TORONTO -- Public health staff are scrambling to conduct contact tracing for nearly 500 COVID-19 patients that were not flagged to local public health agencies in Ontario due to a reporting error.

Ontario Health says after a "thorough investigation," it was discovered that COVID-19 test results from one provincial lab had not been reported to 12 local public health units where the cases reside.

The lab, Ontario Health confirmed, was used by the William Osler Health System and the majority of tests came from the drive-thru assessment centre at Etobicoke General Hospital.

Most of the cases involve residents of Toronto, Peel Region, and York Region.

Officials say the people who were tested had access to their test results online and public health units have now started the process of contact tracing and case management, starting with those who had been tested in the last 14 days.

"Patients who tested positive more than 14 days ago will also be contacted by public health units to enable contact tracing and case management. Many of the cases from more than 14 days ago would now be considered resolved," a statement released by Ontario Health read.

Officials have now "clarified the reporting process" with testing hospitals and assessment centres, the statement continued.

"The provincial network has confirmed with all laboratories that there is a clear and documented understanding of who is informing local public health units to ensure timely case investigation and contact tracing," the statement read.

"All parties are working to ensure this situation does not happen again, and steps taken to prevent re-occurrence."

Health Minister Christine Elliott said that the majority of the approximately 485 patients checked their test results online, and self-isolated as directed.

“All units are aware of the process and this will not be happening again,” she said.

A variety of public health staff are now completing a delayed contact tracing effort for anyone who came into contact with the patients.

“We are following up now, we are doing the contact tracing and case management services to make sure that that has been done and to follow up on any contacts to ensure that they are safe and healthy or perhaps they need to be tested as well but this is something that has been dealt with,” Elliott said.

Communication breakdown 'a huge problem’

Infectious disease specialist Dr. Isaac Bogoch said the error could be responsible for many new cases of the virus in the GTA.

"This is a huge problem," he said in an interview with CP24 on Tuesday morning. "No one would be surprised (if) between April and maybe perhaps a week ago, this is responsible for a lot of the cases in those affected areas."

Bogoch said communication between labs, public health agencies, and patients is "key" to help slow the spread of the virus.

"If someone is positive, you've got to know that they are positive right away and follow up with the contact tracing of all of those people that were in contact with the positive case," he said.

"People need to know that they should be physically distancing from others (and) where to go for help if they need help."

He noted that some people infected with the virus may also be living in a situation with several other people.

"Many people don't have the privilege of physically distancing themselves if they live in a multi-generational home or if there is just a lot of people in the house in a small place, so perhaps they might need assistance with that front as well," he said.

"With a communication breakdown like this, the whole thing falls apart."

Speaking to CP24 on Monday night, Brampton Mayor Patrick Brown said given how strained hospital resources are in the wake of the pandemic, it is no surprise that mistakes were made.

"Our hospitals are at capacity. I look at Osler Health System, they are at 90 per cent right now and they are dealing with a pandemic at the same time," he said.

"It shouldn't happen, we need to be better at communication, but the fact that our hospitals are (at) such high capacity right now, mistakes are bound to happen."

He said he believes it will be a learning experience for all of those involved.

"This is a breakdown of communication and obviously it's disappointing and I think the ministry and the hospitals and public health will learn from this," he said.

"At the end of the day, I'm thankful for everyone working in the hospitals because I know they are doing their best under difficult conditions."