CALGARY -- More than 500,000 additional Albertans will be eligible to be vaccinated against COVID-19 on Tuesday as the province is lowering the benchmark age to qualify for the AstraZeneca vaccine from 55 to 40 years old.

The decision comes on the heels of a similar announcement from Ontario earlier that day, with health officials saying the changes are based on "current supply," as that province grapples with a third-wave of infections.

Manitoba also said on Monday is it lowering the age of eligibility for the AstraZeneca vaccine to 40.

Kenney says the decision is based on growing scientific knowledge about the vaccine and beginning on Tuesday, anyone age 40 and up can book an appointment to get the shot.

Alberta Health says vaccine supply is still limited, including AstraZeneca, and that it is continuing the age-based approach to prioritize those most at risk of severe outcomes and get vaccines into as many arms as possible.

As well, Health Canada has also approved recently the vaccine for use in anyone 18 and older.

There have been recent reports of low uptake from Albertans for the AstraZeneca vaccine.

Only about one-third of Alberta's available doses of AstraZeneca have been administered and the two largest vaccination clinics in Calgary and Edmonton have reported low traffic.

Health officials believe there are concerns over very rare blood clots detected in some jurisdictions — including two Canadians, one of those an Alberta man in his 60s announced Saturday.

Alberta's Chief Medical Officer of Health Dr. Deena Hinshaw says officials continue to monitor the safety of all vaccines and added the risks of blood clots are extremely rare.

She says according to the latest statistics, the chances of blood clots are one case in 100,000 to 250,000 doses of vaccine.

"In comparison, Albertans 55 and older who are diagnosed with COVID-19 have a one-in-200 chance of dying from that infection. They are also at least 1,500 times more likely to be hospitalized from COVID-19," she said.

"We have seen that there is an association between the AstraZeneca and the Johnson & Johnson vaccines, with blood clots, but it's exceedingly rare. And so I think we have to remember that and then you also have to look at what is the risk of getting COVID and what is the risk of severe outcomes associated with COVID, including things like blood clots hospitalization and obviously even death," said Dr. Stephanie Smith, infectious disease expert at the University of Alberta. "And when we look at those risks, then certainly the risk of all those bad outcomes associated with COVID is way higher in those that are 40 and over than it is with the rare event of thrombosis, with the AstraZeneca vaccine."

Officials say lowering the eligibility age means 575,425 more Albertans can be vaccinated, bringing the total eligible population to 2.3 million.

“With COVID-19 cases at high levels throughout the province, we are lowering the age eligibility for this vaccine so that as many Albertans as possible are able to choose the protection this vaccine offers," said Health Minister Tyler Shandro.

"The more people that get vaccinated as quickly as possible, the sooner we can protect our communities, reduce the burden on our healthcare system, and get life back to normal in our province.”

"It certainly makes a lot of sense that we have lowered the age, from 55 to 40 for the AstraZeneca vaccine, if I were to be offered the AstraZeneca vaccine right now I would certainly get it," said Dr. Smith. "I really think that, you know, the very, very rare event of thrombosis associated with AstraZeneca is so much lower than your risk of again, getting COVID and having a severe outcome associated with that."

Canada is not expected to receive any new shipment of the AstraZeneca vaccine over the next seven days.