Also, Ontario's mask requirement will remain in place,

People will have to wear masks indoors in Ontario even after the province moves beyond the current pandemic restrictions.

Details of Ontario's Roadmap to Reopen were released Friday as the province reached one of the milestones it set for reopening: 80 per cent of people over 12 with at least one dose of COVID -19 vaccine. Seventy-five per cent of people in individual health units must also be fully vaccinated and no area can have less than 70 per cent of people over 12 fully vaccinated. Reopening could happen as early as next Friday.

Ontario's preview of reopening plans comes as many provinces have announced they will end pandemic restrictions, including mandatory masking. Alberta, where vaccination coverage is lower than Ontario, has gone much farther, saying it no longer will require people to isolate if they have COVID-19 and will begin treating it the same way seasonal colds or the flu are treated. The move has been heavily criticized inside and outside Alberta.

The preview also comes as the Public Health Agency of Canada (PHAC) is warning that a Delta-driven fourth wave could be starting in the country.

New modelling from PHAC — and new evidence from the U.S. Centers for Disease Control and Prevention (CDC) about the now-dominant Delta variant — underscores that Ontario's benchmark levels of vaccination coverage for reopening are not enough alone to stop a fourth wave of the pandemic.

Ongoing masking or other public health measures such as improved ventilation and contact tracing will be required to prevent another exponential rise in cases, experts say.

“Vaccinations alone won't do it,” said Ottawa epidemiologist Dr. Doug Manuel, who is a member of the province's COVID-19 Science Advisory Table.

Manuel says the estimated reproduction rate for Delta suggests close to 100 per cent vaccine coverage would be needed to achieve herd immunity. That level of coverage is not likely. Not only are children under 12 not yet eligible to be vaccinated, but Manuel says vaccination rates among 20- to 40-year-olds are just about 73 per cent and levelling off. That means it will take more than vaccines to stop the spread of COVID-19 in Ontario and a possible fourth wave.

New information prepared internally by the CDC, leaked to The Washington Post this week, suggests the reproduction rate for Delta could be even higher — meaning even 100 per cent coverage might not provide adequate herd immunity without other public health measures.

Among other things, the CDC reported the Delta variant is more transmissible than SARS, Ebola, seasonal flu and the common cold. It is as transmissible as chickenpox, which has a reproductive value of eight, according to Manuel, meaning one person could infect eight others.

“Delta is different from previous strains,” the CDC document noted.

Crucially, the document also cited studies showing that people who are fully vaccinated can transmit the Delta variant of COVID -19 almost as easily as those who are not vaccinated. In a reversal of earlier advice, the CDC recommended masking in high transmission areas this week, even for people who are fully vaccinated.

Vaccines prevent 90 per cent of severe disease, but may be less effective at preventing infection or transmission of Delta, which means it can lead to more breakthrough infections (in people who are vaccinated) and more community spread, according to the CDC.

The CDC also released results of a study looking at a cluster of cases in Massachusetts in July. Seventy four per cent of those infected were fully vaccinated. The concerning data was pivotal to the CDC's recommendation about masking this week, director Dr. Rochelle Walensky said.

On Friday, Dr. Theresa Tam, Canada's chief public health officer, said models suggested Canada was already at the start of a Delta-driven fourth wave. The trajectory of that wave will depend on increasing vaccination coverage as well as the “timing, pace and extent of reopening.”

With the release of regulations for its reopening plan, the province's ministry of health said face coverings would continue to be required indoors, with some exceptions, because “the Delta variant is the dominant strain in Ontario, which is not the case with some other provinces.”

That decision, the ministry said, is consistent with other jurisdictions such as Quebec and Israel, as well as the CDC.

“The chief medical officer of health will continue to evaluate this need on an ongoing basis.”

Recommendations about schools will be part of the back-toschool plan expected to be released by Education Minister Stephen Lecce next week.

This week, Ottawa's Medical Officer of Health Dr. Vera Etches said she wanted to see mandatory masking continue at least into the fall.