COVID: ‘Fully vaccinated’ should be redefined, experts say

As evidence mounts on the effectiveness of third doses of the COVID-19 vaccine, experts say it’s high time public health officials revised the definition of “fully vaccinated” to include a booster dose.

While most SARS-CoV-2 vaccines available in Canada were originally developed and tested to contain two doses — with the exception of Johnson & Johnson’s vaccine — a primary series should be three doses instead, said Colin Furness, an expert in Infectious Disease Epidemiology from the University of Toronto.

“Someone who has only had two shots will say, ‘I’m fully vaccinated,’ and that’s what the authorities say. [but] authorities got it wrong,” Furness told in a June 1 phone interview. “If we look at the reduction in immunity after the second dose, this is really a three-dose vaccine.”

A newly published meta-analysis conducted by the Chinese University of Hong Kong earlier this year assesses the effectiveness of different vaccine combinations in protecting against COVID-19. The meta-analysis, which included 53 studies involving more than 100 million people, showed that three doses of an mRNA vaccine were 96 percent highly effective in protecting against COVID-19 infection, whether symptomatic or asymptomatic.

Three doses of an mRNA vaccine also had a 95 percent effectiveness rate in reducing COVID-19-related hospitalizations. The study concludes that three doses of a COVID-19 vaccine are required to protect against infection by the omicron variant of the virus.

Despite this, the Health Administration of Canada (PHAC) definition of “fully vaccinated” in relation to COVID-19 vaccines continues to refer to those who have received a basic series of the vaccine. The primary series includes two doses of most vaccines approved in Canada (e.g. mRNA vaccines, AstraZeneca, Novavax and Medicago vaccines) or one dose of Johnson & Johnson’s vaccine.

“As the epidemiological context of COVID-19 evolves, the Canadian government will continue to monitor the efficacy and effectiveness of vaccines used in Canada and may make further recommendations on how to define a fully vaccinated person,” PHAC spokeswoman Anna Maddison wrote on April 1. June in an email to

It’s important to note that as the COVID-19 pandemic has progressed, the SARS-CoV-2 virus has evolved from what was initially developed as a target for vaccines, Furness said. This viral evolution has reduced the effectiveness of current vaccines, particularly when it comes to protecting against infection with Omicron, the latest variant of concern. Even so, three doses of the vaccine still provide solid protection, especially against serious consequences of COVID-19 such as hospitalization and death, Furness said. As a result, he is hoping for a change in the definition of “fully vaccinated”.

“Hypothetically, if we had a vaccine that prevented long-term COVID, hospitalization and death, but didn’t stop anyone from getting infected, I would call that extremely successful. We’d all catch a cold… but no one would actually get sick,” Furness said. “There’s no question we don’t have that, but three doses of a COVID-19 vaccine gets us closer to that.

“When we say ‘fully vaccinated’ we should say three [doses].”

Just over 20 million third and fourth doses of the COVID-19 vaccine have been administered in Canada so far, based on data compiled by According to PHAC, about 55.5 percent of Canadians age 12 and older have been vaccinated with three doses of the COVID-19 vaccine, or about 18.5 million people.


The epidemiologist Dr. University of Saskatchewan’s Nazeem Muhajarine also advises those who are eligible for their third dose of the COVID-19 vaccine to get it. However, rather than tying the definition of “fully vaccinated” to a specific number of doses, health officials should focus on advising Canadians to keep their immunization status “up to date” based on the latest recommendations from the National Advisory Committee for immunization (NAKI).

“‘Fully vaccinated’ no longer works in my opinion as a definition of being up to date on the doses of vaccine you need to be getting… I think it’s high time we [change] that,” he told in a June 1 phone interview. “It’s about staying up to date with all the vaccines that are recommended to you at this stage of the pandemic.”

The current World Health Organization definition of a primary series refers to one or two doses of COVID-19 vaccines approved for emergency use, depending on the product. In connection with international travel, the Canadian government considers those with at least one dose of the Johnson & Johnson vaccine, or two doses of any of the remaining vaccines approved for use in Canada, to be fully vaccinated. However, provincial governments like Ontario’s have updated the wording in their latest guidance to suggest people be vaccinated with “all recommended COVID-19 vaccine doses, including any booster doses if eligible.”

In addition, provinces across Canada have now made third doses available for those ages 12 and older, and NACI strongly recommends third doses for all adults in Canada.

While Furness admits that protection against the COVID-19 vaccine could also wear off after a third dose, he still recommends that those who are eligible for a booster shot do so given the extra protection it offers compared to just two doses.

“It may be fading, but that would still really break up the transmission and it would break outbursts if we could get enough people [vaccinated] with a third dose,” he said.

A third dose of the COVID-19 vaccine will also help further protect the population from new variants or subvariants that may emerge in the future, Muhajarine said.

“As long as multiple variants proliferate, that opens up the possibility for new variants to emerge — that’s how the evolution of those variants happens,” he said. “I see no evidence-based reason to stay ‘fully vaccinated’ at just two base doses.”

Uptake of the third dose of the COVID-19 vaccine was slow compared to the first and second doses, both Furness and Muhajarine said. Part of the reason for this is that the vaccine was originally presented to the public as consisting of two doses, which led people to believe that was all they needed to be adequately protected, Furness said.

“That’s the mental model that most people had,” he said. “When we started saying you need a third dose now… [they] said, ‘They sold us an idea that there’s a two-dose vaccine and I signed, now change the deal and I don’t like that, I’m walking away.'”

Contributing to this lack of enthusiasm for the boost was the lifting of vaccination requirements by provincial governments across the country earlier this year, Muhajarine said. For example, anyone who cannot enter restaurants or cinemas because they have not been vaccinated no longer has access, and proof of vaccination is no longer required.

“Vaccination mandates have worked to actually get people who are eligible for a second dose,” Muhajarine said. “But once governments started rolling back these public health measures and immunization requirements, I think people really lost that incentive.”

Encouraging Canadians to empower themselves

While urging health officials to include third doses in the definition of “fully vaccinated,” Furness said it would take more than a change in definition to encourage people to actually get their booster shot. Part of the solution lies in vaccination regulations implemented and enforced by employers and governments.

“[If] Her employer has mandated vaccination and that’s going to change to three doses, then we’re actually going to have a meaningful public conversation about what the risk and the benefits are,” Furness said. “Once we’ve had enough meaningful public conversations, people will start thinking about it… [otherwise] it will seem like a bureaucratic move.”

Having an open conversation about the role vaccines play in the fight against COVID-19 is especially helpful in addressing those who have doubts or fears about getting vaccinated, Furness said. Part of the solution also lies in increased education about how vaccines work, as well as improved health messaging and more local engagement, Furness said.

“It’s so anxious, doubtful, a bit skeptical [group] that’s not sure who to believe, and anyone in that category by default won’t do anything until you make a good case — that’s where the action is,” Furness said.

Based on his observations, appetite for vaccine mandates across Canada is currently low, Muhajarine said. But one scenario in which he might see these rules reintroduced at the provincial level is if governments see another wave of cases or if another variant or sub-variant would emerge, he said. In this situation, it’s crucial that officials don’t wait too long to act, Muhajarine said.

“Provincial governments have the authority and responsibility to implement policies that ensure the safety of their populations,” he said. “During COVID-19 we have learned that acting quickly is really important. We can’t wait until COVID starts really hurting us to do something about it.”

Furness also said that as pharmaceutical companies get closer to developing an Omicron-specific vaccine, it will likely encourage more people to get their third dose if they haven’t already.

“If we can come back and say, ‘Look, the vaccine is again what a lot of people want to understand,’ which is my reduction in the risk of infection… I think it’s going to bring more wind in the sails,” Furness said. “And it will make it easier to uphold vaccination mandates.”

While Furness said it could be argued to wait until a variant-specific vaccine is developed before considering revising the definition of fully vaccinated, he and other experts recommend that those who qualify get vaccinated sooner rather than later will.


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