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The third dose of vaccine in five questions | Coronavirus: Ontario


1. Why are two doses not enough?

There is evidence that the vaccine’s efficacy decreases over time after the first two doses. Studies carried out in record time by researchers at the Institut Pasteur have even shown that people who received two doses of the Pfizer vaccine or the AstraZeneca vaccine hardly neutralized the Omicron variant five months after vaccination.

However, weaker protection against infection can contribute to an increase in viral transmission, infected people can be a source of infection for others, recalls the National Advisory Committee on Immunization (NACI).

A booster dose therefore offers more lasting protection in order to reduce infection, transmission and, in certain more vulnerable populations, severe disease, concludes the Public Health Agency of Canada.

2. How effective is a 3rd dose?

The Ontario Ministry of Health confirms that real-world evidence shows that the booster dose is associated with good short-term vaccine efficacy and protection similar to that provided by the second dose of the vaccine.

But for now, there is no scientific data on the long-term effectiveness of booster doses; thus, the duration of this protection is not yet known.

However, studies are underway to analyze the effectiveness of the third dose over the long term and to understand why the Omicron variant is more transmissible from one individual to another.

A woman wearing a Christmas hat prepares a dose of vaccine.

Ontarians 18 years of age and older can now make an appointment for their third dose.

Photo: The Canadian Press / Lars Hagberg

The Africa Health Research Institute (AHRI), a South African organization that piloted one of the first studies on the resistance of the new variant to existing vaccines, observed that they may lose some of their effectiveness compared to Omicron variant.

In contrast, considerable immunity is retained in both vaccinated and previously infected individuals, says the study.

For their part, Pfizer-BioNTech and Moderna have insisted on the effectiveness of their vaccine against the Omicron variant after three doses.

Moderna says that a 50 microgram dose of its vaccine – the one already approved by Health Canada – increases the amount of neutralizing antibodies made by the human body by about 37 days after its administration.

Pfizer-BioNTech argues that preliminary laboratory studies indicate that a third dose of vaccine increases neutralizing antibodies 25-fold compared to two doses against the Omicron variant.

3. What is the best time between the 2nd and 3rd dose?

the National Advisory Committee on Immunization, which advises the Public Health Agency of Canada, recommends a booster dose of a messenger RNA vaccine for people 18 to 49 years of age, at least six months after receiving the second dose of their primary series.

A longer interval between the primary series and the booster dose may increase the presence of antibodies, estimates an English study published in late November and cited in the official publications of Canada.

In early December, however, the Ontario government recommended that a booster dose of an mRNA vaccine be offered to people 50 years of age and older at least three months after the last dose of the primary COVID vaccination series. -19.

In addition, immunogenicity studies from England also confirm that booster doses of messenger RNA vaccines given three months or more after the primary series cause a strong immune response.

4. Do we need a 3rd dose for those who have already had COVID?

NACI does not consider an infection with COVID-19 to be equivalent to one dose of the vaccine.

A person who has already been infected should therefore receive the full set of COVID-19 vaccines, including the booster dose, according to his advice.

However, Quebec does not recommend the second dose or the booster dose for people who have already been infected.

5. Can I receive a booster dose of Moderna vaccine after a Pfizer vaccination (and vice versa)?

Health authorities assure that either Moderna or Pfizer-BioNTech are suitable vaccines as a booster dose.

the National Advisory Committee on Immunization however, specifies that for persons aged 18 to 29 years, Pfizer-BioNTech should be preferred due to lower rates of myocarditis or pericarditis reported after vaccination of Pfizer-BioNTech compared to the dose of Moderna.

AstraZeneca or Janssen viral vector vaccines are not currently authorized as booster doses in Canada.

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