Information for Pharmacists: Everything You Need to Know about the COVID Vaccines

Gabrielle Busque

January 12, 2021

A reliable and up to date information source about the COVID vaccine for pharmacists and other healthcare professionals.

Last revised: January 14, 2021

Approval Status of the COVID Vaccines

COVID Vaccine Technologies & Mechanisms:

  • Moderna & Pfizer-BioNTech: mRNA Vaccines 
  • AstraZeneca-Oxford & Janssen: Viral Vector Vaccines 

mRNA vaccines consist of a nucleic acid code (mRNA) for the coronavirus spike protein encased within a lipid coat. Upon vaccination, the lipid coat fuses with the cell membrane of muscle cells at the site of vaccination, allowing the mRNA to enter the cell and to be read by the cell protein machinery. 

With viral vector vaccines, the gene (double stranded DNA) for the coronavirus spike protein is inserted into a viral vector. This viral vector is a safe virus that can infect humans and deliver the gene for the coronavirus spike protein into human cells without replicating and causing disease. The viral vector carries the DNA for the coronavirus spike protein into muscle cells of the vaccinated arm and into the nucleus. The viral DNA is then transcribed to mRNA, which goes to the cytoplasm where it will be read by the cell protein machinery. 

In both cases, vaccinated cells are then able to make a piece of the coronavirus spike protein and show it to the immune system by displaying it on their surface. The immune system begins activating processes to fight off what it believes is an infection, including producing antibodies. Since these antibodies are specific for the novel coronavirus spike protein, the immune system will know how to protect the individual against the virus if they become infected later on.


Two-Dose Vaccines: Prime & Boost

The Pfizer-BioNTech, Moderna, and AstraZeneca-Oxford vaccines all currently require two doses. These dosing regimens were chosen because they were found to produce the best immune response against COVID-19 in early phase studies.

Two-dose vaccines prime and boost the immune system. The first dose primes the immune system to recognize the virus, and the second dose boosts the immune response against the virus. Therefore, the first dose provides some protection, but the second dose provides the highest, most durable immunity against COVID-19 shown in the vaccine studies.  

It is not uncommon for vaccines to require more than 1 dose. Recall that several childhood immunizations require two or more doses to be effective. In addition, when a child gets their first influenza vaccine, or flu shot, they need two doses to ensure that they are protected since their immune system has not seen the influenza virus before. 

Dosing Intervals

In the vaccine trials, the majority of the Pfizer-BioNTech COVID-19 vaccine trial participants received their second dose 21 to 27 days apart; the majority of the Moderna COVID-19 vaccine trial participants received their second dose 21 to 42 days apart.

Minimum Intervals are Critical

It is highly important to adhere to the minimum vaccine interval. Administering the first and second doses of the vaccine too close together does not allow sufficient time for the body to produce an immune response. 

In cases where the second dose of a vaccine is administered sooner than the minimum interval, the efficacy of the second dose is uncertain. The second dose is discounted, and a third dose is administered after at least the minimum interval has passed after the dose given too early. 

Maximum Intervals Don’t Exist (Not Really, Anyway)

In general, extending the interval between doses does not affect the efficacy of the vaccine in producing an immune response. Therefore, a vaccine series does not need to be restarted as a result of delay between doses.

With multi dose vaccines, the greatest short-term protection is achieved with the first dose while additional doses intend to extend protection over the long term. The real problem of extending the vaccine interval is that, until the series is complete, the individual remains at risk for the preventable disease

The National Advisory Committee on Immunization (NACI) recommends following the two-dose vaccine schedule of the COVID-19 vaccines, at the specified interval.

These recommendations reflect the uncertainty for the duration of immunity against COVID-19 conferred by the vaccine, due to new mRNA vaccine technology and short follow-up time in the COVID-19 vaccine trials. However, NACI suggests that an alternate 28 day interval may be more feasible to implement for the Pfizer-BioNTech vaccine.


Efficacy of the Vaccines


Side Effects

Most common: injection site pain, fatigue, headache, muscle pain, joint pain, chills, fever, nausea

→ usually mild to moderate in nature

→ more common & intense after the 2nd dose

→ all symptoms of the immune system kicking into gear and will resolve within a few days. If necessary, can be managed with acetaminophen 


Storage Requirements


Illustrations by: Gabrielle Busque


Gabrielle Busque

Gabrielle is a 4th year pharmacy student at the University of Toronto. At MedMe, Gabrielle is heavily involved in drug/medical information and design. She is passionate about optimizing the delivery of clinical services in pharmacies to advance patient care.

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