The highly infectious Delta variant is on track to become the dominant strain of COVID-19 in Ontario, but there is hope that vaccines offer more protection against it than previously thought, one of Ontario's top doctors says.

“Yes it's true that the Delta variant is growing rapidly in Ontario,” Associate Chief Medical Officer of Health Dr. Barbara Yaffe said during a news conference Monday.

While COVID-19 case numbers overall have been declining, the Delta variant -- first identified in India --  accounts for a growing share of cases that do arise.

And while the Alpha variant first identified in the U.K. has dominated Ontario for months, the Delta variant is quickly gaining ground as the leading strain.

“Rapidly, the UK one is going down and the Delta is going up so we fully expect it will become the predominant strain,” Yaffe said.

She pointed out the Delta variant is around one and a half times more transmissible than the Alpha variant.

While a study suggesting that a single vaccine dose offers around 33 per cent protection against the Delta variant has been widely cited by local health officials, Yaffe said Monday that new research suggests that a single vaccine dose is more effective than first thought.

“But in real life, it appears that, certainly with the mRNA vaccines, that it's more like 47 to 48 per cent effective,” Yaffe said.

She pointed out that that figure speaks to how many people with a single vaccine dose developed any symptom of COVID-19 at all.

“In terms of severe infection, even one dose is quite protective,” Yaffe added.

New research into the Delta variant released by Public Health England this week suggests that after a single dose, Pfizer was 94 per cent effective against hospitalization, while AstraZeneca was 71 per cent effective.

Yaffe said two doses of an mRNA vaccine appear to be around 88-90 per cent effective at preventing symptomatic COVID-19, while two doses of AstraZeneca appear to be around 60-70 per cent effective at preventing any COVID-19 symptoms at all.

“That's one of the reasons we're pushing up the interval between the first dose of AstraZeneca, and the second dose of whichever vaccine the individual chooses, because we want to get people as fully protected as possible as quickly as possible with the Delta strain moving up in Ontario as it is,” Yaffe said.

While Ontario had originally spaced out vaccine doses at 16 weeks to try and extend a basic level of protection to as many people as possible more quickly, the province has begun accelerating second doses as a steady and plentiful supply of vaccines arrives.

The province is also allowing anyone who received a first dose of AstraZeneca to get a second dose of an mRNA vaccine and over the weekend said that people who received AstraZeneca for the first dose would have to wait just eight weeks instead of 12 to receive a second dose.