He said he would discuss the new measures "tomorrow" but reiterated he very much regretted shutting down restaurants.
"But we're going to have further restrictions moving forward, very very quickly".
The letter was signed by Toronto's Medical Officer of Health Dr. Eileen de Villa, Peel Region's Medical Officer of Health Dr. Lawrence Loh and Ottawa's Medical Officer of Health Dr.
The three public health leads also called on Williams to review and trim the essential businesses list, impose travel restrictions between other regions within Ontario, introduce emergency paid sick leave and move schools to online or hybrid learning in areas where school outbreaks are significant.
The decision to close schools in the region did not sit well with Brampton Mayor Patrick Brown.
The Ontario Medical Association (OMA) issued a statement on Monday, saying provincial doctors are concerned that we are entering the worst phase of the pandemic since last spring and urge everyone - all levels of government and every Ontarian - to take the warning seriously.
"Don't close elementary schools".
"Going to the malls is not essential", the Premier said.
A spokesperson from Education Minister Stephen Lecce said the province firmly believes that schools should remain open for in-class learning as they are critical for students' metal health.
They now represent at least two thirds of case growth, but their path to dominance was predicted by the Ontario COVID-19 Science Table as early as mid-February.
Ontario's vaccine rollout began in December and focused initially on immunizing some of the province's oldest residents in long-term care and health-care workers.
ICU doctors have said numerous patients they're treating these days are essential workers who got infected in the workplace.
Ontario's active caseload is only a few thousand cases below its January peak, and only a few hundred patients away from its maximum COVID-19 hospitalization burden. "It means they can join the line if they have not already", she said.
Ontario reports 3,065 new cases of COVID-19 and eight more deaths linked to the virus.
There are 942 people hospitalized with the virus in Ontario, but the Ministry of Health notes that 10 per cent of Ontario's hospitals do not submit data on weekends.