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#292 COVID-19 Vaccine Adverse Events

What does real world evidence tell us about adverse effects from available COVID-19 vaccines?

Best evidence finds systemic adverse events related to COVID-19 vaccines are mostly short-lived and differ by type of vaccine, dose given, age, and gender. Anaphylaxis to mRNA vaccines occurs in 2.5 to 5 per million doses. Vaccine-induced thrombotic thrombocytopenia (VITT) is associated with viral vector vaccines (AstraZeneca or Johnson and Johnson). Best estimates of incidence: ~0.5 to 2 per 100,000 and more commonly in women.

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  • From published prospective or retrospective cohort/case control studies.
  • General systemic adverse events:
    • Best evidence from solicited adverse events up to 8 days post-dose in 655,590 patients with Pfizer (2 doses) or AstraZeneca (1 dose) adults:1
      • Most commonly short lived and last 1-2 days:
        • Fatigue, headache: Each 13-23%.
        • Adverse events more common among:
          • Recipients of AstraZeneca (versus Pfizer), Pfizer second dose versus first.
          • Individuals 55 or younger.
          • Women.
        • Similar findings in clinical trials.2
      • Anaphylaxis risk: Based on ~10 million Pfizer doses, ~8 million Moderna.3
        • Incidence (per million doses): 5 (Pfizer), 2.5 (Moderna).
          • 1/3 reported previous anaphylaxis episodes, ~80% with known allergies.
          • No deaths reported.
  • VITT:
    • VITT leading to rare thromboses, particularly cerebral venous sinus thrombosis (CVST), 4-28 days post AstraZeneca vaccine.
      • Risk varies4,5 from 0.4 per 100,000 in 80+ years to ~2 per 100,000 in 20-49.
      • Majority in women under 60 within 14 days of vaccination.5
    • Cohort of 281,264 AstraZeneca recipients in Denmark/Norway6 calculated thrombosis events in excess of natural background risk:
      • Additional venous thromboembolisms (includes deep venous thrombosis, pulmonary embolism and CVST) possibly attributed to vaccine: 11 per 100,000 doses and mostly in women.
  • Suspect VITT if AstraZeneca or Johnson & Johnson vaccine 4-28 days prior, signs/symptoms or positive imaging of serious thrombosis with thrombocytopenia.7
    • Diagnosis confirmed with positive heparin-induced thrombocytopenia assay.4,7
    • Clinical pathways available to guide approach to suspected VITT8:
  • Patient decision tool for AstraZeneca vaccine available:9
  • To report possible vaccine related adverse events in Canada:10

David Wood June 14, 2021

good information

Steven Hobson July 5, 2021

excellent information

Mona Reck July 30, 2021


Olukayode Fawole August 29, 2021

Very relevant to my practice

Olukayode Fawole August 29, 2021

Very releavnt to my practice

Olukayode Fawole August 29, 2021

Very relevant to my practice

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  • Tony Nickonchuk BScPharm
  • Paul Fritsch MD CCFP
  • Jennifer Young MD CCFP
  • Nicolas Dugré PharmD MSc BCPAC
  • Alex Singer MB BCh BAO CCFP
  • Michael R Kolber MD CCFP MSc

1. Menni C, Klaser K, May A, et al. Lancet Infect Dis. 2021 Apr 27; S1473-3099(21)00224-3. Online ahead of print.

2. Kolber MR, Fritch P, Price M, et al. Can Fam Physician. 2021; 67:185.

3. Shimabukuro TT, Cole M, Su JR. JAMA 2021; 325(11):1101-1102.

4. Cines DB, Bussel JB. N Engl J Med. 2021 Apr 16; NEJMe2106315. Online ahead of print.

5. European Medicines Agency Committee for Medicinal Products for Human Use. Vol 4.; 2021. Available at: Accessed May 15, 2021.

6. Pottegård A, Lund LC, Karlstad Ø, et al. BMJ 2021; 373:n1114.

7. American Society of Hematology. Thrombosis with Thrombocytopenia Syndrome Available at: Accessed May 20, 2021.

8. British Columbia COVID-19 Therapeutics Committee. Vaccine-Induced Immune Thrombotic Thrombocytopenia Guidance. Vol 4.; 2021. Available at: Accessed May 25, 2021.

9. European Medicines Agency Committee for Medicinal Products for Human Use. 2021; 4(23):1-12. Available at: Accessed May 25, 2021.

10. Health Canada. Contact information for Adverse Events Following Immunization (AEFI). Available at: Accessed June 1, 2021.

Authors do not have any conflicts of interest to declare.

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