school because of selfish actions of people having house parties.
“We believe this has all stemmed from a house party at Coatbridge involving a large number of kids. People who are encouraging young people to get involved in parties do not understand the implications on others.”
The child’s father said he had to take his son to be tested at Ravenscraig on Monday morning.
He added: “there were three cars in front and we had to do the test ourselves in the car. By the time we had finished there were another 10-15 cars with kids in them doing tests.”
During the Scottish government coronavirus briefing on Monday, Nicola Sturgeon said: “I am concerned about what appears to be a rising number or cases, albeit that many of these new cases we are seeing are linked to known clusters and outbreaks.
“But nevertheless it’s a really sharp reminder for us that the threat of the virus has not gone away.
“All of us need to be really, really careful, ultra-careful when it comes to abiding by all of the public health advice.”
She said people must be careful about the school connection as they are community outbreaks that involve school pupils but are not school outbreaks.
Dr Christine Tait-Burkard, assistant professor in infection and immunity at Edinburgh University told the BBC’s Good Morning Scotland programme she would not be surprised if young people meeting socially were spreading the virus.
She said: “Kids are kids and they like to gather with each other and as we know from studies that came out from Public Health England last week that secondary school pupils actually shed the virus in similar amounts to adults. The virus is going to spread and transmit in these gatherings.
“The pupils themselves are at very low risk of severe disease but on the other hand, they are probably quite asymptomatic and can carry the virus into their families.”
Under the current Scottish government rules no more than eight people from a maximum of three households are allowed to gather indoors.
Social distancing applies to anyone from separate households, unless they are under the age of 12.