The worker from a home care agency showed up unannounced at Parkland on the Glen, a retirement home in Mississauga, Ont., fighting an outbreak of COVID-19. She was there to visit 12 residents, including one who had tested positive for the virus, before heading back out to see clients at other retirement homes.

The woman was one of 43 workers who travelled through the 21-storey building on the same day last week, visiting several residents at a time, said Jason Shannon, the chief executive officer of Halifax-based Shannex Inc., the retirement and nursing-home operator that owns Parkland.

Mr. Shannon was able to stop the woman from visiting the resident with COVID-19 – he happened to be in the lobby at Parkland when she arrived.

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“She had never met these residents before. She has never been tested for COVID-19 and she is coming into this building during an outbreak,” he said in an interview.

But there was nothing he could do to stop her and the other workers from showing up like a “Trojan horse superspreader,” he said. Retirement homes are private residential complexes where residents make their own arrangements with caregivers.

These individuals are hired by publicly funded home-care agencies in Ontario to provide a range of services for elderly people, including help with bathing, dressing and exercising, allowing them to remain in their own homes or retirement residences.

But health care experts say the sheer numbers of such workers coming into retirement homes are contributing to the rapid spread of the coronavirus throughout the sector.

The pandemic has exposed a gap in the province’s surveillance systems for measuring the impact of COVID-19 on congregate settings. While Ontario is the only province that requires COVID-19 testing every two weeks for staff in retirement homes, there is no requirement to test third-party caregivers.

“All this foot traffic is a recipe for disaster,” said Samir Sinha, the director of geriatrics at the University Health Network and Sinai Health System in Toronto and a co-author of a new study examining the risk factors for COVID-19 outbreaks in Ontario retirement homes, including the prevalence of third-party caregivers.

Andrew Costa, a research chair on aging at McMaster University’s school of medicine and the lead author of the study, which has not yet been peer reviewed, said the fact these workers are not tested is a huge issue.

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“If seven or more [care providers] are coming in every day, the risk of an outbreak goes up almost threefold.”

Since the onset of the pandemic in March, 340 retirement home residents in Ontario have died of the virus. The toll pales in comparison with the devastation in long-term care homes, where 2,537 people have perished from COVID-19 to date. But the number of outbreaks in retirement homes – affecting 81 of the province’s 770 facilities – has jumped 76 per cent since September.

The Ontario Retirement Communities Association (ORCA), an advocacy group that represents home operators, has asked the provincial government to make COVID-19 testing mandatory for all volunteers and visitors to the facilities.

“These protections and best practices have already been implemented across Ontario’s long-term care sector, and our seniors and staff deserve the same level of consideration and support,” ORCA CEO Cathy Hecimovich said in a statement to The Globe and Mail.

Ms. Hecimovich said the government sent a memo to the province’s local health integration networks (LHINs) on Dec. 11 saying home-care workers should follow the same testing guidelines as staff in retirement homes.

Third-party caregivers are responsible for bringing the virus into some homes, according to updates on outbreaks posted by Revera Inc. Two of its homes declared outbreaks last week after a third-party caregiver tested positive: Granite Landing Retirement Residence in Cambridge and Glynnwood Retirement Residence in Thornhill.

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At Revera’s Forest Hill Place in Toronto, 32 residents, eight staff members and 25 third-party caregivers have all tested positive since an outbreak was declared on Nov. 4. Three residents have died of the virus.

Donna Rubinstein said her 84-year-old mother, Rodica Rubinstein, came in contact with an outside caregiver who ended up testing positive. Her mother contracted the virus but has since recovered. She has been confined to her suite for the past seven weeks.

“There’s nowhere to go, nothing to do,” Ms. Rubinstein said. “For the first time in her life, I heard her say that she would rather die than live like this.”

Mr. Shannon said the LHIN responded to his concerns by dispatching a small group of caregivers to work full-time in the home.

“This should have happened months ago,” he said. “It was a ridiculous amount of risk.”

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