1. How COVID-19 Vaccines Work
    1. mRNA vaccines
    2. AstraZeneca
  2. COVID-19 Vaccine Development and Safety
    1. mRNA
    2. AstraZeneca
    3. Real-world considerations: comparing the efficacy of vaccines
  3. The Benefits of Vaccination Against COVID-19
  4. The Risks of Not Being Vaccinated Against COVID-19
  5. Possible Side Effects of COVID-19 Vaccination
    1. mRNA
    2. AstraZeneca
    3. Allergies
  6. Additional Information
  7. e-Learning Module
  8. Still Have Questions?

The information below is used from the COVID-19 Vaccination in Canada: an educational series for primary care professionals developed by the University of Toronto.

Watch the ask the experts video series provided by the Government of Canada.

How COVID-19 Vaccines Work

mRNA vaccines

A COVID-19 mRNA vaccine contains the genetic material to make the “spike protein” which is found on the COVID-19 virus. This spike protein itself isolated from the virus cannot cause disease. Our immune system sees the spike protein, recognizes it as foreign and builds antibodies to it. After the body is able to recognize and attack the spike protein, it will then attack the covid virus if someone were to come in contact with it.

The vaccine does not stay in your body and does not change your own body in any way. After the protein is made, the cell breaks down the mRNA. The immune system memory lasts long after the vaccine products are gone.


  • Viral vector vaccines use a weakened non-replicating version of a live “common cold” virus that has been modified to carry the genetic material needed to make the COVID-19 spike protein. This is a well-established approach.
  • The particular virus used does not replicate or cause illness in humans – it just acts as a “vector” to carry the vaccine’s instructions. The viral vector vaccines cannot cause COVID-19 or change DNA.
  • Once the virus goes inside a human cell, the cell uses the instructions it carries to make the COVID-19 spike protein.
  • The immune system sees the COVID-19 spike protein, knows that it is foreign, and produces an inflammatory response that will destroy that infected cell.
  • The immune response leads to “immune memory” to recognize the spike protein, which provides immunity against COVID-19 in the future.

One of the major advantages of the viral vector vaccines compared to mRNA vaccines is that they can be transported and stored at regular fridge temperatures. This characteristic allows for them to more easily be distributed and administered by small centres including primary care offices and community pharmacies and in rural communities.

Like the mRNA vaccines, the viral vector vaccines do not contain eggs, animal products, nuts, gelatin, latex, or preservatives.

COVID-19 Vaccine Development and Safety

The COVID-19 vaccines have gone from development to approval in record time. But rest assured that the vaccines have been held to the same safety and effectiveness standards as other vaccines.

All vaccines are tested in Phase 1, 2, and 3 trials. Phase 3 trials include testing on thousands of volunteers. Once Phase 3 trials are completed, the manufacturer submits an application to Health Canada for review. A team of experts at Health Canada then conducts a thorough and independent review of all vaccine data. Health Canada only approves a vaccine if it is confident that it is safe and effective.

There are many reasons why things went quickly including strong international collaboration, increased global funding, experience with the technology, large numbers of volunteers, and regulatory reviews being conducted in tandem with the research.


In December 2020, after careful review, Health Canada approved two mRNA vaccines for COVID-19, one from Pfizer-BioNTech and the other from Moderna. Both vaccines are approved for use in adults and require two doses. Both have specific cold storage requirements and limited viability at room temperature.

Both vaccines contain polyethylene glycol (PEG). Neither vaccine contains eggs, gelatin, latex or preservatives.

Both vaccines were evaluated in randomized controlled trials that included upwards of 30,000 participants. Trials included people from diverse racial and ethnic backgrounds and individuals with some types of chronic conditions. However, trials excluded children, pregnant and breastfeeding individuals, those who were immunocompromised, and those with a previous allergy to a vaccine. Both vaccines were found to have ~95% efficacy after the second dose.

On June 17, NACI released updated guidance allowing a different mRNA vaccine to be used to complete a vaccine series: “individuals who received a first dose of an mRNA vaccine (Pfizer-BioNTech, Moderna) should be offered the same mRNA product for their second dose. If the same product is not readily available, or the product used for the first dose is unknown, another mRNA vaccine is considered interchangeable and should be used to complete the series.”


At the time the AstraZeneca vaccine was approved in Canada, we had information on safety and efficacy/effectiveness both from trial data and also real-world use, for example, from countries like the UK where the vaccine was first administered in early January 2021.

Interim results from four randomized controlled trials of the AstraZeneca vaccine were published in the Lancet in December 2020. There were 11,636 participants included in the interim primary efficacy analysis.

An additional trial with 32,449 participants reported in March 2021. Approximately 20% of participants were 65 years and over, and approximately 60% had co-morbidities associated with an increased risk for progression of severe COVID-19, such as diabetes, severe obesity or cardiac disease. This trial showed 76% efficacy for preventing symptomatic COVID-19 and 100% efficacy for hospitalizations.

The trials suggest that the AstraZeneca vaccine is equally effective to mRNA vaccines at preventing severe disease and death from COVID-19.

  • Among all trial participants who received the vaccine, there were no cases of hospitalization after 21 days of receiving the vaccine (100% efficacy in the trials for hospitalization from COVID-19)
  • Real world data from Scotland suggests that even a single dose of the AstraZeneca vaccine is highly effective at preventing hospital admissions, similar to the Pfizer-BioNTech vaccine

Real-world considerations: comparing the efficacy of vaccines

Although the efficacy of the viral vector vaccines for preventing any symptomatic case appears lower than the results from the mRNA studies, there are four points to keep in mind:

  • Apples and oranges: the mRNA studies were done in a different context, generally prior to the emergence of variants of concern
  • Focus on what matters most: all the COVID-19 vaccines are highly effective at preventing hospitalizations
  • Availability: when COVID-19 case counts are high, there is real risk and questionable benefit with waiting an indeterminant amount of time to access a vaccine that might be more effective at preventing mild disease
  • Transmission: it is very likely that all the vaccines help with this, meaning all can play a role in helping us achieve herd immunity

The Benefits of Vaccination Against COVID-19

In addition to collective benefits of the vaccine, there are also benefits to the individual being vaccinated. The approved vaccines are all effective at preventing an individual from acquiring COVID-19 and the related morbidity and mortality. Early evidence also suggests that in the uncommon circumstance where a vaccinated individual did get the virus, the vaccine likely reduces their chance of getting seriously ill from COVID-19. It may also reduce the chances of passing an infection to a loved one.

The vaccine also confers benefit for people who have already been infected from COVID-19. Infection with COVID-19 may offer some natural immunity, but current evidence suggests that reinfection is possible and experts do not know for sure how long protection from natural immunity might last.

Ensuring widespread uptake of the COVID19 vaccine is a crucial step in ending the pandemic. The faster we do so, the more COVID-19 deaths and illnesses can be prevented and the sooner we can move toward economic, educational, and societal recovery.

The Risks of Not Being Vaccinated Against COVID-19

If you are not vaccinated against covid you have a much higher chance of catching covid. Statistics are currently showing that unvaccinated people have a higher rate of covid cases, hospitalizations, and deaths when compared to those that are fully vaccinated. You are also much more likely to develop long term health complications associated with covid such as lung damage and memory problems.

Possible Side Effects of COVID-19 Vaccination


The most common solicited side effects were pain at the site of injection, fatigue, and headache. Other side effects included swelling and redness at the site of injection as well as fever, chills, muscle and joint pain.

Side effects were more commonly reported after the second dose and there was a lower reported rate among those >55 years (Pfizer BioNTech) and >65 (Moderna). No cases of anaphylaxis were reported in the trials.

With other vaccines, severe reactions usually occur within days or weeks after administrations. Long-term side effects are quite rare. Participants in the COVID-19 mRNA vaccine trials were observed for 8 weeks which was thought to be long enough to identify severe reactions. In the coming months, scientists will continue to monitor for extremely rare events and if these occur, they will carefully examine them to determine if they are due to the vaccine or note.

Remember, as we vaccinate millions of people, there will be bad events that coincide with vaccine administration due to chance alone. Scientists will need to evaluate these events carefully to understand if they are truly a result of the vaccine versus another cause.


Tenderness, pain and warmth are common local side effects following injection while fatigue, headache, muscle pain and malaise are common systemic effects.

Issues related to long-term safety and immunity are similar to mRNA vaccines and recipients of the vaccine should continue to adhere to all public health measures. It is not until 3-4 weeks after inoculation that these vaccines generate effective protection.

As with mRNA vaccines, there is limited data on the efficacy against Variants of Concerns (VoCs). At this time, NACI recommends that a vaccine series be completed with the same COVID-19 vaccine product. Visit the module “Emerging Topics” for the latest information on transmission and VoCs.

Is the AstraZeneca vaccine associated with blood clots?

In early March 2021, the adverse event reporting system led many European countries to pause administration of AstraZeneca because of concerns over a small number of thromboembolic events. Through careful review of reported cases, we now know more about these events and their possible relationship to the vaccine.

On March 18, 2021, the European Medicines Agency issued a statement saying:

  • The AstraZeneca vaccine is not associated with an increase in the overall risk of blood clots (thromboembolic events) in those who receive it.
  • However, the vaccine may be associated with very rare cases of blood clots associated with thrombocytopenia including rare cases of clots in the vessels draining blood from the brain (i.e. Cerebral Venous Sinus Thrombosis or CVST). The Ontario Science Table has called this adverse event Vaccine-Induced Thrombotic Thrombocytopenia (VITT)



As of January 18, 2021, the US Centres for Disease Control estimates the rate of anaphylaxis is about 5 per million for the Pfizer vaccine and 2.5 per million for the Moderna vaccine.

Although this is more than the typical 1 per million for most vaccines, the anaphylaxis risk is still extremely low and much lower than the risk of anaphylaxis for penicillin which is estimated to be 1 in 5000. Experts speculate that anaphylactic incidents may be linked to polyethylene glycol (PEG). Prior anaphylaxis in response to things other than the COVID-19 vaccine or its components or container is NOT a contraindication — out of an abundance of caution, such people may be monitored for 30 rather than 15 minutes after vaccine administration.


The AstraZeneca and Janssen vaccines are contraindicated for anyone with a history of anaphylaxis to a previous dose of an adenovirus-based vaccine, or to any of its components or its container. Health Canada notes that individuals who have experienced cerebral sinus vein thrombosis (CVST) with thrombocytopenia or heparin-induced thrombocytopenia (HIT) may be at increased risk of VITT. Other contraindications are the same as with the mRNA vaccines.

The potential allergic ingredient in the AstraZeneca and Janssen vaccines is polysorbate 80 which is found in cosmetics and many medical preparations such as vitamin oils, tablets and anticancer agents. Neither the AstraZeneca vaccine or the Janssen vaccine includes polyethylene glycol.

Additional Information

e-Learning Module

Watch the Sunnybrook Health Sciences Centre e-learning module.

Still Have Questions?

If you still have questions regarding COVID-19 vaccines and being vaccinated, contact Health and Counselling Services to arrange for a virtual appointment with a doctor.