Oxford-AstraZeneca COVID-19 Vaccine

(ChAdOx1 nCoV-19 vaccine/AZD1222/Covishield/Vaxzevria)

Oxford-AstraZeneca COVID-19 Vaccine

(AZD1222, Covishield, Vaxzevria)
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Oxford-AstraZeneca Vaccine Summary

All information in this page has been gathered from Oxford-AstraZeneca's phase 3 clinical trial, unless otherwise referenced.


Vaccine Effectiveness

  • 100% effective for prevention of severe COVID-19 illness and death
  • 76% effectiveness in preventing symptomatic COVID-19

Vaccine Safety

  • Most common side effects
    • Injection site: Mild-to-moderate injection site pain
    • General: Fever, fatigue, headache
  • Likelihood of serious side effects were the same as placebo
    • Vaccine group: 0.6%
    • Placebo group: 0.5%
  • Individuals who are allergic to vaccine ingredients (see below) should avoid the Oxford-AstraZeneca vaccine

Vaccine Administration

  • 2 intramuscular injections, 4+ weeks apart

General Oxford-AstraZeneca Vaccine Information


What are other names for this vaccine?

COVID-19 vaccines are commonly identified by the name of their manufacturer. In this case that is AstraZeneca. You also may see it referred to as the Oxford-AstraZeneca vaccine because it was initially developed by Oxford and AstraZeneca. The scientific name for this vaccine is AZD1222 and it goes by brand names Covishield or Vaxzevria. It is also called the ChAdOx1 nCoV-19 vaccine based on the viral vector that this vaccine uses.

How does this vaccine work?

AstraZeneca’s COVID-19 vaccine is a viral vector vaccine. This means that it uses a safe virus to deliver instructions to your cells to allow them to create a spike protein, a harmless part of the coronavirus that causes COVID-19. This trains your immune system to recognize and attack the spike protein so that if you are exposed to the real coronavirus, your immune system will react much more quickly and effectively to prevent you from developing COVID-19 (especially severe COVID-19).

For an in-depth explanation of how the AstraZeneca vaccine works, see this article.

What are the ingredients?

Ingredient Purpose
Non-infectious adenovirus Active component of vaccine that trains your immune system to defend against COVID-19
Salt/buffer Keeps the vaccine at proper pH and makes the vaccine injection more comfortable to receive
Surfactant Prevents the virus vaccine from sticking to the glass vial during storage
Amino acids/alcohol Protects the vaccine from degrading
Sugar Protects the vaccine during freezing
Water Makes the vaccine an injectable fluid

Notably, Pfizer-BioNTech’s vaccine does NOT contain preservatives, aluminum, or mercury.

How many doses are required?

AstraZeneca’s COVID-19 vaccine was studied as two intramuscular injections. In the initial clinical trial, the interval between doses ranged from 4 weeks to more than 12 weeks.

However, Canada’s National Advisory Committee on Immunization (NACI) (an expert advisory body to the Public Health Agency of Canada [PHAC]) has advised that second doses of COVID-19 vaccines can be administered up to 4 months after the first dose. This NACI recommendation is based on data that showed a single dose of COVID-19 vaccines were still effective in preventing COVID-19, and to allow more people to receive their first dose during the current vaccine shortage (NACI summary).

What temperature does it need to be stored at?

  • AstraZeneca’s COVID-19 vaccine should be stored under refrigeration between 2°C to 8°C.
  • Punctured vials can be stored for up to 48 hours if kept under refrigeration

In what types of people has this vaccine been studied?

The AstraZeneca phase 3 vaccine trial was a combination of smaller studies that all had different inclusion criteria. In general adults 18 years of age or older were included if they had not had COVID-19 before. People who were sick, immunosuppressed, or had an unstable pre-existing medical condition were generally not included in these initial trials but more data are being gathered as countries around the world vaccinate people with these characteristics.

Are there any groups of people that this vaccine is NOT recommended for?

The AstraZeneca vaccine should not be given to people who are allergic to the active ingredient or to any ingredients in the formulation (see above for the Moderna vaccine ingredient list). Various countries have also restricted the use of the AstraZeneca vaccine in younger adults due to the small risk of a rare blood clotting disorder.

Is the Oxford-AstraZeneca vaccine safe?


What are the side effects and the frequency that they occur?

The most common side effect of the AstraZeneca vaccine is pain/tenderness at the site of injection and general fatigue. Other less common side effects include muscle pain, headaches or chills/feverishness. Serious side effects can occur, but these are very rare and in the phase 3 clinical trial there was no difference between the rates of severe reactions between those that received the AstraZeneca vaccine and those that did not.

There is a link between the AstraZeneca vaccine and a rare form of blood clot called Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT). VITT seems to be occurring in somewhere between 1 in every 125,000 to 1 in 1 million people vaccinated with the AstraZeneca vaccine, so the risk of VITT remains very low.

Will this vaccine make me “sick”? 

No. The side effects that are experienced after receiving any vaccine, including AstraZeneca;s COVID-19 vaccine, are caused by the body’s immune system responding to the vaccine to prepare to protect the body from the virus.

How do I treat side effects of the vaccine?

To treat fever/pain, you can take tylenol (acetaminophen). Rest, adequate diet and fluid intake will help as well.

What are the long-term effects of the vaccine?

Since this vaccine is so new, it’s impossible to be entirely sure what the long-term effects may be. What we do know is that millions of people have been vaccinated with viral vector vaccines similar to AstraZeneca’s vaccine and we have no indication that there are any long-term negative health impacts. Governments around the world have reporting systems in place to identify negative health events following vaccines (i.e. the Canadian Adverse Events Following Immunization Surveillance System [(CAEFISS]) and have not identified any long term negative side effects yet.

Despite how new these vaccines are, it is important to place the potential vaccine risks within the context of the current pandemic. The known negative health and population impacts of COVID-19 are much greater than the potential negative health impacts of the Moderna vaccine.

Is the Oxford-AstraZeneca vaccine effective for preventing COVID-19?


What does vaccine “efficacy” mean and how was it measured?

Efficacy in relation to COVID-19 vaccines is the measure of how effective the vaccine is in preventing COVID-19 infection compared to a placebo. Since this is hard to calculate in the real world, the efficacy numbers that we know are what is found in phase 3 clinical trials.

How effective is the Oxford-AstraZeneca vaccine? (What is the vaccine’s efficacy?)

In the early results of the AstraZeneca phase 3 clinical trial from the UK, Brazil and South Africa, the AstraZeneca vaccine was 62.1% effective in preventing symptomatic COVID-19 compared to the control group that did not receive the vaccine. However, in results from a trial in the US, AstraZeneca has said that their vaccine has a 76% efficacy, although the supporting data has not yet been made public.

In all the studies (UK, Brazil and South Africa, and the US) the AstraZeneca vaccine has 100% efficacy in preventing severe COVID-19. Starting 14 days after the second dose, there were no participants that received the AstraZeneca vaccine and then had severe COVID-19 infection or required hospitalization. This indicates that the AstraZeneca vaccine is very effective in protecting people from severe COVID-19 disease.

How long after injection before this vaccine starts working?

Once the AstraZeneca vaccine is injected, your body takes time to react and develop an immune defense. The exact amount of time required for someone to be protected from developing COVID-19 is not known, however, the trial did show that there was some protection starting 22 days after the first dose. Full vaccine efficacy in the phase 3 clinical trial is measured 14 days after the second dose.

Will the vaccine prevent asymptomatic COVID-19?

In the phase 3 trial, there were a total of 69 participants who had asymptomatic COVID-19 as discovered by weekly COVID-19 tests. This included 29/3288 (0.9%) of the vaccine group participants and 40/3350 (1.2%) of the group that did not receive the COVID-19 vaccine (see Table 2 in the phase 3 trial). However, these numbers were not large enough to reach “statistical significance”, which means the difference between groups may have been due to random chance. We will have to wait for the study to include more data from more people to draw more conclusions.

Can I still transmit COVID-19 after being vaccinated?

There is not enough data available to answer this question.

Does the Oxford-AstraZeneca vaccine work against new COVID-19 variants of concern?

The most common COVID-19 variant in Canada and the US is the B.1.1.7 (UK) variant. The AstraZeneca vaccine is likely effective against this variant, but is thought to be less effective against P.1.351 (South Africa) variant. There have not been any studies examining the AstraZeneca vaccine’s efficacy against the P.1 (Brazil) variant.