COVID-19 Prevention

The prime way to prevent an infection with the respiratory syndrome coronavirus 2 (SARS-CoV-2) that causes COVID-19 is vaccination, as directed by your local health authority. There is also one prevention therapy for use in those who are immunocompromised and unlikely to mount an adequate immune response to COVID‐19 vaccination, or in those for whom COVID-19 vaccination is not medically recommended.

COVID-19 Background

Droplets containing SARS-CoV-2 can enter your body mainly through respiratory pathways after an infected person coughs, sneezes, or simply exhales near you. Typically, these droplets could linger in the air, and you might unknowingly breathe them in, which is why masks and physical distancing are so important for preventing the spread of infection. These droplets could also be on a surface that you touch before touching your eyes, nose, or mouth. For this reason, hand sanitizing and washing with soap and water help stop the virus from spreading.

The virus enters your respiratory tract via your mouth, nose, and/or throat and makes its way into your lungs. It infects your body by latching its surface spike proteins to the angiotensin converting enzyme 2 (ACE2) receptors on the surface of your cells. Once the virus is bound to a cell in your body, it fuses with it, inserting genetic material, making many copies of itself (using the viral RNA as a blueprint), and spreads throughout your body, eventually infecting and killing many of your own cells and causing some body systems to malfunction.

How do the mRNA vaccines work?

The Pfizer-BioNTech (Comirnaty®) and Moderna (Spikevax®) COVID-19 vaccines rely on messenger ribonucleic acid (mRNA) technology, which scientists have been researching for more than a decade. In fact, scientists have studied mRNA vaccines for the flu, Zika virus, rabies, and cytomegalovirus.1 Using mRNA technology, the COVID-19 vaccines provide our bodies with the instructions to make non-harmful pieces of the spike protein similar to those that exist on the surface of SARS-CoV-2, after which our body produces antibodies against the spike protein. This trains the immune system to fight the virus’ spikes, should it later encounter them, and is triggered by exposure to the actual virus’ spike protein. While SARS-CoV-2 infects the body by inserting its RNA in our cells, these vaccines use mRNA to prevent the virus from doing this. The mRNA is immediately degraded in the cells and never enter the nucleus. Importantly, the vaccines do not contain a live virus so people who receive these vaccines cannot develop COVID-19 from them.

The New England Journal of Medicine published the Pfizer-BioNTech clinical trial results, demonstrating a two-dose regimen conferred 95% protection against COVID-19 in persons 16 years of age or older. Safety over a median of two months was similar to that of other viral vaccines.2 Moderna’s vaccine demonstrated 94.1% efficacy against the virus, including severe illness, with a two-dose regimen in trial participants 18 years of age or older.3

Health Canada, the federal authority that regulates drugs and health products in Canada, has approved the mRNA vaccines based on their safety, efficacy, and quality. Non-mRNA vaccines are not as popular due to evidence of lower efficacy.

Vaccine Efficacy

The original Pfizer-BioNTech Comirnaty® and Moderna Spikevax® vaccines have demonstrated 95% effectiveness after a two-dose series for healthy individuals older than 16 years of age and a three-dose series for those with moderate to severely weakened immune systems.2,3 However, they have diminished effectiveness4 for newer variants. For example, they offer approximately 60% effectiveness against Omicron strains after a booster dose.5 The vaccines are still highly effective at preventing hospitalization and severe disease.

Further studies are looking into how long protection lasts. but there are studies of up to 6 months efficacy in those with healthy immune systems.6 Check your local guidelines on vaccine schedules for when you can get your booster. If you are taking immune-suppressing medications or have a condition that affects your immune system, you may require more frequent boosters compared to what is suggested for those who are not immunocompromised.

Recent studies have shown that the use of vaccines in children is safe, efficacious, and helps to protect young individuals from COVID-19 infection. The mRNA vaccines have high efficacy in protecting individuals over 18 years of age against COVID-19 and 100% effectiveness for children ages 12-15 years old (12-17 years old for Moderna).7,8 They also have the broadest range of approval for different age groups, conditions (pregnancy, immunocompromised), and booster dosing according to rigorously researched evidence. These vaccines have also been studied in children between 6 months and 12 years of age.

Vaccines such as the Pfizer-BioNTech and Moderna have been updated to include the Omicron variants in addition to the original SARSCoV-2 virus. These are known as bivalent vaccines. The National Advisory Committee on Immunization recommends receiving a bivalent Omicron-containing mRNA COVID-19 vaccine as your booster, if you are eligible.9

Each vaccine has received approval from Health Canada for specific populations.

Moderna Spikevax® COVID-19 vaccine

  • as primary vaccination series among individuals 6 months and older
  • as a booster dose for those 12 years of age and older

Moderna Spikevax® Bivalent COVID-19 vaccine Original/Omicron B.1.1.529 (BA.1)

  • as a booster for those 18 years of age and older

Moderna Spikevax® Bivalent COVID-19 vaccine Original/Omicron BA.4/5

  • as a booster for those 18 years of age and older

Pfizer-BioNTech Comirnaty® COVID-19 vaccine

  • as primary vaccination series among individuals 6 months and older
  • as a booster dose for those 5 years of age and older and at least 6 months after the last dose of the primary vaccination series

Pfizer-BioNTech Comirnaty® Original and Omicron BA.1, bivalent COVID-19 vaccine

  • as a booster for those 12 years of age and older

Pfizer-BioNTech Comirnaty® Original and Omicron BA.4/BA.5, bivalent COVID-19 vaccine

  • as a booster for those 5 years of age and older

As new evidence is gathered, the official guidelines are changing. Vaccine studies in all populations are ongoing, including against new variants. Eligibility and access criteria still have some variability, depending on where you live in Canada.

Other Types of Vaccines are Available

Other vaccines, which may not be as effective and have a narrower range of uses (e.g., not for use in young children), are available in specific circumstances. They are approved for certain populations in Canada, such as those who are unable to receive mRNA vaccines due to an allergy.

Viral vector vaccines are types of vaccines that use a harmless virus, in many cases the adenovirus, as a delivery system. This viral vector is not the virus that causes COVID-19, and it does not cause any infection, it is simply a carrier for information that causes your body to create antibodies against the COVID-19 spike protein.

The AstraZeneca COVID-19 vaccine (Vaxzevria®), which is approved for the prevention of COVID-19, was used quite often at the beginning of the COVID-19 vaccination period but appears to be less effective than the mRNA vaccines. It requires two doses of injections to fully protect an individual, and may be given as a first dose, with a second dose given as a Pfizer-BioNTech or Moderna mRNA vaccine instead.

Since AstraZeneca’s vaccine is a viral vector vaccine, it requires an extra step of transcribing the genetic material of the virus into mRNA to eventually make similar spike proteins to those that Pfizer-BioNTech and Moderna’s vaccines produce. The efficacy of the AstraZeneca vaccine is less than that of the Pfizer-BioNTech and Moderna vaccines, reported at 74%. Rare side effects of the AstraZeneca vaccine include blood clots, which ultimately influenced the government to offer Pfizer-BioNTech and Moderna vaccines in preference over the AstraZeneca vaccine as a second dose. However, currently, the AstraZeneca vaccine is still available and offered to those who may be allergic to Pfizer-BioNTech and Moderna mRNA vaccines.

Another similar vaccine is the Janssen (Johnson and Johnson) vaccine (Jcovden®). It is also a viral vector vaccine and works similarly to the AstraZeneca vaccine, but it only requires one dose for full protection. It also has reduced efficacy compared to the mRNA vaccines at 60%. Currently, Johnson and Johnson’s vaccine is only available to those who are allergic to mRNA vaccines and protein vaccines (described below).

Eligibility and access criteria to these vaccines have some variability, depending on where you live in Canada.

The last type are protein vaccines, which use pieces or fragments (protein particles) of the virus that causes COVID-19. This could be pieces of the spike protein itself, as the main component of the vaccine. The vaccine also contains another ingredient called adjuvants to help the immune system respond to the virus particles. There are two vaccines approved in Canada in this category, the Novavax (Nuvaxovid®) and Medicago (Covifenz®) vaccines. In general, they are not as efficacious as the Pfizer-BioNTech and Moderna mRNA vaccines, which is why they are usually offered to patients who are allergic to mRNA vaccines or for some other reason cannot take the mRNA vaccines.

The Novavax is approved in those older than 12 years of age, and the Medicago is only approved in those 18-64 years of age. The reported efficacy of Novavax is 90% while that of Medicago is 71%. Medicago is also made using plant-based viral-like particles, which is different from the other vaccines that strive to use actual viral particles to replicate the same effect from the virus itself. Due to the high efficacy of the Novavax vaccine, in certain provinces, patients can elect to receive this injection if they choose not to receive mRNA vaccines.

Health Canada is currently reviewing the available vaccines in different age populations. Vaccines are generally tested against the prevailing infectious variant of COVID-19 and often the efficacy carries over to other variants and subvariants. Ongoing surveillance and testing continue as new variants evolve.

Vaccine Availability by Province and Territory

For province-specific information on COVID-19 vaccines, please see our province outlines on vaccine eligibilities and treatments.

Vaccine Doses and Boosters

Currently, for most of the population, a full vaccination course of two doses of Pfizer-BioNTech and/or Moderna vaccines is adequate protection. However, most healthcare practitioners recommend a third dose (or booster dose) for improved protection.

In general, most healthcare practitioners would recommend that everyone who is eligible should receive a booster (third dose) vaccination for improved protection. Rather than getting a second booster of the same vaccine or a vaccine produced for the same variant, there will be new vaccines available targeting the prevailing variants of COVID-19 and those vaccines would be preferred over a second booster of the initial vaccine formulation.

Vaccine eligibility for third/booster doses are province specific. To find out the vaccine eligibility in your province or territory, please see the information specific to the region where you reside.

The recommendation for the immunocompromised population is a three-dose primary series to address the potential for a diminished immune response (not considered a booster). For the immunocompromised population, the National Advisory Committee on Immunization (NACI) recommends that individuals five years of age and older, who are moderately to severely immunocompromised, have a primary series of three doses of an mRNA COVID-19. This includes extremely high-risk persons, such as those who are severely immunocompromised, living in care homes, or of advanced age. Refer to Vaccination of specific populations, immunocompromised persons for more information.

On January 4, 2021, the Canadian Association of Gastroenterology (CAG) released the following guidelines for the use of mRNA COVID-19 vaccines, namely the Pfizer-BioNTech and the Moderna COVID-19 vaccines, for individuals with inflammatory bowel disease (primarily Crohn’s disease and ulcerative colitis):6

  • strongly recommend the vaccination of persons with IBD not receiving immunosuppressive therapies (i.e., azathioprine, cyclosporine, mercaptopurine/6-MP, and methotrexate sodium)
  • individuals on immunosuppressive drugs may receive a mRNA COVID-19 vaccine after consultations with their treating physician on its safety and appropriateness for their condition, medical history, and an assessment of benefits and risks

It is important to note that an IBD diagnosis does not make an individual immunosuppressed, instead, some IBD treatments might cause immunosuppression or alter the body’s immune response.

COVID-19 Prevention Medication

The best approach to preventing infections from SARS-CoV-2 and other respiratory viruses is to follow your local health advisories when there is a pandemic or endemic. Generally, these include not touching your face unless your hands are clean, washing your hands frequently or using hand sanitizer, wearing a face mask in indoor or large public settings, keeping physical distance from others, and avoiding large crowds. You should be sure that your vaccinations are up to date against COVID-19, influenza, pneumonia, and other vaccine-preventable diseases. Talk with your healthcare provider to learn more about the right vaccines for you.

Cilgavimab and Tixagevimab (Evusheld™)7

This combination is currently the only medication for COVID-19 prevention. It is administered as two intramuscular injections at the same time in separate locations of the body. This medication is for individuals who are not currently infected with COVID-19 who have not had recent contact with an infected person. It is approved for use in those who are immunocompromised and unlikely to mount an adequate immune response to COVID‐19 vaccination or in those for whom COVID-19 vaccination is not medically recommended. This treatment consists of proteins that bind to the spike protein of SARS-CoV-2 and stop it from replicating in the body. Recent studies show that protection lasts at least 6 months after treatment administration. However, it is not expected to have significant protection against newer Omicron variants.

Alberta: AHS no longer recommends Evusheld™ due to lack of efficacy against newer variants. However, it is still available on a case-by-case basis. Please consult your doctor.

British Columbia: On November 23, 2022, BC CDC announced that it does not recommend the use of Evusheld™ for the prevention of COVID-19, including its use among severely immunocompromised individuals.12

Manitoba: https://sharedhealthmb.ca/covid19/treatment/

New Brunswick: N/A

Newfoundland and Labrador: N/A

Northwest Territories: N/A

Nova Scotia: https://policy.nshealth.ca/Site_Published/covid19/document_render.aspx?documentRender.IdType=6&documentRender.GenericField=&documentRender.Id=95489

Nunavut: N/A

Ontario: Effective November 2022, Ontario Health no longer recommends the use of Evusheld™ for the prevention or treatment of COVID-19.13

Prince Edward Island: N/A

Quebec: On December 16, 2022, INESSS issued a notice to not recommend Evusheld™ for the prevention and treatment of COVID-19.14

Saskatchewan: N/A

Yukon: N/A

Be cautious of false claims for prevention strategies and treatments that evidence does not support. Products advertised as treatment for COVID-19 that do not have approval from Health Canada are promoting baseless, and even harmful, claims. For further scientific information, the World Health Organization published Coronavirus disease (COVID-19) advice for the public: MythBusters.

Updated: 2023-01-25
Sources
• Government of Canada. AstraZeneca Vaxzevria COVID-19 vaccine. June 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/astrazeneca.html.
• Government of Canada. Drug and vaccine authorizations for COVID-19: List of applications received. August 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/authorization/applications.html#wb-auto-4.
• Government of Canada. Janssen Jcovden® (Johnson & Johnson) COVID-19 vaccine. August 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/janssen.html.
• Government of Canada. Medicago Covifenz COVID-19 vaccine. June 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/medicago.html.
• Government of Canada. Moderna Spikevax COVID-19 vaccine. July 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/moderna.html.
• Government of Canada. Novavax Nuvaxovid COVID-19 vaccine. June 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/novavax.html.
• Government of Canada. Pfizer-BioNTech Comirnaty COVID-19 vaccine. August 2022. https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/pfizer-biontech.html.
• Government of Canada. Vaccines for COVID-19. August 2022. https://www.canada.ca/en/public-health/services/diseases/coronavirus-disease-covid-19/vaccines.html.
• The Immunisation Advisory Centre. How the AstraZeneca vaccine works and the protection it provides. April 2022. https://covid.immune.org.nz/covid-19-vaccines-nz/astrazeneca-vaccine/how-astrazeneca-vaccine-works-and-protection-it-provides.
References
1. US Centers for Disease Control and Prevention. Understanding mRNA COVID-19 Vaccines page. Available at: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/mrna.html. Accessed 2021-01-04.
2. Polack FP et al. Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine. N Engl J Med 2020; 383:2603-2615, DOI: 10.1056/NEJMoa2034577 Accessed 2021-01-04.
3. Baden LR et al. Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine. N Engl J Med 2020. DOI: 10.1056/NEJMoa2035389. Accessed 2021-01-05.
4. De Serres G, Febriani Y, Ouakki M, Talbot D, Gilca R, Deceuninck G, et al. Efficacité du vaccin contre la COVID-19 causée par le variant Omicron au Québec. INSPQ. 2022 Feb 16. https://www.inspq.qc.ca/covid-19/vaccination/efficacite-omicron.
5. Tseng HF, Ackerson BK, Luo Y, Sy LS, Talarico CA, Tian Y, et al. Effectiveness of mRNA1273 against SARS-CoV-2 Omicron and Delta variants. Nat Med. 2022 May;28(5):1063,1071. doi:10.1038/s41591-022-01753-y.
6. Chemaitelly H, Ayoub HH, AlMukdad S, Coyle P, Tang P, Yassine HM, et al. Duration of mRNA vaccine protection against SARS-CoV-2 Omicron BA.1 and BA.2 subvariants in Qatar. Nat Commun. 2022 Jun 2;13(1):30822.
7. Health Canada. Moderna Spikevax COVID-19 vaccines. Available at: https://www.canada.ca/en/health-canada/services/drugs-health-products/covid19-industry/drugs-vaccines-treatments/vaccines/moderna.html. Accessed 2022-09-20.
8. Steenhuysen J. Moderna says COVID-19 vaccine shows 100% efficacy in teens aged 12-17. Global News. Available at: https://globalnews.ca/news/7890482/moderna-covid-vaccine-teens-study/. Accessed 2022-09-20.
9. https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/guidance-covid-19-vaccine-booster-doses.html
10. Canadian Association of Gastroenterology. COVID-19 Vaccination in Patients with Inflammatory Bowel Disease: Communiqué from the Canadian Association of Gastroenterology page. Available at: https://www.cag-acg.org/images/publications/CAG-COVID-19-Vaccines-in-IBD-Patients.pdf. Accessed 2021-01-04.
11. Health Canada. AstraZeneca Canada Inc. Evusheld™. April 2022. https://covid-vaccine.canada.ca/evusheld/product-details.
12. BC COVID Therapeutics Committee. Clinical Practice Guide for the Use of Tixagevimab/Cilgavimab (Evusheld™) for Pre-exposure Prophylaxis against COVID-19. BC Centre for Disease Control. November 2022. http://www.bccdc.ca/Health-Professionals-Site/Documents/COVID-treatment/ClinicalPracticeGuide_Evusheld.pdf.
13. What you need to know about Evusheld: For people who have received Evusheld in the past. Ontario Health. December 2022. https://www.ontariohealth.ca/sites/ontariohealth/files/2022-05/Patient%20Handout%20-%20Evusheld%20-%20EN.pdf.
14. Tixagévimab/Cilgavimab (Evusheld) en prophylaxie préexposition. Institut national d’excellence en santé et en services sociaux. December 2022. Available at: https://www.inesss.qc.ca/en/covid-19/traitements-specifiques-a-la-covid-19/tixagevimab-/-cilgavimab-evusheld-en-prophylaxie-preexposition.html.
Photo: WiR_Pixs | pixabay.com
This information is vetted by a number of healthcare experts and updated regularly to remain relevant. However, we cannot guarantee that all external links are current. We thank all of the content contributors who helped develop this page.