EDMONTON -- Alberta's plan for a great summer could give way to a not-so-great fall, experts are warning, because of the fast-growing delta COVID-19 variant.

The strain, which was first found in India, is thought to be up to twice as transmissible as the alpha variant first found in the United Kingdom, according to early evidence. The former is behind a sharp spike in cases in parts of the U.K., even as other strains drop.

Studies there show one shot of vaccine – on which Alberta's reopening plan is founded – offers less protection.

"The emerging evidence is showing that one shot, although offers good protection, is not as strong against some of these variants," infectious diseases expert Dr. Craig Jenne told CTV News Edmonton, pointing out variants are Alberta's most dominant strains.

According to Alberta Health, a first dose of an mRNA vaccine, like the Pfizer and Moderna products, offers 73 per cent protection against the alpha variant. That immunity is boosted to 91 per cent with a second dose.

A first shot of those vaccines is also 75 per cent effective, and a second shot 89 per cent effective, against the gamma variant, first found in Brazil.

But against the delta strain, one dose only offers 33 per cent protection, and two doses 88 per cent protection.

"What’s happening in the United Kingdom at the moment is they’re seeing very rapid spread of this delta variant and they had adopted a very similar approach to Canada in emphasizing to everyone to get their first vaccine," Dr. Noel Gibney explained. "But we're moving relatively slowly on the second vaccines, although they are slightly ahead of Canada." 

He co-chairs the pandemic planning committee of the Edmonton Medical Zone Staff Association and says the province is at risk of a fourth wave in the fall without higher second-dose rates or other protective measures.

"It seems kind of strange to be able to give that forecast so far ahead but that was very much the case with the U.K. variant where it was apparent in mid-February we were going to be in trouble in April," Gibney said. "With that kind of warning, we do have the opportunity to make sure that we can stave this off."

PANDEMIC NOT OVER: DOCTOR

According to the latest data, Alberta has identified 126 cases of the variant – but there could be more because not all positive cases were tested for variants of concern during the third wave.

"The problem with the current reopening is … approximately 40 per cent of the population – so the 30 per cent who are not vaccinated as well as those 12 and under – do not have protection. So there is a risk that the virus will transmit in that part of the community," Jenne told CTV News Edmonton.

The province could move to Stage 2 as soon as June 10, seeing as it has already met the threshold of having 60 per cent of the eligible population vaccinated and fewer than 500 hospitalizations.

Stage 3 will start at a minimum of two weeks later, once 70 per cent of the eligible population has had one dose. Almost all restrictions will be dropped then, and the province plans on allowing big events such as the Calgary Stampede to run.

"Anyone who thinks that it’s over just because we’ve had 60 to 70 per cent of our population vaccinated with one shot is mistaken," Gibney said.

Premier Jason Kenney has acknowledged a first dose of vaccine doesn't give full protection, but has said he expects all Albertans who were given a second dose to be fully immunized by the end of the summer.

Currently, anyone vaccinated in March can book their second dose. In one week, Albertans who were vaccinated in April can book for a second dose.

Two shots of AstraZeneca is considered 66 per cent effective against the delta variant, Gibney said, "way more effective" than any of Alberta's flu shots.