TORONTO -- Officials and experts emphasized the interchangeability of the Moderna and Pfizer-BioNTech vaccines on Monday as many more Ontarians became eligible to book accelerated second shots.

The province said its shipment of Pfizer COVID-19 vaccines for the week had been delayed by a few days, so a large supply of Moderna would be used at many appointments, meaning residents may end up with a different mRNA shot for their second dose.

The government has been encouraging residents to get fully immunized as soon as possible by taking the most readily available second dose.

"If you had Moderna or Pfizer for your first dose you can safely take either for your second. If you had AstraZeneca for your first dose you can safely take AstraZeneca, Moderna or Pfizer for your second," Health Minister Christine Elliott said on Twitter on Monday.

Residents who got a first dose of an mRNA vaccine on or before May 9 were able to book second doses starting at 8 a.m. Monday, while those in Delta variant hot spots who got their initial jabs on or before May 30 can do the same on Wednesday.

Canada's National Advisory Committee on Immunization has found "no important difference" between Moderna and Pfizer, which both use similar mRNA technology.

Peel Region highlighted that point in a social media statement on Monday.

"Moderna and Pfizer -- their labels may be different, but they use the same technology to protect us," it wrote on Twitter.

Experts encouraged people to get whichever of the two mRNA shots they're offered as their second dose.

"With this week's delayed Pfizer vaccine shipments, I'm concerned about people delaying dose 2 because they are being offered Moderna vaccine," Dr. Jeff Kwong, an epidemiologist with the University of Toronto, said on Twitter. "The last thing we want is any loss of momentum in our flourishing vaccine rollout."

Kwong said analyses of data compiled by the independent research organization ICES show that two doses of Moderna are "just as good" as two of Pfizer in preventing infections.

As a result, he said there's no reason to think one dose of Pfizer and a second of Moderna would be any worse than two doses of the Pfizer vaccine.

Other experts expressed similar sentiment, noting people regularly get different brands of other vaccines without thinking twice.

"If you got a flu shot this year, you likely don't know the brand. Neither do I. They're made by separate companies," Dr. Zain Chagla, an infectious disease specialist at St. Joseph's hospital in Hamilton, Ont., said in a tweet. "Moderna and Pfizer are interchangeable."

The province noted that youth will continue to received the Pfizer shot since it is currently the only one approved in Canada for those under 18.

Ontario's vaccine rollout is progressing well, with 76 per cent of adults vaccinated with one dose and 24 per cent of adults fully immunized as of Monday.

Cases have been dropping -- the province reported 270 new infections Monday and three more deaths. But there are concerns about the more transmissible Delta variant, which is poised to become dominant, since science shows people with one dose are less protected against it.

Health units covering Toronto, Peel, Halton, Porcupine, Wellington-Dufferin-Guelph, Waterloo and York, Hamilton, Simcoe-Muskoka and Durham are considered hot spots for the more infectious variant.

- With files from Paola Loriggio

This report by The Canadian Press was first published June 21, 2021.