Public health authorities are recommending that Quebecers have a COVID-19 mRNA booster shot as soon as they are eligible. Booster doses:
- can increase the immune response
- are expected to offer enhanced protection against infection and severe disease
- may help reduce spread of infection
Public health has also made it clear that it is safe to mix and match Pfizer and Moderna COVID vaccines for booster shots. CIUSSS West-Central Montreal is currently offering Pfizer for individuals between the ages of 5 and 29 and Moderna for those 30 and older. As 
outlined on the Government of Canada web site, the rationale is as follows:
- Both the Pfizer-BioNTech and Moderna mRNA vaccines work the same way
- During clinical trials, both mRNA vaccines (Pfizer-BioNTech, Moderna)demonstrated similar safety profiles and side effects (NACI).
- Both mRNA vaccines showed similar vaccine efficacy in clinical trials against symptomatic COVID-19 disease following the second dose, 95% and 94% respectively for Pfizer-BioNTech and Moderna vaccines (NACI).
- There is no reason to believe that mRNA vaccine series completed with a different authorized mRNA vaccine product would result in any additional safety issues or reduction in immune protection against COVID-19 at this time (NACI).
- Public health authorities have determined that if the same product is not readily available, or the product used for the first dose is unknown, the available mRNA vaccine product should be used for the subsequent (“booster”) doses.
Furthermore, research from the Centers for Disease Control (CDC) found that using the mix-and-match strategy elicited either a similar or a higher response as compared to using the same vaccine.
A recent study out of the United Kingdom indicates a third dose of Moderna offered more effective protection against Omicron, compared to Pfizer. The study from the UK Health Security Agency showed that among those who received a Pfizer primary course, vaccine effectiveness was around 70% after a Pfizer booster, dropping to 45% after 10-plus weeks and stayed around 70 to 75% after a Moderna booster up to 9 weeks after booster.