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Middlesex-London Health Unit

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COVID-19 Vaccine Eligibility

Keep your COVID-19 vaccines up to date

The COVID-19 vaccine is safe and effective and will continue to protect us against COVID-19. Please note: the Middlesex-London Health Unit follows the Ontario Ministry of Health guidance on eligibility for the COVID-19 vaccine in Ontario.


Primary Series

A primary series is the initial number of doses of a COVID‑19 vaccine that a person needs to develop a strong initial immune response. Most people need two doses of an mRNA vaccine (Pfizer, Moderna) or Novavax (for those over 18 years of age) to complete their primary series. If you are immunocompromised, you may need a three-dose primary series. Each dose in a primary series should be given at an interval of eight weeks between doses.

Individuals aged 6 months and over are eligible for a primary series of COVID-19 vaccine

  • Children aged six months to under five years can receive the paediatric Moderna or Pfizer COVID-19 vaccine. To receive the vaccine, children must be at least six months old.
  • A booster dose is not available for children aged six months to under five years.

Recommended spacing between COVID-19 vaccine doses in a primary series

The Ontario Ministry of Health recommends an interval of eight weeks (56 days) between first and second doses instead of the minimum required for Pfizer-BioNTech Comirnaty (21 days) and Moderna Spikevax (28 days). There is emerging evidence that longer intervals between the first and second doses of COVID-19 vaccines result in more robust and durable immune response and higher vaccine effectiveness.

Vaccines can be safely mixed

The mRNA vaccines (Moderna and Pfizer) are interchangeable for those who are 18 years of age or older. Pfizer and Moderna are essentially different brands of the same vaccine and you will have the same protection against COVID-19 regardless of what you get for your first or second dose.

The interchangeability of mRNA vaccines has been supported by experts at both the Public Health Agency of Canada and the National Advisory Committee on Immunization. Interchangeability means that you can receive either the Pfizer or Moderna mRNA vaccine as your second dose, regardless of the type of vaccine you received for your first dose. Learn more →

Nuvaxovid (Novavax) COVID-19 vaccine

The Health Unit has a limited supply of the Novavax Nuvaxovid COVID-19 vaccine. If you are interested in receiving the vaccine, please call 1-226-289-3560 to be added to the waitlist. We will contact you when vaccine is available for you.

Booster Doses

Booster doses are doses of a COVID‑19 vaccine received after the primary series. Protection after a primary series may decrease over time, especially against new variants. Booster doses help keep you protected from severe outcomes from the virus.

Booster doses should be given at an interval of six months between completion of a primary series and a first booster dose or between booster doses. The minimum interval at which a booster dose can be administered is three months after an individual completed their primary vaccine series or received their most recent booster dose.

Individuals aged 5 and over are eligible for booster doses of COVID-19 vaccine

  • Children between the ages of 6 months and under 5 years are not eligible for a booster dose.

Bivalent Booster Doses

The individuals listed below are eligible for the bivalent COVID-19 booster.

  • Individuals aged 18 and over
  • Moderately to severely immunocompromised individuals aged 12 and over

What is the bivalent COVID-19 vaccine in Canada?

The bivalent COVID-19 vaccine in Canada is an adapted version of the Moderna Spikevax COVID-19 vaccine, which was authorized for use by Health Canada in September 2022. This vaccine targets the original SARS-CoV-2 virus from 2019 and the Omicron (BA.1) variant and triggers a strong immune response against both virus strains. It also generates a good immune response against the Omicron BA.4 and BA.5 subvariants, and it’s expected to extend the durability of protection. To learn more, visit Health Canada’s website at www.canada.ca.

Immunocompromised individuals

Three-dose primary series and booster doses

Some individuals who are immunocompromised are eligible for a three-dose primary series, which means that three doses of COVID-19 vaccine are received to complete an initial vaccination series. This is because underlying health conditions could put these individuals at high risk of severe illness due to COVID-19. A three-dose primary series enhances the immune response and establishes an adequate level of protection against COVID-19.

  • Individuals aged 6 months and over who are immunocompromised are eligible for a three-dose primary series.
  • Following a three-dose primary series, immunocompromised individuals aged 5 years and over are eligible for booster doses of COVID-19 vaccine at a recommended interval of six months since their previous dose.
  • Immunocompromised individuals aged 12 years and over should receive bivalent booster doses at a recommended interval of six months since their previous dose.

Who is considered immunocompromised?

You might be eligible as someone who is immunocompromised if you are:

  • a transplant recipient (including solid organ transplant and hematopoietic stem cell transplants)
  • receiving stable, active treatment (chemotherapy, targeted therapies, immunotherapy) for a malignant hematologic disorder or solid tumor
  • in receipt of chimeric antigen receptor (CAR)-T-cell
  • an individual with moderate or severe primary immunodeficiency (for example, DiGeorge syndrome, Wiskott-Aldrich syndrome)
  • in Stage 3 or advanced untreated HIV infection and those with acquired immunodeficiency syndrome
  • undergoing active treatment with the following categories of immunosuppressive therapies: anti-B cell therapies (monoclonal antibodies targeting CD19, CD20 and CD22), high-dose systemic corticosteroids, alkylating agents, antimetabolites, or tumor-necrosis factor (TNF) inhibitors and other biologic agents that are significantly immunosuppressive or are taking specific immunosuppressant medications (PDF)
  • receiving dialysis (hemodialysis or peritoneal dialysis)

If you believe you are eligible for a three-dose primary series, please contact your health care provider. When receiving your COVID-19 vaccine, please bring a completed patient referral form from your health care provider or a relevant prescription or pill bottle.

Re-vaccination with a new COVID-19 vaccine primary series

The following individuals are eligible for a new COVID-19 vaccine primary series.

  • Post-transplantation individuals who received hematopoietic stem cell transplants (HSCT), hematopoietic cell transplants (HCT) (autologous or allogeneic), and recipients of CAR-T-cell therapy, due to the loss of immunity following therapy or transplant.
Date of creation: February 18, 2021
Last modified on: September 28, 2022