COVID-19 vaccine update: Recommendations on expanded third dose eligibility for moderately to severely immunocompromised individuals

Advisory Alert

September 16, 2021

*Cette information est seulement disponible en anglais.

To: Local health system partners


Expanded eligibility for third dose of COVID-19 vaccine

This Advisory Alert outlines new recommendations from the Ontario Ministry of Health and the National Advisory Committee on Immunization (NACI) regarding the expansion of third dose eligibility for the COVID-19 vaccines.

On September 14, 2021, the government of Ontario announced expanded eligibility for third doses of the COVID-19 vaccines and issued their COVID-19 Vaccine Third Dose Recommendations. This direction aligns with recommendations recently shared by the National Advisory Committee on Immunization (NACI) on third doses for individuals who are moderately to severely immunocompromised.

A third dose for moderately to severely immunocompromised

Individuals who are moderately to severely immunocompromised are at an increased risk of severe outcomes from COVID-19. Per emerging evidence, this group demonstrated a sub-optimal immune response to the COVID-19 vaccine series. Due to their underlying conditions and vaccine response, certain immunocompromised individuals may benefit from a third dose to complete their vaccine series.

The National Advisory Committee on Immunization (NACI) strongly recommends that those who are moderately to severely immunocompromised who have not yet been immunized, receive a primary series of three doses of an authorized mRNA vaccine, permitted they are in the authorized age groups. Strong recommendations were also specified for those who are moderately to severely immunocompromised who have already received a complete primary series (whether 1 or 2 doses, whether with a homologous or heterologous schedule using an mRNA or a viral vector vaccine) indicating that an additional dose of an authorized mRNA COVID-19 vaccine should be offered.

In alignment with NACI, the Ministry of Health recommends that third doses of the COVID-19 vaccines be offered to an expanded group of moderately to severely immunocompromised individuals to complete their extended COVID-19 vaccine series. It is important to distinguish that this third dose should not be considered a booster dose and rather an extension of the primary vaccine series.

Individuals who are immunocompromised will qualify for a third dose if they meet one of the following conditions:

The Ministry of Health recommends that those who are severely immunocompromised receive their third dose eight weeks following their second dose. Per NACI recommendations, the minimum interval should be 28 days; however, an interval longer than the minimum is likely to result in a better immune response. Exact timing of third doses should be decided with treating providers to optimize immune response and minimize impact on treatment.

Continued eligibility for vulnerable elderly in high-risk congregate settings

Per the August 17, 2021 direction, residents of high-risk congregate settings including long-term care homes, higher-risk licensed retirement homes (as defined by the Ministry of Health), and First Nations elder care lodges are eligible for a third dose of the COVID-19 vaccine to provide an extra layer of protection against the Delta variant.

The recommended interval for residents of long-term care, high-risk retirement homes, and elder care lodges is at least 5 months (20 weeks) after the second dose. Vaccinations will be provided within the home, either by the home’s staff or through a mobile clinic offered by Public Health Sudbury & Districts.

It is strongly recommended that all people who come into close contact with residents of these high-risk settings complete their COVID-19 vaccine series to provide a ring of protection for this more vulnerable group.

Contact your eligible patients about a third dose

Health care providers are asked to contact their eligible patients regarding a third dose. Details on this process were previously outlined in an Advisory Alert dated August 31, 2021. As a reminder, a referral form (English, French) will be required to receive a third dose for moderately and severely immunocompromised individuals, unless vaccination occurs onsite with the physician or specialty program responsible for patient care. In addition to health care providers’ offices and pharmacies, individuals can access a third dose at Public Health led vaccination clinics provided they have a completed referral form in hand.

Individuals eligible for a third dose of the COVID-19 vaccine should receive a third dose of an mRNA vaccine (Pfizer-BioNTech or Moderna), and the same vaccine product as their second dose if readily available (available at the time of vaccination without delay or wastage). An mRNA vaccine is also recommended as the best third dose option for individuals that received AstraZeneca/COVISHIELD COVID-19 vaccine for their first and second dose unless contraindicated.

In the event that individuals are unable to receive an mRNA vaccine due to contraindications, they may be offered a viral vector vaccine and informed consent must be obtained. In this scenario, informed consent must include a discussion about the lack of evidence of an additional dose of a viral vector vaccine in immunocompromised populations as well as potential adverse events.

The Canadian Immunization Guide (Part 3 – Vaccination of Specific Populations) provides guidance for the immunization of immunocompromised persons. Details include timing of vaccination for transplant recipients and those requiring immunosuppressive therapies, including a list of conditions leading to primary immunodeficiency, and information on immunosuppressive therapies. Additional guidance on third dose products and informed consent for viral vector vaccines are including in the Ministry of Health’s COVID-19 Vaccine Third Dose Recommendations guidance document.

Additional information

For more information on the COVID-19 vaccines, as well as for specific answers to your frequently asked questions visit our FAQ web page or call Public Health Sudbury & Districts at 705.522.9200 (toll-free 1.866.522.9200).



Original Signed By

Dr. Penny Sutcliffe
Medical Officer of Health and Chief Executive Officer

NOTE: All Advisory Alerts are found on our website.

  1. Active treatment for individuals with solid tumours or hematologic malignancies includes those who have completed treatment within three months.
  2. Active treatment for individuals receiving B-cell depleting therapy includes those who have completed treatment within 12 months.

This item was last modified on September 16, 2021