When the first round of COVID-19 vaccines arrived in late 2020, millions of unprotected Canadians rushed to get a shot as soon as they could.
Nearly two years later, updated vaccines — which target the dominant Omicron family of variants — are rolling out across the country.
And at this point in the pandemic, the immunity landscape is far more complicated.
“Now what we have are different people who’ve been vaccinated at different times, maybe even with different vaccines. Many people have been infected and recovered from COVID-19 at various points in time as well,” said Dr. Isaac Bogoch, an infectious diseases specialist with the University Health Network in Toronto.
“So it’s much harder to have a one-size-fits-all approach to the vaccine rollout.”
The timing of your next dose now depends on your risk level, and when you were last vaccinated or infected — with federal guidance suggesting most Canadian adults should be waiting up to six months or so before getting another shot, or around three months in situations where the risk of serious illness is higher.
To make sense of the best approach, CBC News spoke to a slate of medical experts — including infectious disease specialists, immunologists and virologists — and reviewed the latest guidance from Canada’s top vaccine advisers.
We’ve broken down their advice into a step-by-step process. You can take the quiz below to see how it applies to you. Or keep reading to learn the science behind how to time your next booster to ward off a COVID infection as long as possible.
Gauge your risk
For Canadians who want a booster, knowing when to get one starts with understanding your own health — since that can change your risk of serious illness if you catch SARS-CoV-2, and determines the optimal timing for another dose.
Canadians who are frail and elderly, and those with serious underlying medical conditions, are at an elevated risk of getting severe COVID, and should consider getting a booster sooner than others, said McMaster University immunologist and vaccine researcher Matthew Miller.
Federal officials list a variety of risk factors: Being an older adult, particularly over the age of 60; being pregnant; being obese; having chronic medical conditions like moderate-to-severe asthma, diabetes, high blood pressure, or heart, kidney, liver or lung disease; or being immunocompromised, including people who have cancer or are on chemotherapy.
None of those mean you’ll definitely have a severe case of COVID, but they do increase your likelihood of poor outcomes — particularly if you’re dealing with multiple risk factors at once.
More than eight in 10 in-hospital COVID deaths in the U.S. were among adults aged 65 and up, and nearly three-quarters involved people with three or more underlying medical conditions, according to a Centers for Disease Control report released in mid-September.
Another big risk factor: Not being vaccinated at all.
In Canada, federal data from the month of August 2022 shows unvaccinated individuals were five times more likely to be hospitalized and seven times more likely to die from their illness, compared to people with a completed primary COVID vaccine series — that’s typically two doses.
During the same four-week period, when compared to people with a completed primary vaccine series and at least one booster dose, unvaccinated individuals were seven times more likely to be hospitalized and eight times more likely to die of COVID.
“If you haven’t been vaccinated at all — even if you’ve been infected — the first thing that you’re going to want to do is get the primary series of vaccinations,” stressed Angela Rasmussen, a virologist with the University of Saskatchewan’s Vaccine and Infectious Disease Organization (VIDO).
Time it right
As for the majority of adult Canadians who’ve had two or three doses already, there’s another key factor to consider when getting your next shot: Timing.
While many Canadians scrambled to get each round of COVID shots as soon as they were eligible in the early days of the country’s vaccination campaign, a growing body of research suggests waiting a bit longer between doses is a better approach.
The best timing for a booster depends on a few factors — your risk level and when you were last infected or vaccinated, since the shots are more effective if they’re spaced out from an active COVID infection or previous vaccination.
For those who are high risk, Canada’s vaccine advisory body, the National Advisory Committee on Immunization (NACI), suggests waiting three months or more after your last COVID vaccination or infection before getting another dose.
“If you’re an individual who is living in a high-risk area or has comorbidities and you feel that you want to inch toward the three-month mark, have a consultation with your primary care physician or reach out to your public health unit,” said Dr. Nitin Mohan, a physician and epidemiologist in London, Ont.
For the bulk of the population, the recommendation is to wait at least six months or more between doses, or since your last infection.
Those factors mean the timing of COVID boosters will start to vary more from person to person. And it means some people shouldn’t be getting a fall booster at all, but rather waiting until later in the winter, or even early spring.
Let’s say you’re young and healthy, and were last vaccinated against COVID a year ago, but tested positive for the virus on a rapid test in September. In that case, you could consider waiting around six months post-infection — or until March — for your next booster.
Another scenario: You’ve never caught COVID at all, consider yourself at high risk of serious illness because you’re elderly and juggling multiple other health issues, and had your last dose in July. If you’re following the suggestions for higher-risk individuals, you could consider getting a booster around the three-month mark — so, early fall — with whatever type of shot is available in your area.
Those long waits might be a bit anxiety-inducing for Canadians who are eager to stay up-to-date on their shots. So what’s the scientific reasoning behind waiting months and months for your next dose?
Over the last few years, researchers in several countries — including Canada — have put forward multiple studies showing a longer interval between doses of various types of COVID vaccines can help provoke a better immune response.
The original Pfizer mRNA vaccine generated antibody responses 3½ times larger in older people when a second dose was delayed to 12 weeks after the first, one U.K. study showed back in 2021, before the Omicron family became dominant.
Canadian findings released later that year showed protection against infection from two doses of the Pfizer vaccine spiked when the first and second shots were spread out — from roughly 80 per cent after a few weeks, to more than 90 per cent after four months.
“You’re not waiting — you’re getting your dose at the right time,” stressed infectious diseases specialist Dr. Lisa Barrett, an assistant professor at Dalhousie University in Halifax. “‘Right dose, right time’ is a great message for people.”
Hit the 'sweet spot'
The goal is hitting a “sweet spot” for maintaining protection against future infections as much as possible, said Miller.
“What we’re trying to do is find that middle ground whereby the immunity from your prior exposure has waned enough that the vaccine can optimally work,” he explained, “but not so long that your protection is gone altogether.”
One small U.S. study showed that our immune system’s ability to respond to a booster dose may actually be blunted by a recent infection.
The paper, co-authored by Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, and published in late September in the journal Cell, looked at 66 people with different infection histories over a two-month period.
The researchers studied the subjects’ B-cells, which are a part of the human immune system known for producing antibodies that can attack threats like viruses and bacteria, and found having a recent infection impeded B-cell responses to a booster shot.
Rasmussen, from VIDO, likened it to overloading your brain during school.
Both infections and vaccinations are a way of training your immune system to recognize a particular pathogen, as if your body is taking a class all about identifying and fighting SARS-CoV-2.
“If you go and start a new class, while the immune system is still in the middle of a previous class and still taking the final exam — it’s not the right time to start a new class,” she said.
“Because the immune system is still in the process of processing that information and adapting to the lesson that [you] got from that prior infection or prior vaccine.”
Know your options
On top of the original vaccines that helped boost immunity throughout much of the pandemic, there’s now a slightly different type of shot entering the market: Bivalent vaccines.
These vaccines, including ones approved by Pfizer and Moderna, are engineered to target both the original virus and certain Omicron subvariants.
Moderna’s shot, approved for Canadian adults, is tailored to the subvariant BA.1, which began spreading in late 2021 and drove the largest wave of the pandemic. (It’s no longer circulating, but is still part of the Omicron family and more closely related to what’s spreading right now.)
Pfizer’s new vaccine, which was approved on Friday for use as a booster for people aged 12 and up, targets the BA.4 and BA.5 subvariants, which together make up nearly all of Canada’s recent COVID cases. Moderna also has an application in for a similarly updated option.
Clinical trial data showed these shots could provide slightly greater protection against infection in the Omicron era, but for now, there’s a lack of real-world evidence. So for the time being, there’s no rationale to recommend one over the other at this stage, said Miller.
“Going forward, if we find out that one of these is significantly better, I’m very confident that the guidance will change to reflect that,” he said.
Right now, bivalent vaccines are only approved in Canada for use as a booster dose.
Meanwhile, for the roughly one in 10 Canadians aged five and up who still haven’t had a single shot, the original vaccines are still available to start that process.
Consider your lifestyle
As you’re trying to time your next booster shot, some medical experts say you can also factor in your lifestyle and schedule.
COVID vaccines help ward off serious illness, but spreading out boosters is also meant to offer spikes in protection against getting infected in the first place — which might come in handy if you have major life events coming up, or a return to the office for the first time in years.
“Some of those people that are low risk, that’s the added benefit they want to receive, right?” said Miller. “They basically want to avoid the inconvenience and discomfort associated with getting infected overall.”
There’s also the hassle of missing work or school; the potential of passing the virus onto vulnerable family members; and the possibility of acquiring chronic symptoms, whether it’s a long-lasting cough, a loss of smell or taste, or rare-but-debilitating health issues from more serious forms of long COVID. Up-to-date vaccinations are a key tool to try and prevent those outcomes.
That means there’s good reason to time your vaccination around higher-risk events, said Miller, since you’ll likely have more protection against infection for several months after getting a shot.
“Upcoming travel or a big indoor wedding as we move into the colder months of the year is a reason to accelerate the timing for which you might have been vaccinated — but probably not a reason to delay it,” he said, since that could leave you vulnerable to infection in the interim if it’s been six months or more since your last exposure.
It’s also still unclear just how long spikes in protection against catching COVID actually last, noted Barrett. And while these vaccines offer durable protection against serious illness, they’re not a foolproof tool for avoiding infection in the first place, which means other precautions, like quality masks and hand hygiene, can still be important tools during respiratory virus season.
“The primary job of a booster is not to protect against all infection,” Barrett said. “It’s to refresh your response to the latest form of the virus that’s circulating.”
Federal officials are also warning against delaying a booster to wait for a particular bivalent vaccine, and say anyone who chooses that route should “carefully assess their individual risk.”
The original shots, multiple medical experts agreed, do still offer high levels of protection against serious illness and death.
The future of boosters
As the next generation of shots go into arms this fall, what’s still hazy is the long-term future of COVID vaccination efforts.
Will Canadians still be offered shots multiple times a year? Or can we expect annual or semi-annual boosters?
It all depends on how this virus evolves, what types of new variants emerge and when the spread of SARS-CoV-2 settles into a more predictable seasonal pattern instead of the roller-coaster the world has been riding on since early 2020.
Timothy Evans, executive director of the COVID-19 Immunity Task Force — a collaboration between scientists and experts from universities and health-care facilities across Canada — said his team is now aiming to follow groups that have different patterns of immunity and study the benefits of the latest booster shots, in order to learn how those differ depending on someone’s combination of vaccinations and infections.
“And so, we are hoping to get some better sense of what actually transpires and then use that information to understand what the next question is going to be — do we need an annual booster for this, or not?”
With files from Adam Miller, Ask CBC | Interactive graphics by Graeme Bruce, Wendy Martinez | Main photo by Turgut Yeter