The AstraZeneca COVID-19 vaccine, known commercially as Vaxzevria, emerged early in the global response to the pandemic. Developed through a partnership between Oxford University and the pharmaceutical company AstraZeneca, it became one of the first vaccines to be authorized and widely distributed worldwide. The vaccine played a significant part in international immunization efforts, particularly due to its relatively low cost and ease of storage compared to some other vaccine types. This overview examines the specific science behind the vaccine’s function, its documented effectiveness in clinical use, its safety profile, and its current status in global health initiatives.
How the Vaccine Works
The AstraZeneca vaccine uses a delivery method known as a non-replicating viral vector. This platform employs ChAdOx1, a modified version of an adenovirus which is a common cold virus isolated from chimpanzees. This specific virus was chosen because most humans have no pre-existing immunity to it, ensuring the body’s immune system focuses its response on the target.
Scientists genetically altered the adenovirus to be unable to replicate within human cells, making it harmless as a delivery vehicle. The vector carries the genetic instructions, or DNA, for the distinct spike protein found on the surface of the SARS-CoV-2 virus. Once injected, the viral vector enters human cells, which then use the provided genetic code to manufacture the spike protein. The presence of the newly produced spike proteins on the cell surface triggers an immune response. This process trains the immune system—specifically T-cells and B-cells—to recognize the actual SARS-CoV-2 spike protein. If the vaccinated person is later exposed to the real virus, the immune system is primed to rapidly neutralize the threat, preventing severe illness.
Understanding Efficacy and Protection
The effectiveness of the AstraZeneca vaccine was demonstrated across initial clinical trials and extensive real-world studies. While initial trial data for preventing symptomatic COVID-19 showed an overall efficacy of approximately 62% to 70%, its protection against the most serious outcomes was consistently high. The vaccine’s primary strength was its ability to prevent severe disease, hospitalization, and death.
Early analyses indicated 100% efficacy against severe or critical disease and hospitalization beginning 22 days after the first dose. Subsequent real-world data confirmed that the two-dose regimen offered equivalent protection against hospitalization and death when compared to mRNA vaccines, with rates consistently in the 91% to 93% range. The standard schedule involved two doses administered intramuscularly, with an interval of 4 to 12 weeks between injections. Evidence suggested that extending the interval between the two doses to 12 weeks or more resulted in a higher overall efficacy against symptomatic infection. This flexibility in dosing proved beneficial for large-scale public health programs.
Safety Profile and Rare Side Effects
Like all vaccines, the AstraZeneca shot caused common, generally mild side effects, which typically resolve within a few days. These included local reactions such as pain or tenderness at the injection site, and systemic reactions like fatigue, headache, muscle pain, and fever. More serious adverse events were extremely rare but became the focus of intense public scrutiny and regulatory review.
The most notable rare adverse event is Thrombosis with Thrombocytopenia Syndrome (TTS), also referred to as Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT). TTS is a condition characterized by the presence of blood clots (thrombosis) in unusual locations, such as the brain or abdomen, combined with a low platelet count (thrombocytopenia). This condition was found to be statistically linked to the vaccine, particularly following the first dose.
The frequency of TTS was estimated to be very low, occurring in approximately 2 to 3 people per 100,000 vaccinated individuals under the age of 60. For those aged 60 and older, the rate was slightly lower. Symptoms of TTS typically appeared between 4 and 42 days after vaccination and included:
- Severe, persistent headaches that did not respond to common painkillers.
- Blurred vision.
- Shortness of breath.
- Chest pain.
- Unexplained bruising away from the injection site.
Early recognition and specialized treatment significantly improved outcomes for those affected by this rare syndrome.
Global Use and Current Status
The AstraZeneca vaccine was instrumental in the initial global vaccination effort, with over three billion doses supplied worldwide. Its logistical advantages—specifically its ability to be stored at standard refrigerator temperatures—made it particularly suitable for deployment in low and middle-income countries through initiatives like COVAX.
Concerns regarding the rare TTS event led to various regulatory decisions in many countries, often resulting in preferential recommendations for other vaccine types for certain age groups. Some regulatory bodies, including the European Medicines Agency (EMA), concluded that the benefits of the vaccine in preventing COVID-19 significantly outweighed the small risk of TTS, but age restrictions were sometimes implemented to mitigate the risk in younger populations.
The landscape of COVID-19 vaccination has since evolved with the development of newer, variant-adapted vaccines. This surplus of updated options has led to a decline in demand for the original AstraZeneca formulation. Consequently, AstraZeneca has begun the process of voluntarily withdrawing its marketing authorization globally, and the vaccine is no longer manufactured or supplied in many regions, including its withdrawal from the European Union and Australia.