TORONTO -- Some experts call the novel coronavirus delta variant "a wild card."

It’s fuelling a sharp spike in cases in parts of the U.K. -- as other strains drop.

“The delta variant has grown at the fastest rate of any of the other variants that have appeared,” said David Bauer of Britain’s Francis Crick Institute and the National Institute for Health Research.

“The number one reason why it falls into everyone's radar is simply the observation that it's a spreading really quickly.”

Growing evidence shows the delta variant is up to twice as transmissible as the original U.K. variant, B117, now named alpha.

And that has moved the delta strain -- which first emerged in India -- to the top of the list of concern for Harvard University scientist Bill Hanage as well.

“(Its) transmissibility is really worrying for the world, because it may be able to infect people before we're able to vaccinate them,” he said.

Hanage is a professor with Harvard’s Center for Communicable Disease Dynamics, and among a growing number of scientists warily watching the effects of this variant, and monitoring the most recent Public Health England report.

The report concluded it may not only spread more quickly, but there could be an increased risk of serious illness.

“Hospitalizations were more likely for people infected with the delta variant. I want to emphasize this is early days,” said Hanage.

Both English and Scottish analyses continue to support the finding of “reduced vaccine effectiveness.”

That comes with a paper published in the Lancet that found that people vaccinated with Pfizer produced lower level of antibodies against the delta variant -- particularly if they only had one dose.

After a single dose of Pfizer-BioNTech, 79 per cent of people had a quantifiable neutralizing antibody response against the original strain, but this fell to 50 per cent for alpha variant, 32 per cent for delta variant.

“What we found was that the delta variant was less able to be neutralized by those antibodies. And that, in particular the levels of those antibodies dropped off depending on how old the person was,” said David Bauer.

“The issue is that we're starting from a much lower level against the delta variant, and therefore this becomes more concerning that you might have people who become vulnerable to getting infected.”

It raises the possibility of the need for more rapid delivery of a second shot, or even possibly a booster shot in the coming months.

All of the data from the U.K. has bearing here in Canada -- that’s because the delta variant is implicated in some cluster of cases in B.C. and an outbreak in a long-term care home. And also in Ontario -- where more cases of delta infections are being noted in Ontario.

“This is the next variant that will become dominant, most likely globally, or in many places in the world actually. And it's just a matter of time,” said Dr. Peter Juni, head of Ontario’s science advisory table.

What we see right now for the delta variant, it's flat, meaning only when we start to open up it will start to increase and start to grow with case counts going up. But what it also means is, ]with B117 declining, that its relative proportion will increase now over time and it's clear that the delta variant will eventually become the next dominating variant in the province.”

Fortunately, the U.K. data is also showing that 73 per cent of delta infections in Britain are being reported in unvaccinated people, and only 3.7 per cent in those fully vaccinated.

“That is the take-home message: we need to get people vaccinated,” said Bauer. “That, you know, having no vaccine is clearly the biggest risk factor, full stop.”

That’s fuel to closing the gap between first and second doses in Canada.

“We are absolutely in a position to avoid the fourth wave, we just need to be smart now with our vaccination rollout. And we should just be, you know, realistic with our reopening program: not too fast,” said Juni.

He also said that in some of Ontario’s COVID-19 hotspots – Peel Region and Waterloo Region – “as many people as possible” need to get second doses.

Dr. Lawrence Loh, the medical officer of health for Peel Region, said Wednesday at a press conference that delta cases are "quickly replacing" the alpha variant as the most prevalent form of the virus locally.

"The trends being observed are concerning," he told reporters.

With files from Ryan Flanagan