An injection of hope: Herd immunity – where are we now?

A photo with a black backgroud that features two vials of COVID-19 vaccine and a syringe

Deep into the third wave of the COVID-19 pandemic, Canada is racing to catch up with a virus spreading and mutating across the globe at an incredible pace. No longer is the collective goal to “flatten the curve” – this time, armed with vaccines, the aim is to break the chain of infection altogether.

In this three-part series, YFile investigates the COVID-19 vaccine as an injection of hope for recovery. Today, in part one of this series, we look at herd immunity.

By Ashley Goodfellow Craig, deputy editor, YFile

In a country of more than 37.5 million people, what would it take for Canada to achieve herd immunity against COVID-19, and just how close are we?

The hard truth is herd immunity may not happen at all – and if it does, we might never actually know. Despite efforts to vaccinate populations en masse, herd immunity is not going to be what saves us from COVID-19. At least, not yet.

In simple terms, herd immunity is reached when a certain percentage of the population is considered to have immunity against infection, either by vaccination or previous infection. This percentage is calculated by looking at the infection’s reproduction number, or R0, and plugging it into a standard equation: 1-1/R0.

So, if R0 is two, herd immunity is reached when 50 per cent of the population is immune to infection.

Jane Heffernan
Jane Heffernan

York University Professor Jane Heffernan explains that with COVID-19, however, it is not quite that simple.

Heffernan, the inaugural York Research Chair (Tier II), Multi-Scale Methods for Evidence-based Health Policy in the Faculty of Science, studies the effect of vaccines and vaccine hesitancy. She’s investigating what the level of immunity looks like in an individual after vaccination or infection by modelling the cells in the immune system to determine when individuals have mild, moderate or severe outcomes.

She explains that the reproduction number for the novel coronavirus SARS-coV-2 is estimated to be between two and five, putting herd immunity on a scale between 50 and 80 per cent. However, there is more to consider. The variants of concern (VOCs) of COVID-19 are more infectious, for example, with an R0 approximately 1.5 times that of the original strain. This means to achieve heard immunity against all strains of the virus, up to 87 per cent of the population needs to be immune.

That translates to roughly 32.6 million people. Statistics as of June 7 indicate 9.7 per cent (three million) of Canada’s population are fully vaccinated, and 54.3 per cent (20.5 million) are inoculated with at least one dose but not considered fully immune.

That’s not all. Heffernan says there are complicating factors in measuring herd immunity for COVID-19, and here’s why:

  • Only some of the under-18 age population are eligible for vaccination – a demographic that makes up more than seven million Canadians or roughly 20 per cent of the country’s population. Further to this, the youngest of these populations may not have access to a vaccine for some time or at all.
  • There is no way to truly and accurately track those who have developed immunity due to previous infection in individuals who were: asymptomatic, unaware of infection, or had mild symptoms and did not get tested.
  • There is no clear evidence that rules out re-infection of COVID-19 with respect to variants and new strains of the virus.

“To achieve herd immunity, immunity in individuals needs to prevent infection – but we know that individuals might be able to get infected again and we hear information that vaccines have some efficacy against infection and then some efficacy against disease,” Heffernan explains. “So, considering that 87 per cent of individuals might need to have immunity against infection (for herd immunity) given the variants of concern – and with the vaccines, if our immunity wanes over time – we are not going to achieve that, especially if we can’t vaccinate younger age groups.”

A group of people wearing face masks
To achieve heard immunity against all strains of the COVID-19 virus, up to 87 per cent of the population needs to be immune

With adults making up roughly 80 per cent of Canada’s population, reaching the upper threshold of 87 per cent for herd immunity presents a challenge, and these numbers offer a dose of perspective. Even if the entire adult population was vaccinated, and the vaccine protected them from infection (not just from symptoms), we still wouldn’t reach the required herd immunity threshold because younger children aren’t yet able to be vaccinated.

Additionally, says Heffernan, there is a population of Canadians who will decline vaccination altogether. Vaccine hesitancy – a broad term used to describe those who are critical of vaccines, those who have barriers to accessing vaccines and those who delay vaccines – also must be considered in projections of herd immunity.

York University Professor Steven J. Hoffman says previous studies, before COVID-19, indicate about three per cent of Canadians are vaccine critical and seven per cent are hesitant but likely to accept a vaccination with more information or better access.

Hoffman is the Dahdaleh Distinguished Chair in Global Governance and Legal Epidemiology, a professor of global health, law and political science, and the director of the Global Strategy Lab at York University.

Steven Hoffman
Steven Hoffman

“It’s an important part of the puzzle because it means there is 10 per cent of the population that would not be immediately excited about vaccination, and that’s important because when you are trying to open a society, like we are now, you need as many people as possible to be vaccinated for that to happen,” says Hoffman.

As well, looking beyond Canada’s borders, the faster vaccinations happen worldwide, the better the chance to contain the virus. The reality is, he says, if this virus continues to mutate, it becomes increasingly likely that COVID-19 will return in different forms, requiring either a new vaccine or a booster shot.

Heffernan agrees. She says vaccination uptake needs to be maximized to lesson severe outcomes. Her current research looks at the effect vaccine hesitancy has on generating immunity in the population and aims to quantify the immunological effects on those who will not be vaccinated and those who will delay their first or second vaccination versus those who are vaccinated on schedule.

“I am trying to determine what the level of immunity is in the population by age, whether from infection or infection and vaccination, or one dose of vaccination or two doses of vaccination,” she says.

This research is particularly significant when different types of vaccines – in this case, mRNA and viral vector – are administered. Different vaccines will generate different outcomes in immunity, she says, and this information will continue to be relevant as more vaccines are developed – including those for young children and possible booster vaccines. Hoffman says the development of second-generation COVID-19 vaccines are already in clinical trials.

A nurse administers a vaccine
The development of second-generation COVID-19 vaccines are already in clinical trials

Heffernan’s research also has her asking how the vaccine uptake rate could be affected by decreases in reported positive COVID-19 cases, whether that will be a sustained trend, and what happens if there’s vaccine hesitancy around the second dose.

“Something we are looking at is how likely is it that people will get the second dose if COVID-19 numbers continue to decline,” she says.

And while vaccine campaigns are beginning to administer second doses for some, there are still many Canadians without their first shot. Hoffman says despite vaccine confidence being relatively high in Canada – in both absolute and relative terms – there are many reasons why people might not get vaccinated.

“Most of those reasons have to do with things like inconvenience or lack of awareness around the need, or misunderstanding around risks involved,” says Hoffman. “The bigger part of the problem is making it easy for people to get vaccinated.”

Overall, however, positive COVID-19 case counts in Canada, which reached a peak of over 9,000 cases daily in mid-April, are steadily declining and currently trending at just over 2,500 positive cases per day – a hopeful sign that vaccinations are working to protect us.

And though Canada may have been off to a sluggish start in getting vaccines into arms, the country is now on track to surpass vaccination rates in the U.S.

“I’m pleased with the rate at which Canadians are getting vaccinated,” says Heffernan. “Canada is looking good.”

For part two of this series, visit: To read part three, see: