Efficacy & Safety of the COVID Vaccines: A Closer Look

Gabrielle Busque

January 14, 2021

Detailed efficacy & safety data for the Pfizer-BioNTech, Moderna, and AstraZeneca-Oxford vaccines.


Pfizer-BioNTech Vaccine

  • First dose efficacy of 85% (95% CI 66-93%) in preventing COVID-19 within 11-21 days after vaccination
  • Second dose efficacy 95% (CI 90-97%) 7 days after vaccination

Moderna Vaccine

  • First dose efficacy of 94% (95% CI 76-99%) starting 14 days after vaccination
  • Second dose efficacy 94.1% 2 weeks after vaccination
  • May be slightly less effective (86.4%; 95% CI 61.4%-95.5%) in older adults 65+

AstraZeneca-Oxford Vaccine

  • First dose efficacy of 64.1% (95% CI 50.5-73.9%) against symptomatic COVID-19 21 days or more after vaccination with one standard dose
  • Second dose efficacy of 70.4% (95% CI 54.8-80.6%) across two dosing regimens (low dose [LD]+ standard dose [SD]; SD + SD)
  • Second dose efficacy of 90% (95% CI 67.4-97%) in LD + SD group
  • Second dose efficacy of 62.1% (95% CI 41-75.7%) in SD + SD group


Side Effects


Injection site pain (84.1%), fatigue (62.9%), headache (55.1%), muscle pain (38.3%), chills (31.9%), joint pain (23.6%) and fever (14.2%) - all usually mild or moderate in intensity


Injection site pain (92.0%), fatigue (70.0%), headache (64.7%), myalgia (61.5%), arthralgia (46.4%), chills (45.4%), nausea/vomiting (23.0%), axillary swelling/tenderness (19.8%), fever (15.5%), swelling at the injection site (14.7%), and erythema at the injection site (10.0%)


  • Very common (>10%): tenderness, pain, warmth, redness, itching, swelling or bruising where the injection is given, generally feeling unwell, fatigue, chills or feeling feverish, headache, nausea, joint pain or muscle aches
  • Common (10%): a lump at the injection site, fever, vomiting, flu-like symptoms (e.g. high temperature, sore throat, runny nose, cough and chills)
  • Uncommon (1%): feeling dizzy, decreased appetite, abdominal pain, enlarged lymph nodes, excessive sweating, itchy skin, or rash.
  • In clinical trials there were very rare reports of events associated with inflammation of the nervous system, which may cause numbness, pins and needles, and/or loss of feeling. However, it is not confirmed whether these events were due to the vaccine.



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.