Information on COVID-19 Vaccine Second Doses


  • For individuals who received their first dose of an mRNA vaccine (Pfizer-BioNTech Comirnaty or Moderna Spikevax), their second dose appointment should be at least 8 weeks following the first dose. 
    • The recommendation of an 8 week interval will provide increased and longer-lasting protection from the vaccine and may be associated with a lower risk of myocarditis and/or pericarditis. This recommendation is for everyone 5 years of age and older.
  • For individuals who received a first dose of AstraZeneca Vaxzevria vaccine, their second dose appointment must be at least 8 weeks following the first dose. See the AstraZeneca Vaxzevria section below for more details.


Interchangeability of mRNA Vaccines

Individuals can safely receive different mRNA vaccines (Pfizer-BioNTech Comirnaty and Moderna Spikevax) for their first, second and subsequent doses depending on their age. This supports individuals in getting fully immunized and receiving their booster doses more quickly and conveniently. This also helps offset issues related to vaccine supply and delivery.

The National Advisory Committee on Immunization (NACI) and Ontario Ministry of Health have confirmed that a mixed vaccine schedule for the COVID-19 vaccine is safe and effective. This means that individuals who received a first dose of an mRNA vaccine can receive a different mRNA vaccine for their second and subsequent doses.

Mixing of mRNA vaccines is reasonable given the similarity of these vaccines in their mechanism of action, side effect profile and effectiveness against severe COVID-19 infection.

At this time, the mixed schedule does not apply to youth 12 to 17 years of age.

Individuals are encouraged to get any dose they are eligible for as soon as they are able, and to get the vaccine available at the clinic most convenient for them, regardless of which mRNA vaccine is offered at that clinic.


Individuals who Received the AstraZeneca Vaxzevria Vaccine for their First Dose

As of June 12, 2021, Ontario accelerated the second dose interval for those who received a first dose of AstraZeneca Vaxzevria to eight weeks with informed consent, from 12 weeks. See the media release for details.

An mRNA vaccine is now preferred as the second dose for individuals who received a first dose of the AstraZeneca Vaxzevria vaccine, based on emerging evidence of a potentially better immune response from this mixed vaccine schedule and to mitigate the potential risk of VITT associated with viral vector vaccines. As of September 3, 2021, AstraZeneca Vaxzevria is no longer available at local pharmacies.

Current evidence recommends waiting 12 weeks to receive a second dose as this interval allows for a better/optimal immune response to be achieved. However, the province recognizes that some may want to receive their second dose sooner to have the increased protection provided by an earlier second dose.

Those who choose to book an mRNA vaccine (Pfizer-BioNTech Comirnaty or Moderna Spikevax) appointment should visit the COVID-19 Vaccine Appointments page to find a clinic.

Additional information for those who choose an mRNA vaccine (Pfizer-BioNTech Comirnaty or Moderna Spikevax) as their second dose:

  • The National Advisory Committee on Immunization (NACI) updated its guidance to recommended that a first dose of the AstraZeneca Vaxzevria vaccine can be followed by an mRNA vaccine (Pfizer-BioNTech Comirnaty or Moderna Spikevax).
  • The updated NACI guidance is based on emerging research from Spain and the United Kingdom that found mixing and matching the AstraZeneca Vaxzevria and Pfizer-BioNTech Comirnaty vaccines was both safe and effective at preventing COVID-19.
  • Health Canada is monitoring the situation and reviewing new research from around the world as it becomes available.
  • Individuals with questions should speak to their health care provider about their own specific situation.
  • Individuals will be considered “fully immunized” if they choose an mRNA vaccine (Pfizer-BioNTech Comirnaty or Moderna Spikevax) for their second dose.
Last Updated: 13/01/2022