Why the delta variant is spreading COVID-19 so quickly — and what that means for Canada

It’s the word on everyone’s mind right now when it comes to the COVID-19 pandemic: Delta.

The highly contagious variant, which was first discovered in India in late 2020, has spread around the world and now accounts for the majority of cases in Canada and various other countries. 

The recent spread in the United States has led the U.S. Centers for Disease Control and Prevention (CDC) to recommend that Americans wear masks in areas with substantial transmission “regardless of vaccination status.”

So, why is this particular variant spreading so rapidly? And what’s the mechanism at play behind its apparent success at infecting human hosts?

The answers are complicated, and there are still unknowns around how, exactly, the delta variant was able to take hold at such a brisk pace, throwing a wrench in global efforts to wind down the pandemic.

But we are getting closer to understanding how it works — and why vaccines, thankfully, still hold up.

How contagious is delta?

The delta variant is thought to be significantly more contagious than the early strain of SARS-CoV-2, the virus which first swept around the world. Delta has been dubbed a variant of concern by the World Health Organization (WHO).

“We are fighting the same virus but a virus that has become fitter and better adapted to transmitting amongst us humans,” said Dr. Michael Ryan, executive director of the WHO’s health emergencies program, during a Friday briefing.

Scientists estimate it’s spreading roughly 50 per cent faster than the alpha variant, which was 50 per cent more contagious than the original virus strain, according to the Yale School of Public Health.

That means every person infected is capable of passing the virus on to more people than before, helping this variant spread through populations quickly — and even faster among those whose immune systems aren’t already on high alert due to a previous COVID-19 infection or vaccination.

WATCH | COVID-19 delta variant a ‘dangerous virus,’ warns WHO:

The World Health Organization says the trajectory for the COVID-19 delta variant in some countries is almost ‘vertical’ and warns that even partially vaccinated countries are at great risk because of its high transmissibility. (Willy Kurniawan/Reuters) 0:56

Why is it more transmissible?

While the exact mechanism making delta more transmissible isn’t entirely clear, emerging research is hinting at possible reasons why it’s so infectious.

One lab-based study published in the journal Cell Host & Microbe, from researchers at the Kumamoto University and Weizmann Institute of Science, suggests mutations on the spike protein of this SARS-CoV-2 variant can evade cellular immunity and may increase its infectivity. 

The spike protein is a crucial feature on the surface of the coronavirus that allows it to gain access to our cells, explained University of Ottawa epidemiologist Raywat Deonandan. 

“It fits into a receptor on our cells and then it enters the cell via that receptor. Something about the mutation has changed the shape or a feature on the spike protein that makes it fit a bit better,” he said.

“As a result, you need fewer viruses present to achieve infection.”

People wear masks while walking in Grand Central Terminal in New York City on July 27. Due to the rapidly spreading delta variant, the U.S. Centers for Disease Control and Prevention now recommends that fully vaccinated people begin wearing masks indoors again in places with high COVID-19 transmission rates. (Spencer Platt/Getty Images)

Another study from a team in China, which has not yet been peer-reviewed, found people infected with the delta variant carried, on average, a more than 1,000 times higher amount of virus in their nose compared to the original strain — which likely means they’re shedding more of it.

The researchers also found people carrying this variant test positive faster: around four days after exposure, compared to around six for the original strain. That suggests delta replicates at a quicker pace inside someone’s body.

“You may actually excrete more virus and that’s why it’s more transmissible,” microbiologist Sharon Peacock, who runs the U.K.’s efforts to sequence the genomes of coronavirus variants, recently told Reuters.

How well do vaccines protect against it?

If carrying around delta means people may be shedding more of the virus and transmitting it to others more easily, vaccines definitely have a tougher task — since people’s immune systems are now facing a larger army and need to ramp up to mount a defence.

In the U.S., the CDC warned Friday that emerging data from a county in Massachusetts suggests the higher viral loads could mean vaccinated people can still transmit delta to others.

But the good news is that leading vaccines, including those approved for use in Canada, do seem to ward off serious disease that can lead to hospitalization or death.

A recent study in the New England Journal of Medicine found two doses of the Pfizer-BioNTech vaccine were 88 per cent effective against the delta variant, while two shots of the AstraZeneca-Oxford vaccine were 67 per cent effective. 

It marked a drop in the vaccines’ ability to curb infections of any severity level — whether mild or more severe — when compared to the earlier alpha variant, but the researchers said there were only “modest differences.”

Health-care providers administer the Pfizer-BioNTech COVID-19 vaccine at a pop-up clinic in Toronto on July 27. Leading vaccines do seem to ward off serious disease that can lead to hospitalization or death, even against the delta variant. (Evan Mitsui/CBC)

Recent data from Israel also shows the Pfizer shot reduced the risk of severe disease by a whopping 91 per cent, and hospitalization by 88 per cent. The level of protection against symptomatic infections in general was less than half that, but there are questions about how the government collected its data and how many infections were involved.

Still, it’s a hopeful real-world snapshot of how a leading mRNA vaccine is warding off serious illness, and it is likely the highly similar Moderna shot also being used in Canada would fare similarly.

“I don’t want to minimize the risk of things like ‘long COVID,’ but one of the biggest risks is how likely you’ll develop serious disease after being infected,” stressed Angela Rasmussen, a virologist with the University of Saskatchewan’s Vaccine and Infectious Disease Organization.

“The bottom line there,” she said, “is vaccination is still very protective.”

How widespread is delta?

This variant has definitely taken hold around the world since being first reported in October 2020 in India, with other countries later reporting high levels as well.

“It’s out-competing all other viruses, because it just spreads so much more efficiently,” virologist Shane Crotty, of the La Jolla Institute for Immunology in San Diego, told Reuters.

In the U.S., delta now represents more than 80 per cent of new infections. Cases are spiking in many areas with low vaccination rates, and data shows unvaccinated individuals total nearly 97 per cent of all severe cases.

Across Africa, meagre vaccination rates, coupled with the rise of the delta variant, are putting major pressure on various countries’ hospital systems.

The variant is now being found in at least 26 African nations, and 21 countries have seen cases rise by more than 20 per cent for at least two weeks running, the WHO announced in late July.



Countries from the U.K. to Singapore are also dealing with delta surges — including Canada. Variants of this virus now make up the bulk of our SARS-CoV-2 infections.

By early July, delta made up roughly 70 per cent of the country’s cases, according to the latest available whole genome sequencing data from the Public Health Agency of Canada.

That marked a massive jump from early May, when delta still represented less than 10 per cent of sequenced infections.

So what does this all mean for Canada?

There are now growing signs that the rise of delta could spark another overall surge in cases — even though nearly six in 10 Canadians are now fully vaccinated, millions still remain unprotected.

Ontario and Quebec have largely plateaued for new daily COVID-19 cases after weeks of falling numbers, while the number of new cases reported each day in B.C. has actually tripled during the last three weeks.

The virus is also spreading faster in Alberta than it did during the peak of the province’s third wave — all while that province is set to loosen its tightest restrictions around mask-wearing and isolation requirements.

WATCH | Alberta removing most COVID-19 isolation, testing requirements:

Alberta will be pulling back on requirements for COVID-19 testing, contact tracing and quarantines, despite rising cases in the province. 2:05

Even if overall case growth starts to spike, some experts are hopeful Canada, as a whole, will avoid the most dire outcomes, including overwhelming pressure on the country’s hospital networks and a death toll similar to earlier waves. 

Most people may still be able to avoid getting infected if they’re both vaccinated and playing it safe, even in high-density, urban areas, Rasmussen said, though she does expect school outbreaks if mitigation measures aren’t in place.

Many Canadians are still wearing masks and taking precautions in social settings, said Deonandan.

“I’m fairly optimistic that if this good decision-making continues, we could be world leaders in this endeavour.”

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