The COVID-19 vaccine Vaxzevria, developed by the University of Oxford and AstraZeneca, was a tool used during the global pandemic. In India, it was manufactured and supplied under the name Covishield. Authorized in the UK in late 2020, it eventually reached more than 170 countries. Administered via intramuscular injection, it was used for preventing COVID-19.
How the Vaccine Works
The AstraZeneca vaccine is a viral vector vaccine that uses a modified, harmless virus to deliver genetic instructions to the body’s cells. The delivery vehicle is a modified chimpanzee adenovirus, chosen because humans are unlikely to have pre-existing immunity to it. This adenovirus is altered so it cannot replicate or cause illness in humans. Scientists inserted DNA that codes for the spike protein of the SARS-CoV-2 virus into this adenovirus vector.
Once injected, the vector enters human cells and releases this genetic blueprint. The cells’ machinery then uses the DNA to produce copies of the spike protein. These proteins are harmless, but their presence is recognized by the immune system as foreign. This recognition prompts the creation of antibodies and T-cells trained to target the spike protein, allowing the immune system to mount a defense if later exposed to the actual coronavirus.
Efficacy and Protection
Clinical trials and real-world data showed the AstraZeneca vaccine’s effectiveness. Initial studies in 2020 showed an efficacy of 76% in preventing symptomatic COVID-19 after the first dose, which increased to 81.3% after the second dose. A subsequent US trial reported 74% efficacy. The vaccine was effective at preventing severe outcomes, with data showing 91-93% protection against hospitalization and death, a level comparable to mRNA vaccines.
The vaccine’s performance against new viral variants was also monitored. Against the Alpha variant, real-world data from Scotland indicated 81% effectiveness against symptomatic infection, while other data showed an 86% reduction in hospitalizations. When the Delta variant became dominant, two doses of the vaccine were found to be 92% effective against hospitalization, with no deaths reported among vaccinated individuals in the study.
The vaccine’s effectiveness against other variants varied. Its efficacy against the Beta variant was lower for symptomatic infection but improved against severe disease after a second dose. Against the Omicron variant, protection from symptomatic infection was reduced, though a third booster dose was shown to increase antibody levels against this variant.
Safety Profile and Side Effects
The AstraZeneca vaccine’s safety profile included common side effects that resolved within a few days. These were signs of a normal immune system response and included:
- Pain at the injection site
- Headache
- Fatigue
- Muscle aches
- Fever
A rare but serious side effect was identified as Thrombosis with Thrombocytopenia Syndrome (TTS), also known as VITT. This condition involves the formation of blood clots in unusual locations, such as the brain or abdomen, combined with low levels of platelets, which are cells that help blood to clot. The biological mechanism is believed to be an immune response triggered by the vaccine in some individuals.
The frequency of TTS was very low. Data from the UK and European Union estimated the risk to be between 1 in 100,000 and 4 per million doses. Australian data reported a rate of about 2 to 3 cases per 100,000 doses in people under 60, and 2 per 100,000 in those 60 and older. Symptoms appeared between 4 and 42 days after the first dose and included:
- Severe, persistent headaches
- Blurred vision
- Shortness of breath
- Unexplained bruising
This risk, though rare, led some countries to recommend the vaccine primarily for older age groups, where the risk of TTS was lower.
Global Distribution and Impact
The AstraZeneca vaccine was part of the initial global pandemic response due to its logistical advantages. It could be stored and transported at normal refrigeration temperatures (2-8°C or 36-46°F), making it suitable for areas with limited healthcare infrastructure. This contrasted with mRNA vaccines that required ultra-cold storage.
Its low cost and AstraZeneca’s commitment to provide it on a not-for-profit basis during the pandemic made it accessible to low- and lower-middle-income countries. The vaccine was the biggest initial supplier to the COVAX initiative, a program for equitable vaccine access. Through COVAX, hundreds of millions of doses were shipped to 142 countries, with the first deliveries arriving in Ghana and Cote D’Ivoire.
By November 2021, two billion doses had been supplied to over 170 countries, with approximately two-thirds going to low- and lower-middle-income nations. The vaccine was used in national immunization campaigns in countries like the United Kingdom, Australia, and India, where it was manufactured as Covishield. It is estimated that the vaccine saved over 6.5 million lives in its first year of use.
Evolving Role and Withdrawal
The role of the AstraZeneca vaccine shifted as the pandemic evolved. A reason for its declining use was the availability of updated mRNA vaccines. These newer vaccines were formulated to target emerging variants, offering more effective protection. The surplus of alternative vaccines led to a drop in demand for Vaxzevria.
Public and regulatory perceptions also changed following the identification of the rare TTS side effect. Although the risk was low, these concerns influenced vaccination policies and contributed to a preference for mRNA vaccines. The transition of the pandemic to a phase of higher population immunity also altered the public health landscape.
In March 2024, AstraZeneca initiated the withdrawal of the vaccine’s marketing authorization in Europe, citing a surplus of updated vaccines and lack of demand. The company stated the decision was for commercial reasons, as the vaccine was no longer being manufactured or supplied. This marked the conclusion of its use in many parts of the world.