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Helping You Understand COVID-19 Vaccines

Medcan’s Medcan Advisory Services (MAS) team explores the difference between efficacy and effectiveness and shares new developments in vaccine dose timing, side effects and variants of concern.

A medical hand in a glove holds an ampoule with a vaccine and a syringe with illustration

Our Medical Advisory Services (MAS) team is the foundation of our Safe at Work System, providing evidence-based guidance to develop and implement time-relevant COVID-19 strategies. The following primer, compiled by the MAS team, was designed to educate and empower employees and their families by addressing their most common vaccine-related questions. The information in this primer reflects the evidence as of July 15th, 2021. Please note that the COVID-19 situation and related vaccination guidance is constantly developing. Refer to your municipal, rovincial and federal updates for the most up-to-date information.

Here, Medcan’s chief medical officer, Dr. Peter Nord, provides vaccine guidance, addresses concerns about the AstraZeneca vaccine, and discusses how we arrived here and what to expect in the coming weeks.

Why do we need vaccines?

All vaccines approved by Health Canada are proven to prevent severe illness from COVID-19, resulting in fewer ICU admissions and deaths.

Preventing COVID-19 hospitalizations and deaths will help end this pandemic and vaccination is a massive step forward in this direction. By getting vaccinated, you protect yourself and help your community by reducing the severity of COVID-19 infection.

Vaccine Efficacy and Effectiveness

Much has been reported on COVID-19 vaccine efficacy and effectiveness. But what do they really mean—and what’s the difference between them? In this case, efficacy refers to how well a vaccine works under controlled circumstance, like a clinical trial. Alternatively, effectiveness refers to how well that vaccine works in the real world. To break it down further:

Vaccine Efficacy

  • Refers to the percentage reduction of disease in an immunized group of people compared to the placebo group, which is calculated in clinical trials
  • The comparison of efficacy of one vaccine to another is not possible due to varying trial conditions (i.e. Pfizer and Moderna in the U.S. from August to November 2020, J&J in South Africa and Brazil from October 2020 to January 2021)

Vaccine Effectiveness

  • Refers to how the vaccine works in the real world, where conditions cannot be controlled
  • Captures effects from previous exposure to the virus, immune status of the individual, and receipt of recommended doses
  • Will continue to be evaluated as the COVID-19 immunization program rollout progresses
Covid-19 Vaccine Comparison Pfizer-BioNTech Moderna AstraZeneca-Oxford Johnson & Johnson
Health Canada-Approved? Yes Yes Yes Yes
Number of Doses Two doses Two doses Two doses Single dose
Minimum Spacing Between Doses 21 days 28 days 28 days None
Vaccine Type mRNA mRNA Adenovirus Vector Adenovirus Vectore
Efficacy rate against severe disease 75-100% 100% 100% 85.4%
Disease Prevention in Clinical Trials (Efficacy) 95% efficacy 94% efficacy 76% efficacy 66% efficacy
Disease Prevention in Real-World Studies (Effectiveness) Real world effectiveness of 91% after first dose according to the recent UK study 70 to 80% effective in preventing infection in England, Quebec, British Columbia and Israel 94% effective in preventing hospitalization (after first dose) in the UK 77% effective for prevention of Covid infection. (US study of 2,000 individuals)
Variants of Concern Likely effective against the B.1.1.7 Variant but likely have a reduced effectiveness against the B.1.351 variant Likely effective against the B.1.1.7 Variant but likely have a reduced effectiveness against the B.1.351 variant 10.4% efficacy against South African B.1.351 variant 57% effectiveness in South Africa
Vaccine Efficacy in Kids Confirmed for kids aged 12-15 Confirmed for kids aged 16-18 Study has begun for those aged 6-17 Pediatric studies still in planning
Storage Requirements Subzero Temperature Subzero Temperature Standard Refrigeration Temperature Standard Refrigeration Temperature
Canadian Doses Ordered Up to 76 M 44 M 22 M Up to 38 M

How do vaccines work?

Vaccines teach your immune system how to fight a specific virus by introducing parts of the invader or weakened/killed pathogen in a way that doesn’t make us ill but instead produces antibodies that can fight that virus. This prepares the immune system to recognize and remember how to fight the virus if exposed in the future. Booster vaccinations stimulate the production of more antibodies and longer lasting memory.

  mRNA Vaccines
Messenger ribonucleic acid (mRNA) are genetic instructions for our cells to make a harmless piece of the spike protein found on the surface of the COVID-19 virus. The mRNA is encased in a lipid coat so it can enter our cells, which read the instructions like a recipe to create the spike protein. Our bodies create antibodies that can recognize the spike protein and the mRNA is then broken down.
Known Vaccines: N/A
COVID-19 Vaccines: Pfizer, Moderna, CureVac
  Viral Vector
A modified version of a harmless virus (not COVID-19) that delivers spike protein instructions to the cells. In this case, the instructions are DNA instead of mRNA. The vector virus is a type of adenovirus (common cold virus) that has been modified to carry the instructions and to prevent replication.
Known Vaccines: Zika, HIV, Ebola
COVID-19 Vaccines: AstraZeneca, Johnson & Johnson, Sputnik V
  Protein Subunit
These vaccines introduce the spike protein rather than just delivering instructions to make it.
Known Vaccines: Hepatitis B
COVID-19 Vaccines: Novavax
  Inactivated (attenuated live virus or inactive dead virus)
These vaccines are created by taking the coronavirus and removing its ability to replicate. The spike proteins are left intact, allowing the body to recognize the virus and create antibodies.
Known Vaccines: Inactivated (Influenza), Attenuated live virus (MMR)
COVID-19 Vaccines: Sinopharm, Sinovac Biotech, Covaxin

Side Effects and Contraindications

Common vaccine side effects include a sore arm, fatigue and a mild headache. It seems that these symptoms can be worse after the second injection, however, the sore arm should improve within 48-72 hours. One in 10 people will feel muscle aches, chills or a mild fever following vaccination, which is consistent with what is experienced with a normal flu vaccine. While the rate of severe allergic reactions to the mRNA vaccines has been higher than the normal flu vaccine, it remains exceedingly rare, affecting around one in 100,000 people (or 0.001%) in the Pfizer-BioNTech vaccine, for example. Please note that if you are allergic to PEG (polyethylene glycol) or any PEG derivatives like polysorbates, you should speak with your healthcare provider before taking the vaccine. This potential allergen may be found in bowel preparation products for colonoscopy, laxatives, cough syrup, cosmetics, skin products and some food and drinks.

You can find current allergy information and COVID-19 vaccine information on the Canadian Society of Allergy and Clinical Immunology website.

Myocarditis/Pericarditis and mRNA Vaccination

The Public Health Agency of Canada (PHAC) and Health Canada are closely monitoring the rare correlation of heart conditions like myocarditis and pericarditis after mRNA vaccinations (Pfizer, Moderna). Myocarditis is the inflammation of the heart muscles whereas pericarditis is the inflammation of the outlining of the heart.

With Canadian cases is still being observed, reports from international data suggests myocarditis and pericarditis after vaccination occurs more after second dose, usually is active within a week of vaccination, and is more prevalent in males than females.

Health Canada released on statement a statement strongly recommending all eligible adults receive the mRNA vaccines (Moderna, Pfizer). In Addition, the list of symptoms can be found here.

The MAS team acknowledges the risk of myocarditis and pericarditis after vaccination. While these cases are still being examined, it is still important to follow the recommendations of your local public health unit or medical doctor. The MAS team agrees with Health Canada that the benefits of COVID-19 vaccines continue to outweigh their potential risk.

Immunocompromised, Pregnant and Breastfeeding Women

The vaccine clinical trials did not study the safety nor efficacy in pregnant women nor immunocompromised individuals, which has generated hesitancy and questions.

However, the American Journal of Obstetrics and Gynecology has found that the COVID-19 vaccines do prompt a vigorous immune response in pregnant and breastfeeding women which is passed on to their babies. The Society of Obstetricians and Gynecologists of Canada (SOGC) also released a statement: “Women who are pregnant or breastfeeding should be offered vaccination at any time during pregnancy if they are eligible and no contraindications exist.”

Also, those with an auto-immune disorder should undergo a risk assessment with their primary health care provider or medical specialist to determine that the benefits outweigh the potential risks.

Variants of Concern

Like influenza, COVID-19 is likely to remain endemic, requiring an updated vaccine annually to address mutations and the new variants of concern. Vaccine manufacturers have confirmed that they can adjust the vaccines in real time to target the new variants of concern, with Moderna already applying for regulatory approvals (clinical trials not required).

Vaccine Dose Timing

NACI recently recommended implementing an extended four-month interval strategy—but why?

  • Interruption of vaccine series (a.k.a., extending the interval) does not require restarting the series
  • Clinical trials showed single dose efficacy of 92% up to the second dose (19-42 days) and real-world evidence supports 70-80% effectiveness (for up to 2 months, and continuing to sustain)
  • Experience with multi-dose vaccines after a single dose suggests persistent protection could last for six months or longer in adolescents and adults
  • Vaccine distribution and coverage will be optimized leading to short-term public health benefits in reduced symptomatic disease, hospitalizations and deaths

Cumulative COVID-19 Incidence Rate vs. Days after dose 1

Previous recommended dose separation (21 days) and the new separation recommendation (4 months)

A Post-Vaccine World

The CDC has relaxed guidelines for fully vaccinated individuals, suggesting they do not need to isolate or be tested if exposed to a positive case (unless symptoms develop). As per CDC guidance, vaccinated individuals are able to interact with any number of other vaccinated individuals with unvaccinated family members, however, the Public Health Agency of Canada has yet to issue similar updated guidance.

At the moment, we’re unsure whether vaccinated individuals who contract COVID-19 are still able to transmit the infection to those who are unvaccinated. Current evidence suggests that the viral load and the transmission risk is reduced, but not zero. Overall, vaccinated Individuals must continue to behave, in the community (i.e. work environment), with the same precautions as they did pre-vaccination—wearing masks, physical distancing, and practicing personal hygiene.

These measures will generally remain in place until 75-85% of the population has been fully vaccinated (i.e. received both shots) or has contracted the COVID-19 virus—also known as herd immunity.

To learn more about Medcan’s Medical Advisory Services, send a note to [email protected].

You can refer to our COVID-19 Vaccine Updates for the latest information on vaccine distribution at Medcan.


Center for Disease Control and Prevention (CDC) –

Center for Effective Practice (CEP)-

National Advisory Committee on Immunization (NACI) – 

Health Canada updates Pfizer-BioNTech and Moderna COVID-19 vaccine – labels to include information on myocarditis and pericarditis. (2021, June 30). Retrieved from alert-rappel-avis/hc-sc/2021/75959a-eng.php 

Myocarditis and Pericarditis Following mRNA COVID-19 Vaccination. (n.d.). Retrieved from 

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