More Ontario residents will be able to book earlier second shots of a COVID-19 vaccine next week as the province prepares to receive millions of doses this month.

Officials encouraged people to get fully immunized as quickly as possible in order to curb the spread of a highly transmissible variant that's threatening reopening plans in a few communities.

"The best vaccine for your second dose is the vaccine that is available first," Health Minister Christine Elliott said on Thursday. "It is critical that everyone sign up sooner for your second dose when it's your turn."

Anyone who received their first shot of an mRNA vaccine on or before May 9 can book a second dose starting Monday. Residents of 10 Delta variant hot spots who got first mRNA shots on or before May 30 can schedule second doses starting Wednesday.

Second doses will be offered based on available supply, not the vaccine type individuals received for their first shot, officials said.

People with one vaccine dose are believed to be less protected against the Delta variant. The strain is on track to become dominant in Ontario and the province has been accelerating second doses – which were originally offered four months after the first – to fight it.

The government initially identified seven regions as Delta hot spots: Toronto, Peel, Halton, Porcupine, Wellington-Dufferin-Guelph, Waterloo and York.

It added three more – Hamilton, Durham and Simcoe-Muskoka – to the list Thursday.

The province also said all adults who got their first mRNA dose on or after May 10 can book an earlier second shot starting the week of June 28, with exact details to be confirmed.

That timeline doesn't apply to youth between the ages of 12 and 17. Officials said further details are to come on accelerated second doses for that group, who are currently only eligible to receive the Pfizer-BioNTech vaccine.

People who received the Oxford-AstraZeneca vaccine as a first dose can book second shot appointments eight weeks after their initial jab. They can receive another AstraZeneca shot, or a dose of Moderna or Pfizer.

The acceleration of second doses comes as Ontario's vaccine effort continues to expand, with record numbers of daily shots administered this week.

The province continues to face criticism, however, for the lack of a centralized booking system and for placing the onus on individuals to schedule earlier second doses. That has led to long lines at pop-up clinics and some having to travel far from home for jabs when nearby appointments are snatched up fast.

Nonetheless, more than 75 per cent of adults in Ontario had received one vaccine dose as of Thursday and 19 per cent were fully vaccinated.

The numbers are very close to province's target for entering the second stage of its reopening plan, which would see restrictions on businesses, gatherings and other activities rolled back further on July 2.

The Delta variant, however, has posed a threat to those plans in some areas.

The northeastern Porcupine Health unit, which covers the city of Timmins and several First Nations communities, did not enter Step 1 of the reopening with the rest of the province last week due to an ongoing spike in cases, suspected to be variant-related.

In Waterloo Region, the area's top doctor warned this week that a variant-driven spike in cases could hamper further reopening and might mean a return to stricter public health measures.

Ontario is sending mobile vaccination teams to help respond to the situation in Waterloo, officials said on Thursday.

Premier Doug Ford said he's been in contact with officials there and the decision to reopen further would depend on the local top doctor's wishes.

The rest of the province "is moving forward," the premier said.

"There's no one wants to open this economy more than I do. I just want to make sure we do it cautiously."

Ontario reported 370 new cases of COVID-19 Thursday and seven more deaths linked to the virus.