Canada’s National Advisory Committee on Immunization (NACI) is recommending a third dose of a COVID-19 vaccine for certain immunocompromised Canadians. However, no decision has been made yet on additional shots for the broader population.

The advisory body said on Friday that a third dose of an mRNA vaccine like Moderna or Pfizer can help immunocompromised individuals develop antibodies. 

“Recent studies have demonstrated that some people who do not respond after two doses, particularly those who are moderately to severely immunocompromised, develop antibodies after a third dose of an mRNA vaccine; and that there are increases in antibody titres following a third dose for some of those who do respond to an initial primary series,” NACI said in its updated guidance document. 

NACI previously preferentially recommended that two-doses of an mRNA COVID-19 vaccine should be offered to individuals in the authorized age group who are immunocompromised due to disease or treatment. 

In its latest update the advisory body also made a clear distinction between a third dose and booster show.  

“The additional or third dose being considered for moderately to severely immunocompromised persons should be distinguished from that of a booster dose,” stated NACI. “The intent of a booster dose is to restore protection that may have waned over time in individuals who responded adequately to an initial 1- or 2-dose primary vaccine series. Additional doses beyond the standard primary vaccine series, such as discussed in this statement, provide an opportunity for individuals who may not have achieved an adequate level of protection from the standard primary vaccine series to develop a better immune response.”

NACI said its updated guidance is based on the review of direct and indirect evidence available up to August 11, 2021 on efficacy/effectiveness, safety and immunogenicity of the standard 1- or 2- dose primary series, and the safety and immunogenicity of an additional dose of COVID-19 vaccine following a standard primary series in individuals who are immunocompromised due to disease or treatment.

Several countries including Israel, the United States, France, Germany, the United Kingdom, Denmark and Norway have implemented, or are planning to implement, the administration of third doses of COVID-19 vaccine in some immunocompromised populations in order to reduce the risk of breakthrough infections among vulnerable groups.

Ontario and Alberta are also getting ready to offer a third dose for certain vulnerable populations.

 

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