Table of ContentsView AllTable of ContentsHow It WorksHow Effective Is It?When Was It Available?Who Can Get the AstraZeneca Vaccine?Side Effects and Adverse EventsFunding and DevelopmentNext in COVID-19 Vaccine Brand GuideAn Overview of the Johnson & Johnson COVID-19 Vaccine
Table of ContentsView All
View All
Table of Contents
How It Works
How Effective Is It?
When Was It Available?
Who Can Get the AstraZeneca Vaccine?
Side Effects and Adverse Events
Funding and Development
Next in COVID-19 Vaccine Brand Guide
While some vaccine makers used newer approaches to fight COVID-19, others turned to more established methods. The pharmaceutical company AstraZeneca partnered with Oxford University to develop an adenovirus-based vaccine, officially named ChAdOx1 nCoV-19 (AZD1222).
The vaccine is a viral vector vaccine. This means it uses a modifiedadenovirus—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:
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.
This article explains how the AstraZeneca-Oxford vaccine works, its effectiveness, side effects, and why it was later withdrawn.
zoranm / Getty Images
The AstraZeneca-Oxford vaccine—also known as Vaxzevria—is arecombinantadenoviral 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 haveweakened immune systems.
The AstraZeneca-Oxford vaccine uses a weakened form of a common cold virus to deliver genetic material. One drawback of recombinant adenoviral vector vaccines is that people may require booster shots over time.
The Johnson & Johnson/Janssen COVID-19 vaccine is also an adenovirus-based vaccine.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.
Dosing
The recommended dosage was two doses, with the second dose given four to 12 weeks after the first dose.The vaccine was also approved as a booster dose after completing a primary series of Vaxzevria or an mRNA vaccine.
Researchers tested the AstraZeneca-Oxford vaccine using two doses given about a month apart. The dosing used in the trials was unclear.
Reporters uncovered the discrepancy after preliminary results were published. However, it was noted that those who received a half-dose of the first shot had a higher level of protection against COVID-19 than those who received the full dose.Overall, despite this error, the results of the trials looked promising.
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.It remains unclear why a half-dose of vaccine might have worked better than a full first dose.
The vaccine has been shown to provide protection against theDelta (B.1.617.2) variantas well. A study published inThe New England Journal of Medicinefound one dose of the AstraZeneca-Oxford vaccine to be 30% effective against this variant and two doses to be 67% effective.
One study published inThe Lancet Regional Health Southeast Asiain 2023 found that the AstraZeneca-Oxford vaccine was 73% effective at preventing Omicron-related infections after the fourth dose—results similar to that seen with the mRNA vaccines. This fourth dose was on top of any previous primary or booster vaccine.
Europe
The AstraZeneca vaccine received authorization from the EMA on January 29, 2021. It was 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.
Since its authorization in 2021, the vaccine has been released to more than 170 countries, including Australia, Brazil, and Canada.
U.S.
In the U.S., the other authorized vaccine options were able to meet the demand, although demand has declined over time. AstraZeneca cited the availability of other vaccines targeting newer variants as a reason for withdrawing its vaccine.
Although the AstraZeneca vaccine was not authorized in the U.S., millions of doses were manufactured in the country, for export.
After the first COVID-19 vaccines were authorized in the U.S., healthcare workers and people living in long-term care facilities were the priority groups to receive them. Now, three COVID-19 vaccines are available and recommended for everyone ages 6 months and over.
COVID-19 Vaccines:Stay up to date on which vaccines are available, who can get them, and how safe they are.
The AstraZeneca vaccine was only authorized for use in adults ages 18 years and older.
Because it had not received authorization from the FDA, no one in the United States could 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. In February 2021, the University of Oxford initiated a new children’s trial that enrolled 262 participants ages 6 to 17.
In May 2024, AstraZeneca began the global withdrawal of its COVID-19 vaccine due to a decline in demand. It is no longer being manufactured or distributed.
Generally, side effects of COVID-19 vaccines have included minor reactions that are typical of most vaccines. These have included things like:
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 EffectsA side effect is a physical response to a medication, whereas anadverse eventis a more medically significant, less common reaction linked to the medication.
Adverse Events vs. Side Effects
A side effect is a physical response to a medication, whereas anadverse eventis a more medically significant, less common reaction linked to the medication.
The preliminary report on the vaccine published inThe Lancetrevealed generally good results in terms of side effects. But there were a few concerning adverse reactions.
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.
Although rare, the vaccine has also been linked to thrombosis withthrombocytopeniasyndrome (TTS) andGuillain-Barré syndrome.
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.
17 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.European Medicines Agency.Vaxzevria (previously COVID-19 Vaccine AstraZeneca).U.S. Department of Health and Human Services.Vaccine Types.Voysey M, Clemens SAC, Madhi SA, et al.Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (Azd1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK.The Lancet. 2021;397(10269):99-111. doi:10.1016/S0140-6736(20)32661-1AstraZeneca.FDA authorizes restart of the COVID-19 AZD1222 vaccine US Phase III trial.European Medicines Agency.COVID-19 Vaccine AstraZeneca: benefits still outweigh the risks despite possible link to rare blood clots with low blood platelets.World Health Organization.WHO statement on AstraZeneca COVID-19 vaccine safety signals.Sadoff J, Gray G, Vandebosch A, et al.Safety and efficacy of single-dose Ad26.COV2.S vaccine against Covid-19.N Engl J Med. 2021;384(23):2187-2201. doi:10.1056/NEJMoa2101544U.S. Department of Health and Human Services.Vaccine types.World Health Organization.The Oxford/AstraZeneca (ChAdOx1-S [recombinant] vaccine) COVID-19 vaccine: what you need to know.AstraZeneca.Vaxzevria approved in the EU as third dose booster against COVID-19.Lawton G.Trial results come under fire.New Scientist. 2020;248(3311):8. doi:10.1016/S0262-4079(20)32105-9Lopez Bernal J, Andrews N, Gower C, et al.Effectiveness of Covid-19 vaccines against the B.1.617.2 (Delta) variant.N Engl J Med. 2021;385(7):585-594. doi:10.1056/NEJMoa2108891Intawong K, Chariyalertsak S, Chalom K, et al.Effectiveness of heterologous third and fourth dose COVID-19 vaccine schedules for SARS-CoV-2 infection during delta and omicron predominance in Thailand: a test-negative, case-control study.The Lancet Regional Health - Southeast Asia. 2023;10:100121. doi:10.1016/j.lansea.2022.100121AstraZeneca.Two billion doses of AstraZeneca’s COVID-19 vaccine supplied to countries across the world less than 12 months after first approval.Precision Vaccination.Vaxzevria COVID-19 vaccine.Food and Drug Administration.Coronavirus (COVID-19) Update: August 6, 2021.Li G, Cappuccini F, Marchevsky NG, et al.Safety and immunogenicity of the ChAdOx1 nCoV-19 (AZD1222) vaccine in children aged 6-17 years: a preliminary report of COV006, a phase 2 single-blind, randomised, controlled trial[published correction appears in Lancet. 2022 Jul 2;400(10345):24].Lancet. 2022;399(10342):2212-2225. doi:10.1016/S0140-6736(22)00770-X
17 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.European Medicines Agency.Vaxzevria (previously COVID-19 Vaccine AstraZeneca).U.S. Department of Health and Human Services.Vaccine Types.Voysey M, Clemens SAC, Madhi SA, et al.Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (Azd1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK.The Lancet. 2021;397(10269):99-111. doi:10.1016/S0140-6736(20)32661-1AstraZeneca.FDA authorizes restart of the COVID-19 AZD1222 vaccine US Phase III trial.European Medicines Agency.COVID-19 Vaccine AstraZeneca: benefits still outweigh the risks despite possible link to rare blood clots with low blood platelets.World Health Organization.WHO statement on AstraZeneca COVID-19 vaccine safety signals.Sadoff J, Gray G, Vandebosch A, et al.Safety and efficacy of single-dose Ad26.COV2.S vaccine against Covid-19.N Engl J Med. 2021;384(23):2187-2201. doi:10.1056/NEJMoa2101544U.S. Department of Health and Human Services.Vaccine types.World Health Organization.The Oxford/AstraZeneca (ChAdOx1-S [recombinant] vaccine) COVID-19 vaccine: what you need to know.AstraZeneca.Vaxzevria approved in the EU as third dose booster against COVID-19.Lawton G.Trial results come under fire.New Scientist. 2020;248(3311):8. doi:10.1016/S0262-4079(20)32105-9Lopez Bernal J, Andrews N, Gower C, et al.Effectiveness of Covid-19 vaccines against the B.1.617.2 (Delta) variant.N Engl J Med. 2021;385(7):585-594. doi:10.1056/NEJMoa2108891Intawong K, Chariyalertsak S, Chalom K, et al.Effectiveness of heterologous third and fourth dose COVID-19 vaccine schedules for SARS-CoV-2 infection during delta and omicron predominance in Thailand: a test-negative, case-control study.The Lancet Regional Health - Southeast Asia. 2023;10:100121. doi:10.1016/j.lansea.2022.100121AstraZeneca.Two billion doses of AstraZeneca’s COVID-19 vaccine supplied to countries across the world less than 12 months after first approval.Precision Vaccination.Vaxzevria COVID-19 vaccine.Food and Drug Administration.Coronavirus (COVID-19) Update: August 6, 2021.Li G, Cappuccini F, Marchevsky NG, et al.Safety and immunogenicity of the ChAdOx1 nCoV-19 (AZD1222) vaccine in children aged 6-17 years: a preliminary report of COV006, a phase 2 single-blind, randomised, controlled trial[published correction appears in Lancet. 2022 Jul 2;400(10345):24].Lancet. 2022;399(10342):2212-2225. doi:10.1016/S0140-6736(22)00770-X
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
European Medicines Agency.Vaxzevria (previously COVID-19 Vaccine AstraZeneca).U.S. Department of Health and Human Services.Vaccine Types.Voysey M, Clemens SAC, Madhi SA, et al.Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (Azd1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK.The Lancet. 2021;397(10269):99-111. doi:10.1016/S0140-6736(20)32661-1AstraZeneca.FDA authorizes restart of the COVID-19 AZD1222 vaccine US Phase III trial.European Medicines Agency.COVID-19 Vaccine AstraZeneca: benefits still outweigh the risks despite possible link to rare blood clots with low blood platelets.World Health Organization.WHO statement on AstraZeneca COVID-19 vaccine safety signals.Sadoff J, Gray G, Vandebosch A, et al.Safety and efficacy of single-dose Ad26.COV2.S vaccine against Covid-19.N Engl J Med. 2021;384(23):2187-2201. doi:10.1056/NEJMoa2101544U.S. Department of Health and Human Services.Vaccine types.World Health Organization.The Oxford/AstraZeneca (ChAdOx1-S [recombinant] vaccine) COVID-19 vaccine: what you need to know.AstraZeneca.Vaxzevria approved in the EU as third dose booster against COVID-19.Lawton G.Trial results come under fire.New Scientist. 2020;248(3311):8. doi:10.1016/S0262-4079(20)32105-9Lopez Bernal J, Andrews N, Gower C, et al.Effectiveness of Covid-19 vaccines against the B.1.617.2 (Delta) variant.N Engl J Med. 2021;385(7):585-594. doi:10.1056/NEJMoa2108891Intawong K, Chariyalertsak S, Chalom K, et al.Effectiveness of heterologous third and fourth dose COVID-19 vaccine schedules for SARS-CoV-2 infection during delta and omicron predominance in Thailand: a test-negative, case-control study.The Lancet Regional Health - Southeast Asia. 2023;10:100121. doi:10.1016/j.lansea.2022.100121AstraZeneca.Two billion doses of AstraZeneca’s COVID-19 vaccine supplied to countries across the world less than 12 months after first approval.Precision Vaccination.Vaxzevria COVID-19 vaccine.Food and Drug Administration.Coronavirus (COVID-19) Update: August 6, 2021.Li G, Cappuccini F, Marchevsky NG, et al.Safety and immunogenicity of the ChAdOx1 nCoV-19 (AZD1222) vaccine in children aged 6-17 years: a preliminary report of COV006, a phase 2 single-blind, randomised, controlled trial[published correction appears in Lancet. 2022 Jul 2;400(10345):24].Lancet. 2022;399(10342):2212-2225. doi:10.1016/S0140-6736(22)00770-X
European Medicines Agency.Vaxzevria (previously COVID-19 Vaccine AstraZeneca).
U.S. Department of Health and Human Services.Vaccine Types.
Voysey M, Clemens SAC, Madhi SA, et al.Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (Azd1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK.The Lancet. 2021;397(10269):99-111. doi:10.1016/S0140-6736(20)32661-1
AstraZeneca.FDA authorizes restart of the COVID-19 AZD1222 vaccine US Phase III trial.
European Medicines Agency.COVID-19 Vaccine AstraZeneca: benefits still outweigh the risks despite possible link to rare blood clots with low blood platelets.
World Health Organization.WHO statement on AstraZeneca COVID-19 vaccine safety signals.
Sadoff J, Gray G, Vandebosch A, et al.Safety and efficacy of single-dose Ad26.COV2.S vaccine against Covid-19.N Engl J Med. 2021;384(23):2187-2201. doi:10.1056/NEJMoa2101544
U.S. Department of Health and Human Services.Vaccine types.
World Health Organization.The Oxford/AstraZeneca (ChAdOx1-S [recombinant] vaccine) COVID-19 vaccine: what you need to know.
AstraZeneca.Vaxzevria approved in the EU as third dose booster against COVID-19.
Lawton G.Trial results come under fire.New Scientist. 2020;248(3311):8. doi:10.1016/S0262-4079(20)32105-9
Lopez Bernal J, Andrews N, Gower C, et al.Effectiveness of Covid-19 vaccines against the B.1.617.2 (Delta) variant.N Engl J Med. 2021;385(7):585-594. doi:10.1056/NEJMoa2108891
Intawong K, Chariyalertsak S, Chalom K, et al.Effectiveness of heterologous third and fourth dose COVID-19 vaccine schedules for SARS-CoV-2 infection during delta and omicron predominance in Thailand: a test-negative, case-control study.The Lancet Regional Health - Southeast Asia. 2023;10:100121. doi:10.1016/j.lansea.2022.100121
AstraZeneca.Two billion doses of AstraZeneca’s COVID-19 vaccine supplied to countries across the world less than 12 months after first approval.
Precision Vaccination.Vaxzevria COVID-19 vaccine.
Food and Drug Administration.Coronavirus (COVID-19) Update: August 6, 2021.
Li G, Cappuccini F, Marchevsky NG, et al.Safety and immunogenicity of the ChAdOx1 nCoV-19 (AZD1222) vaccine in children aged 6-17 years: a preliminary report of COV006, a phase 2 single-blind, randomised, controlled trial[published correction appears in Lancet. 2022 Jul 2;400(10345):24].Lancet. 2022;399(10342):2212-2225. doi:10.1016/S0140-6736(22)00770-X
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