The AstraZeneca COVID-19 vaccine, scientifically known as AZD1222, was developed by the University of Oxford and AstraZeneca. It has been distributed under brand names like Vaxzevria and Covishield, the latter manufactured by the Serum Institute of India. This vaccine was a tool in the global response to the COVID-19 pandemic and is administered as a two-dose course via intramuscular injection.
Initial approval was granted by the UK’s MHRA on December 30, 2020, followed by authorizations in many countries. An Emergency Use Listing from the World Health Organization (WHO) facilitated its inclusion in the COVAX program for global distribution. The vaccine’s formulation is identical regardless of its brand name.
Viral Vector Technology
The AstraZeneca vaccine uses viral vector technology. It employs a harmless, weakened chimpanzee cold virus, Chimpanzee Adenovirus Oxford 1 (ChAdOx1), as a delivery vehicle. This specific adenovirus was chosen because humans lack pre-existing immunity to it, preventing the immune system from attacking the vector before it delivers its genetic material to human cells.
Scientists modified this adenovirus so it cannot replicate or cause disease in humans. They inserted the genetic instructions for building the spike protein found on the surface of the SARS-CoV-2 virus. When injected, the vector enters human cells and uses the cell’s machinery to produce this spike protein without altering the cell’s DNA.
The presence of these spike proteins triggers an immune response. The body recognizes them as foreign and produces defensive cells, including antibody-generating B-cells and T-cells that destroy infected cells. This process primes the immune system, creating a memory of the spike protein to provide a rapid defense against a future SARS-CoV-2 infection.
Efficacy and Dosage Schedule
Clinical trials demonstrated the AstraZeneca vaccine’s effectiveness. Initial 2020 studies indicated 76% efficacy in preventing symptomatic COVID-19 after the first dose, increasing to 81.3% after the second. U.S. trials later showed 74% effectiveness after the second dose. The vaccine was highly effective at preventing severe outcomes, with analysis showing no hospitalizations among participants who received two doses.
The timing between doses was a factor in the vaccine’s performance, with research indicating a longer interval resulted in higher efficacy. Based on this data, the World Health Organization (WHO) recommended an interval of 8 to 12 weeks between the first and second shots to maximize protection.
The vaccine was also effective against SARS-CoV-2 variants. A Scottish study found it was 81% effective against the Alpha variant and 61% against the Delta variant in preventing symptomatic infection. Data from Public Health England showed 92% effectiveness against hospitalization from the Delta variant and 86% from the Alpha variant. However, protection against symptomatic disease from the Omicron variant waned after two doses.
Reported Side Effects and Safety Profile
The AstraZeneca shot is associated with common, temporary side effects that signal an immune system response. These reactions are mild to moderate and resolve within a few days. They include:
- Pain or tenderness at the injection site
- Headache
- Fatigue
- Muscle aches
- A general feeling of being unwell or feverish
A rare but serious adverse event linked to the vaccine is Thrombosis with Thrombocytopenia Syndrome (TTS), also known as VITT. This condition involves the combination of blood clots (thrombosis) and a low count of platelets (thrombocytopenia). The clots associated with TTS can form in unusual locations, such as the veins of the brain or abdomen.
The incidence of TTS is very low, with Australian data estimating a risk of about 2.6 per 100,000 people after the first dose and a lower rate after the second. The condition appeared more frequently in younger adults. In February 2024, AstraZeneca acknowledged in legal documents that the vaccine “can, in very rare cases, cause TTS”. Symptoms requiring immediate medical attention appeared between 4 and 42 days after vaccination and included:
- Severe, persistent headaches
- Blurred vision
- Shortness of breath
- Chest pain
- Leg swelling
- Persistent abdominal pain
Global Role and Current Status
The AstraZeneca vaccine played a part in the initial global pandemic response due to its practical advantages. Its lower cost and ability to be stored at standard refrigerator temperatures (2-8°C) made it an accessible option for many nations, especially low- and middle-income countries. These factors made it an element of the COVAX initiative, a global effort for equitable vaccine access, with millions of doses shipped worldwide.
Over time, the vaccine’s role shifted as many higher-income countries gained access to mRNA vaccines from Pfizer-BioNTech and Moderna. These countries gradually phased out the AstraZeneca shot, influenced by the availability of alternatives and concerns over the rare TTS clotting risk. Citing a surplus of updated vaccines, AstraZeneca began withdrawing its marketing authorizations in Europe in March 2024 and globally by May 2024. The vaccine is no longer in production.