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Health Canada has approved a booster of Pfizer-BioNTech’s COVID-19 vaccine for people over the age of 18, though third doses have already been given to high-risk people across the country.

The booster is designed to help people with their first two COVID-19 vaccine doses maintain their protection against the virus over time.

The booster is identical to the regular Pfizer-BioNTech vaccine, and is to be given at least six months after the first two vaccine doses. Clinical trials have shown side-effects more or less the same as those experienced with the original two doses of the vaccine.

Health Canada’s review found the booster dose met the regulator’s safety, efficacy and quality requirements, according to a news release from the department.

Different provinces have employed different rollout strategies for booster doses, and mRNA vaccines like Pfizer-BioNTech have already been used to offer longer-lasting protection to high-risk people in several parts of the country.

Those provinces have so far been using the vaccines “off label,” since until now they were not authorized for a three-dose regimen, according to Health Canada.

Provinces and territories are able to use health products outside the scope of what they are specifically approved for. Health Canada said it’s called the “practice of medicine,” and it is regulated by provincial and territorial governments.

Most boosters have so far been administered to the immunocompromised, people of advanced age, people in long-term care, Indigenous populations, and front-line health workers.

The National Advisory Committee on Immunization has also recommended a booster for people who received two doses of Oxford-AstraZeneca or one dose of the Janssen vaccine, because of evidence of waning immunity over time.

There is no evidence of waning protection over time against severe COVID-19 in the general vaccinated population, according to NACI’s latest advice.

However at a briefing last Friday, chief public health officer Dr. Theresa Tam suggested some areas of the country with a high number of COVID-19 cases and good vaccine coverage may be suffering from the effects of waning immunity.

“Some of these populations have had the vaccine the earliest and therefore the longest time since vaccination,” she said. “There’s been reports of some observations of waning immunity against infection, and in some cases, they have had some severe outcomes.”

Many provinces have already begun to look ahead to booster campaigns for the entire population.

Ontario is eyeing early 2022 to expand booster doses to all adults, based on a six-to-eight month interval after people received their second doses.

Officials there still note that two doses provide very good protection against COVID-19, especially against serious illness.

Moderna has also applied to Health Canada to approve its COVID-19 booster shot, which uses a smaller amount of vaccine than the first two doses.

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