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#280 COVID Vax Fast Facts: Say That 10 Times Fast

What are the benefits and risks of the three COVID-19 vaccines likely available soon in Canada?

Interim results of two large randomized, placebocontrolled trials (RCTs) demonstrate ~95% relative efficacy in preventing COVID-19 (Pfizer, Moderna). The AstraZeneca/Oxford vaccine has ~70% relative efficacy. Absolute benefits will vary with baseline risk and time but if annual risk of developing COVID-19 is 20%, then vaccine would decrease risk to 1% (6% with AstraZeneca/Oxford). These vaccines appear safe and may decrease the likelihood of severe COVID-19. Ongoing studies should provide further details.

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Interim results from FDA submissions,1,2 or peer reviewed publications.3,4 Median follow-up 2 months.2,3,4 Cases were symptomatic and had laboratory confirmed COVID-19.5-7 Severe COVID-19 definitions included needing high-flow oxygen or ICU admission.5-7 Pfizer/BioNTech: double-blind, multi-country RCT.1,3,5 Two doses given 21 days apart to 40,137 adults (median age 51):1,3
  • COVID-19 cases: vaccine 9, placebo 169. Relative risk reduction (RRR): 95% (statistically different).
    • Severe COVID-19:3 vaccine 1, placebo 4.
  • Adverse events:
    • Unsolicited patient reporting:1 injection pain (11%), fatigue (6%), myalgia/ headache (5%).
    • Solicited reporting (patients asked about specific adverse events): 5-10 times more common.
      • Example fatigue:1,3 vaccine 34-59%, placebo 17-33%.
    • Serious adverse events (~0.5%) and deaths similar between groups.1,3
Moderna: double-blind RCT.2,6 Two doses given 28 days apart to 27,817 USA adults (median age 51).2
  • COVID-19 cases: vaccine 11, placebo 185. RRR: 94% (statistically different).
    • Severe COVID-19: vaccine 0, placebo 30.
  • Adverse events:2
    • Unsolicited patient reporting: headache (3%), fatigue (2%), lymphadenopathy (1.2%), myalgia (1%).
    • Solicited reporting: ~5-20 times more common.
      • Example headache: vaccine 25-63%, placebo 18-29%.
    • Serious adverse events (0.6%) and deaths similar between groups.
AstraZeneca/Oxford: multiple single-blind RCTs with multiple arms [including variable first dose and timing (4 to >12 weeks) of second dose]. Two doses given to 11,636 Brazil or United Kingdom adults.4,7
  • COVID-19 overall: vaccine 30, placebo 101. RRR=70% (statistically different).
    • Low dose regimen RRR=90%, standard dose RRR=62%.
      • Low dose given to only 18-55 year-olds, ~90% health care workers.
    • Severe COVID-19:4 vaccine 0, placebo 2.
  • Serious adverse events:4 vaccine 0.7%, placebo 0.8%.
    • 3 cases of transverse myelitis (2 vaccine, 1 placebo): deemed unrelated to vaccine.4
    • Overall mortality similar.4
Limitations: efficacy in children and duration of response unknown. Context:
  • Storage requirements:4,8,9 Pfizer (-70°C), Moderna (-20°C), AstraZeneca/Oxford (2- 8°C).
  • Baseline risk of COVID-19 varies substantially with location and time, impacting potential absolute benefit.
    • For example, if annual risk of COVID-19 is 20%:
      • Pfizer/Moderna vaccine would decrease risk to 1%.
      • AstraZeneca/Oxford to 6%.

Olukayode Fawole April 5, 2021

Very educative

Gilbert Bretecher June 6, 2021

Pfier/Moderna more effective than Astra-Zeneca

Jason Price September 12, 2021


Martin Withers January 1, 2022

Good review

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  • Michael R Kolber MD CCFP MSc
  • Paul Fritsch MD CCFP
  • Morgan Price MD PhD CCFP
  • Alexander Singer MB BAO BCh CCFP
  • Jennifer Young MD CCFP
  • Nicolas Dugré PharmD MSc BCPAC
  • Sarah Bradley MD CCFP
  • Tony Nickonchuk BSc Pharm

1. Pfizer-Biontech, Vaccines and Related Biological Products Advisory Committee Meeting Dec 10, 2020. FDA Briefing Document. Available at: Accessed Dec 7, 2020.

2. Moderna TX Inc. Vaccines and Related Biological Products Advisory Committee Meeting December 17, 2020. FDA Briefing Document. Available at: Accessed Dec 15, 2020.

3. Polack FP, Thomas SJ, Kitchin N, et al for the C4591001 Clinical Trial Group. New Engl J Med. Published online Dec 10, 2020. DOI: 10.1056/NEJMoa2034577

4. Voysey M, Costa Clemens SA, et al. Lancet. Published online Dec 8, 2020.

5. Pfizer. A Phase 1/2/3, placebo-controlled, randomized, observer-blind, dose-finding study to evaluate the safety, tolerability, immunogenicity, and efficacy of sars-cov-2 rna vaccine candidates against covid-19 in healthy individuals. PF-07302048 (BNT162 RNA-Based COVID-19 Vaccines) Protocol C4591001. Available at: Accessed Nov 25, 2020.

6. ModernaTx. Protocol mRNA-1273-P301, Amendment 3. 20 Aug 2020. Available at: Accessed Nov 30, 2020.

7. AstraZeneca. Clinical Study Protocol - Amendment 2. AZD122 - D8110C00001. Sept 17, 2020. Available at: 8c32-cece903592b9/D8110C00001_CSP-v2.pdf. Accessed Dec 1, 2020.

8. Pfizer US vaccine distribution fact sheet. Available at: Accessed November 25, 2020.

9. Moderna vaccine shelf-life announcement. Available at: Accessed Nov 25, 2020.

Authors do not have any conflicts of interest to declare.

Les auteurs n’ont aucun conflit d’intérêts à déclarer.