BARRIE -- As the push to vaccinate all eligible Canadians against COVID-19 continues, some people are already due for their booster shots.

On Friday, the National Advisory Committee for Immunization (NACI) released updated guidance regarding COVID-19 booster shots in Canada, outlining several subsections of the country’s population that should receive a third jab at least six months after their primary vaccine series.

The committee said it “strongly recommends” mRNA booster shots be offered to those aged 50 and over, seniors living in long-term care homes and other congregate living facilities and to those who received a complete series of a viral vector COVID-19 vaccine, such as Johnson & Johnson or AstraZeneca.

NACI also strongly recommends booster shots be offered to First Nations, Inuit and Metis peoples, as well as frontline healthcare workers who are in direct contact with patients.

The agency also recommends that booster doses may be offered to those between the ages of 18 and 49, at least six months after they received their primary vaccine series.

But what is a booster shot, and will everyone eventually need one? Here’s what experts say.


COVID-19 booster shots are just a third dose of the initial vaccine, Dr. Matthew Miller, an associate professor at McMaster University’s department of biochemistry and biomedical sciences, explained.

“For example, people who got two doses of an mRNA vaccine, the booster shot would be a third dose of the same vaccine -- same dose and same contents,” he told in early November.

Miller said that is similar to what you see with most conventional booster shots.

He said people receive the same vaccine multiple times in order to “strengthen the immune response that’s elicited against the contents of the vaccine.”

Dr. Isaac Bogoch, an infectious diseases faculty member at the University of Toronto, said “semantics matter,” adding that COVID-19 boosters should really just be referred to as a third dose.

Bogoch said many vaccines are administered in three doses, and pointed to the Hepatitis B vaccine as an example.

“So I think we should be thinking of this as a three-dose vaccine series,” he told in early November.

According to Bogoch, it is “very likely” that most people will require a third COVID-19 vaccine.

“Most of us are two doses in to a three-dose vaccine series,” he said.


Bogoch said there are a few reasons a third dose of COVID-19 vaccine is necessary.

He said some people don’t mount as strong of an immune response as others – namely the elderly and those who are immunocompromised.

Bogoch said they can “certainly benefit” from a third dose of the vaccine “on the sooner end of the spectrum.”

According to Bogoch, another reason for administering a third dose is to combat waning immunity and protection, which could happen over time.

“Waning protection really referring to breakthrough infections,” he said. “Some studies overestimate the degree of it, but it’s still there and it’s clear that people will need a third dose to help prevent that from happening.”

Bogoch said another reason a third dose should be administered is to help further reduce the risk of severe outcomes and death, as well as reduce the risk of getting the infection in the first place.

He pointed to a study from Israel, which found those over the age of 60 who had received a third dose of the vaccine were 19.5 times less likely to have severe COVID-19 than those of the same age who had only received two shots.

“So I think it’s safe to say that most people will be getting a third dose and the reason they will be is because this should really be a three-dose vaccine series,” he said.

Miller echoed Bogoch’s remarks, saying he thinks it is “inevitable” that everyone will eventually need a third dose of the COVID-19 vaccine.

He added that NACI is making “evidence-based recommendations” when it comes to who receives their third dose and when, in a way that will “protect vulnerable populations.”


When it comes to determining whether an additional dose of a vaccine is necessary, researchers look at a few different things, Miller said.

First, researchers will study the effectiveness of the initial vaccine series. They do this by monitoring effectiveness in large populations, Miller explained.

“What we want to see is steady protection as we get months from completion of the primary vaccine series,” he said. “When we start to see that dropping, that’s really the most important signal that a booster may be necessary.”

In addition to monitoring its effectiveness, Miller said some researchers will also look specifically at the immune response the vaccine is producing.

“So they look at things like antibodies in people’s blood, or cells in people’s blood -- immune cells -- and look to see what happens with the numbers of those things,” he said. “That’s a more imperfect measure than looking at effectiveness directly, but it can also give us signs of when a booster might be necessary.”


Both Miller and Bogoch said it’s too soon to know whether Canadians will need COVID-19 vaccines every year.

“It might be an annual thing -- it might not be, no one really knows,” Bogoch said. “I think if anyone speaks with a high degree of confidence on this we should be suspicious because we don’t really know.”

Miller said this is something that “still requires pretty intensive investigation.”

“On one hand, it’s a decision that’s going to be impacted by how long we see immunity last,” he explained. “But that’s also related to how much the virus is circulating [and] how much the virus is evolving.”

Miller said researchers are monitoring these things in real-time, every day to help inform those decisions.


While Canada begins administering third doses to those in need, Miller said the country should also “still be thinking really carefully” about the other places in the world where people have not yet received even their first jab.

Miller said ensuring people in other parts of the world have access to COVID-19 vaccines will “really help Canada too.”

“The lower the infection rates are internationally, the lower the probability of problematic variants emerging will be,” he explained.

Miller said making sure other countries have sufficient quantities of vaccines for their populations, and ensuring third doses are available to those in Canada who require them should be “parallel priorities.”