EDMONTON -- Alberta is following the steps of Ontario and calling the Moderna and Pfizer COVID-19 vaccines "interchangeable" in the face of a two-to-three-day delay in the latter product.

The province is set to receive some 280,000 Pfizer doses the week of June 21, but Canada's shipment of 2.4 million shots from Pfizer and BioNTech isn't slated to arrive until mid-week.

Alberta Health Services is encouraging Albertans who want an earlier appointment to switch their vaccine preference to Moderna in the meantime.

"Both Pfizer and Moderna are mRNA vaccines and work in the same way. They are now considered to be interchangeable," it said in an announcement Sunday afternoon.

"If you book for Moderna, you will be able to get an earlier appointment thus complete your series."

More detailed information on the AHS website called it typical for people get booked for the same product as their first shot, but not essential. The vaccines work the same, are both about 94 to 95 per cent effective in preventing COVID-19 disease, and have similar side effects, the agency said.

"There is no reason to believe that a second dose with a different mRNA vaccine product would result in additional safety issues or deficiency in protection. Two doses of any of the COVID-19 vaccines available in Alberta is considered a complete, safe and protective vaccine series."

An Edmonton epidemiologist echoed this, noting the products' similar technology, efficacy, and side effect profiles. 

"There's no reason to think that mixing and matching them is going to produce either less immune response or more side effects," Dr. Stephanie Smith told CTV News Edmonton. 

"So I would say absolutely, it's important to get that second shot as soon as possible. And I would certainly have no concerns about getting a Moderna vaccine if you've initially had a Pfizer." 

NATIONAL GUIDANCE

The recommendation is in line with advice from the Public Health Agency of Canada and the National Advisory Committee on Immunization, which said on Friday people who received a first dose of an mRNA vaccine should be offered the same for their second dose unless it is unavailable.

"If the same mRNA vaccine is not readily available or unknown, another mRNA vaccine can be considered interchangeable and should be offered to complete the vaccine series."

Ontario announced over the weekend it would use an influx of Moderna supply to "supplement the delayed Pfizer doses."

"Both Moderna and Pfizer vaccines are authorized for use in Canada and use a similar mRNA technology, so the vaccines are interchangeable and safe to mix," a statement from its healthy ministry read.

Canada expects to receive 5.2 million doses in total this week from the two major vaccine suppliers. Of the 2.8 million doses of Moderna, more than 770,000 will land in Alberta.

Second doses have begun to outpace first doses in the province: On Saturday, more than 45,200 second doses and nearly 6,700 first doses were administered.

According to the latest data, 70.6 per cent of the population aged 12 and older have received at least one dose, while the portion of people fully immunized sits around 28.6 per cent.

With files from CTV News Edmonton's Nahreman Issa and Adam Lachacz