An Overview of the AstraZeneca-Oxford COVID-19 Vaccine

While some vaccine makers are using new approaches to fight COVID-19, others are turning to more established methods to end the pandemic.

AstraZeneca and Oxford University have been working on an adenovirus-based vaccine, officially named ChAdOx1 nCoV-19 (AZD1222). The vaccine is a viral vector vaccine. This means it uses a modified adenovirus—a virus that causes the common cold—to deliver genetic material from the SARS-CoV-2 virus.

Scientists were able to develop the vaccine quickly since this vaccine technology has been around for decades.  

Oxford had been studying adenovirus-based vaccines for several other diseases like the Zika virus when COVID-19 emerged. Researchers used a weakened chimpanzee adenovirus to develop the vaccine. A Phase 1 trial began in April 2020, with more than 1,000 vaccinations given in the United Kingdom.

Initial clinical trial results were published in early December and investigated how well the vaccine worked in more than 11,000 people over age 18 enrolled across four trial groups in the United Kingdom, Brazil, and South Africa.

There has been some controversy surrounding this vaccine, including:

  • Dosing discrepancies in some study groups
  • Unexplained complications prompting the Food and Drug Administration (FDA) to pause the U.S. trial (however, the FDA restarted the trial after reviewing the safety data)
  • Potential link to blood clots resulting in about a dozen European countries that had authorized the vaccine to pause vaccinations temporarily

After a review, the European Medicines Agency (EMA) announced on March 18, 2021 that it did not find an overall increased risk of blood clots. However, it did find a possible association with a very rare type of blood clot.

The EMA concluded that the vaccine's benefits still outweigh the risks of possible side effects, especially considering COVID-19 also can lead to blood clots. The World Health Organization (WHO) also released a statement urging countries to proceed with vaccination.

There are not any hearings scheduled yet on an emergency use authorization by the FDA.

This article explains how the AstraZeneca-Oxford vaccine works, its effectiveness, availability, side effects, and who is eligible to receive the vaccine.

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How It Works

The AstraZeneca-Oxford vaccine is a recombinant adenoviral vector vaccine. Recombinant vaccines use a small piece of genetic material from a pathogen (infectious organism), like SARS-CoV-2, to trigger an immune response. In this way, the vaccine can target a specific part of the virus.

Recombinant vaccines are generally safe to use in a large population of people—even those with chronic health problems or people who have weakened immune systems.

The AstraZeneca-Oxford vaccine uses a weakened live pathogen. One drawback of recombinant adenoviral vector vaccines is that people may require booster shots over time.

Examples of similar types of recombinant vaccines (that do not use live pathogens) are:

While recombinant vaccines are common, the only commercially available adenovirus-based vaccine of this type is a rabies vaccine for animals.


Researchers tested the AstraZeneca-Oxford vaccine using two doses given about a month apart. The dosing used in the trials is unclear. Reuters news service first uncovered different accounts of the dosing changes.

AstraZeneca and Oxford gave reporters with the news agency two different answers about whether a subgroup in the U.K. arm of the study received half doses of the vaccine purposefully or by accident. Reporters uncovered the discrepancy after preliminary results were published.

How Effective Is It?

Whether by accident or on purpose, the dosing change appears to have been lucky.

According to the trial report, the two-dose series of the AstraZeneca-Oxford vaccine is about 70% effective, on average. However, researchers calculated this average after finding a 62% effective rate in people who received the full vaccine dose in their first shot, compared with 90% effective in those who received a half dose.

Additional reviews are ongoing. It remains unclear at this time why a half-dose of vaccine might have worked better than a full first dose.

Based on limited studies, the vaccine has been shown to provide protection against the Delta (B.1.617.2) variant as well. A study published in The New England Journal of Medicine found one dose of the AstraZeneca-Oxford vaccine to be 30% effective against this variant and two doses to be 67% effective.

When Will It Be Available?

Availability depends on where you live.


The vaccine is widely authorized across Europe for use in adults 18 and older. This authorization is despite the initial arguments over dosing and temporary pauses in both the trials and vaccinations.

The company announced in November 2020 that trials are still underway around the world. But—pending regulatory approvals—as many as 3 billion doses could be available in 2021.

The vaccine may be easier to distribute than some other COVID-19 vaccine options. That's because rather than requiring extremely cold storage temperatures like some other vaccines, the AstraZeneca-Oxford vaccine needs only standard refrigeration. In addition, it is estimated to cost only around $3 to $4 per dose.


If and when the AstraZeneca-Oxford vaccine might be available in the United States is a big question. The Centers for Disease Control and Prevention (CDC) is leading vaccination efforts, and all orders of the COVID-19 vaccine, regardless of the manufacturer, will go through the agency.

The CDC is overseeing the distribution of vaccines, too. In addition, the CDC’s Advisory Committee on Immunization Practices (ACIP) has made recommendations on prioritizing vaccine supplies.

Healthcare workers and people living in long-term care facilities were the priority groups to receive authorized COVID-19 vaccines. Now, the vaccine is available and recommended for everyone age 12 and over, with authorization expected for children, as well.

The U.S. alone has a population of about 330 million. So far, other vaccine options are meeting the demand.

Who Can Get the AstraZeneca Vaccine?

Because it hasn't received authorization from the FDA, nobody in the United States can get an AstraZeneca vaccine. The initial clinical trial for the AstraZeneca-Oxford vaccine focused on people aged 18 to 55. New trials with people older than 55 began in August 2020.

There were initial trials involving children, but researchers removed that group from trial data in mid-December 2020. AstraZeneca and Oxford have not made any statements on the change.

In February 2021, the University of Oxford announced the launch of a new children's trial that will enroll 300 participants ages 6 to 17.

Side Effects and Adverse Events

Generally, side effects of COVID-19 vaccines have included minor reactions that are typical of most vaccines. These have included things like:

  • Arm pain
  • Arm swelling
  • Redness at the injection site

A few other adverse effects have been reported, including one that resulted in a pause in the AstraZeneca-Oxford clinical trial.

Adverse Events vs. Side Effects

A side effect is a physical response to a medication, whereas an adverse event is a more medically significant, less common reaction linked to the medication.

The preliminary report on the vaccine published in The Lancet revealed generally good results in terms of side effects. But there were a few concerning adverse reactions.

There were three cases of transverse myelitis—a condition where there is spinal cord inflammation—in people who received the vaccine. However, according to the trial report, these illnesses were unlikely to have been caused by the vaccine.

Some deaths were also reported in the study (most in the control group). But those deaths were unrelated to the vaccine and were due to incidents like traffic accidents and homicide.

Funding and Development

Scientists developed the vaccine through a partnership between Oxford and AstraZeneca. Funding support for the study was also provided by:

  • United Kingdom Research and Innovation
  • The Bill & Melinda Gates Foundation
  • The NIHR Oxford Biomedical Research Centre
  • The Thames Valley and South Midland's NIHR Clinical Research Network

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  3. AstraZeneca. FDA authorizes restart of the COVID-19 AZD1222 vaccine US Phase III trial.

  4. European Medicines Agency. COVID-19 Vaccine AstraZeneca: benefits still outweigh the risks despite possible link to rare blood clots with low blood platelets.

  5. World Health Organization. WHO statement on AstraZeneca COVID-19 vaccine safety signals.

  6. U.S. Department of Health and Human Services. Vaccine types.

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