Third vaccine doses now recommended in Canada for people with immunity issues

According to the NACI'S website, the recommendation applies to people with moderate to severe issues with their immune systems, and recommends a regimen of three doses total of an mRNA-based vaccine, that is to say, either a Pfizer or Moderna vaccine.

"There is currently a resurgence of COVID-19 cases in some regions of Canada fueled by the highly transmissible Delta variant that was declared a global variant of concern by the World Health Organization in May 2021. The Delta variant has been observed to increase the risk of infections due to its higher transmissibility."

"Breakthrough infections (infection in fully vaccinated people) have occurred with the Delta variant. This may be due to some degree of evasion of vaccine-induced immunity or waning of vaccine-induced immunity over time, or poor immune response to initial vaccine doses (as might occur amongst those who are moderately or severely immunocompromised)."

The recommendation itself, as posted on the NACIs page, reads:

"NACI previously preferentially recommended that a complete COVID-19 vaccine series (defined as a 2-dose series) with an mRNA COVID-19 vaccine should be offered to individuals in the authorized age group who are immunocompromised due to disease or treatment."

"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."

"There is increasing evidence that antibody titres are related to vaccine effectiveness, (including against viral variants) and may relate to the duration of protection and protection against severe disease. However, a correlate of protection has not yet been defined."

Both the Pfizer and the Moderna vaccines have been found to be well over 90 percent effective in the general population at the two-dose level. The Pfizer vaccine has been recently given full approval by the American FDA, although the organization has yet to pronounce on the efficacy of a potential third dose.

Reportedly, further investigation is still being conducted into this issue, and other population groups may in the future be signaled for a potential booster shot as well.