Ontario has added more public health units to the list of Delta variant hot spots and adults in those 10 regions who received their first shot of an mRNA vaccine on or before May 30 will be eligible to book a second dose starting next week.

Adult residents of Delta hot spots regions, which now includes Hamilton, Simcoe-Muskoka, and Durham Region, who received their first dose of the Pfizer or Moderna vaccine on May 30 or earlier will be eligible to book their second shot on Wednesday.

The three regions join Toronto, Peel Region, Porcupine, Waterloo, Wellington-Dufferin-Guelph, Halton Region, and York Region as the province's Delta variant hot spots.

The province said Thursday that it intends to assist the Region of Waterloo, which is seeing alarmingly high rates of infection due to the Delta variant, administer first and second doses to priority populations with the use of mobile teams.

Adults in public health units not designated as Delta hot spots who received an mRNA shot on May 9 or before will also be eligible to book a second dose on Monday.

Starting on June 28, the province said, the second-dose rollout will gradually expand to adult residents in non-Delta regions who received their first dose on May 10 or later.

Ontario residents who received AstraZeneca are eligible to book their second dose at an eight-to-twelve-week interval. Those who received AstraZeneca as a first dose can choose to receive the same vaccine for a second dose or can move to an mRNA vaccine.

Children between the ages of 12 and 17, who are only eligible for the Pfizer vaccine, are expected to receive their second dose starting sometime between Aug. 9 and 22.

Millions of Moderna doses arriving in Ontario

The accelerated rollout comes as Ontario sees a big boost in vaccine supply, particularly with the Moderna COVID-19 vaccine.

Ontario expects to receive more than 900,000 doses of the Pfizer vaccine every week this month. The province received more than one million doses of the Moderna mRNA vaccine this week, a figure that does not factor in the one million new doses Canada expects to receive from the U.S. this week. Another two million Moderna doses are expected to arrive in Ontario next week.

"We have made tremendous progress, and we are continuing to build on that success with over 3 million doses of Moderna arriving in June. Ontario is continuing to accelerate our vaccine rollout by expanding eligibility for second doses ahead of schedule," Ontario Health Minister Christine Elliott said during a news conference at Queen's Park on Thursday.

Future allocations of the AstraZeneca vaccine have not been confirmed by the federal government.

Due to the influx in Moderna supply, the province says most of the new appointments offered through the provincial booking system will be for the Moderna vaccine.

The federal and provincial governments have said that Pfizer and Moderna can be used interchangeably for first and second doses and officials confirmed Thursday that the provincial booking system will not distinguish which mRNA vaccine will be offered at a given appointment unless it is an appointment for someone under the age of 18.

"I understand that people initially felt that if they had Pfizer as the first shot they believed they needed Pfizer for the second. It's simply not proven to be the case through science," Ontario Solicitor General Sylvia Jones told reporters Thursday.

"I would urge people who have concerns, have those conversations with your medical practitioner, with your pharmacy because the science says messenger RNAs, all of these vaccines are interchangeable."

To date, about 75 per cent of Ontarians have received their first shot of a COVID-19 vaccine and about 19 per cent are fully vaccinated with two doses.