What the steep climb toward herd immunity looks like

Experts doubt we’ll hit the threshold any time soon, but it’s not impossible.

Robson FletcherCBC News • June 14, 2021

As more and more Canadians are vaccinated against COVID-19, it has often been suggested (and sometimes simply assumed) that we’ll soon reach herd immunity.

This is a theoretical point where so many people are immune to the virus that it cannot continue to viably spread, offering indirect protection to the entire population — even those who are not immunized.

Epidemiologists, however, now believe it’s unlikely that we will achieve true herd immunity against COVID-19 anytime soon. It’s possible, but it will be a steep climb to get there.

But don’t worry, they say. It’s not herd immunity or bust.

The severity of the disease is expected to decrease dramatically as the vaccine rollout continues. And life is expected to return, increasingly, toward normalcy.

But stamping out COVID in the way we’ve stamped out some other diseases will be a tall order.

Here’s why.

The herd immunity threshold depends on two variables.

The first variable is the transmissibility of the virus. In other words, how contagious it is.

The second variable is how effective our immune systems — once primed through vaccination or natural infection — are at preventing us from being infected and transmitting the virus to others.

Early in the pandemic, it was estimated that having 60 to 70 per cent of the population immunized could be enough to reach herd immunity.

These estimates account for the fact that immunization is not perfectly effective at preventing transmission.

But the estimates have changed with the arrival of new and more transmissible variants of the virus.

These variants are now dominant and, as a result, epidemiologists believe the herd immunity threshold is likely in the range of 75 to 90 per cent.

These numbers refer to the total percentage of people who have been either fully vaccinated or have immunity through natural infection.

There’s another challenge, too.

No vaccine has yet been approved in Canada for kids under the age of 12.

And these kids are no small group; they represent 13 per cent of the population, which puts a hard ceiling on the number of people who can be vaccinated, for now.

While millions of Canadians have received a first dose of vaccine, only 5.7 per cent of the population had been fully vaccinated as of June 1st.

We don’t know exactly how many people have immunity through natural infection, but some estimates suggest it’s now around 10 per cent of the population.

It’s widely expected that, by the fall, every Canadian will have had the opportunity to get a full complement of vaccinations.

But how many will actually choose to get fully vaccinated?

Let’s look at three scenarios.

The three dots illustrate overall immunity this autumn if 90 per cent of eligible Canadians choose to get vaccinated, versus 80 per cent, versus 70 per cent.

These scenarios include an assumption that four per cent of unvaccinated people will have natural immunity, come autumn. (This is lower than the current estimate of 10 per cent because many people with a past infection are expected to become fully vaccinated between now and then.)

As you can see, in order to have a shot at hitting the threshold, an overwhelming majority of teens and adults will need to opt for vaccination.

Otherwise, we might fall short.

One dose is good, but not good enough

At this point, you might be wondering: What about those millions of Canadians who have already been vaccinated?

It’s important to remember that the vast majority have received only a single dose.

From what we know so far, single doses appear very good at preventing you from getting severely ill but not as good at preventing you from transmitting the virus to others.

And this has a huge effect on the herd immunity calculation.

There’s a wide range of estimates on the effectiveness of first doses against transmission, so it’s hard to pin down a precise figure. But let’s assume some rough numbers and say a first dose is somewhere between 60 and 75 per cent effective.

Relying on first doses alone, that would bump the herd immunity threshold way up into the range of 95 to 125 per cent — in other words, somewhere between extremely hard and mathematically impossible.

This is why experts say first doses won’t be enough, on their own, to reach herd immunity. Still, single doses are expected to suppress COVID-19 to a significant degree, so much so that many provinces have hinged their reopening plans on getting a certain percentage of the population at least partially vaccinated.

But herd immunity, they say, will be a much higher hurdle to clear.

To even have a shot at reaching that goal, tens of millions of Canadians will need to roll up their sleeves and get fully vaccinated.

Methodology

CBC News created these informational graphics after consulting with numerous epidemiologists about the broad concepts of herd immunity. You can read more about these concepts and hear from these experts in this story.

The specific estimates of herd immunity thresholds and the number of people who currently have natural immunity were arrived at in consultation with Ashleigh Tuite, an epidemiologist and mathematical modeller with the University of Toronto's Dalla Lana School of Public Health.

Calculations of the number of people immunized in each of the three scenarios, as well as the assumption that four per cent of unvaccinated people would have natural immunity come autumn, were arrived at via mathematical models developed by Scarsin Corporation and shared with CBC News.

Text: Robson Fletcher • Design and development: Adam Foord, Richard Grasley, William Wolfe-Wylie, CBC News Labs