OTTAWA -- Three weeks after Thanksgiving weekend, a family dinner is Renfrew County is being linked to 13 cases of COVID-19.

Acting Medical Officer of Health Dr. Robert Cushman tells CTV News Ottawa between 15 and 20 people attended an intergenerational Thanksgiving dinner over the holiday weekend.

Dr. Cushman says it appears someone at the dinner was asymptomatic or didn't pay attention to the symptoms.

The Renfrew County and District Health Unit says 13 positive cases are linked to the Thanksgiving dinner, including two new cases Thursday. Not all 13 positive cases attended the dinner.

"What you see is the spread, now into the third group from those at the dinner," said Dr. Cushman, noting there is now second and third generational spread of the virus.

Two family members who tested positive for COVID-19 were high school students.

"Luckily, no further spread yet (at schools), thanks to excellent public health precautions at the school," said Dr. Cushman.

Seventy students at the school were tested for COVID-19, while 90 students returned to school on Friday after being asked to self-isolate for 14 days.

Dr. Cushman says four outstanding students who developed symptoms on days 14 and 15 are now being retested, and will remain in isolation.

The Renfrew County and District Health Unit is also investigating a COVID-19 outbreak at the Canadian Nuclear Laboratory at Chalk River. Six people have tested positive for COVID-19.

"This virus is very wily," said Dr. Cushman, noting CNL has solid public health measures in place.