Vaccine efficacy against symptomatic COVID-19: 79%
Efficacy against severe disease and hospitalization: 100%, consistent across race and age groups, with 80% efficacy in participants aged 65 and older
Favorable reactogenicity and overall safety profile
On March 22, AstraZeneca announced results from the US phase 3 clinical trial of its COVID-19 vaccine developed with the University of Oxford, showing 79% efficacy in preventing symptomatic COVID-19 and 100% efficacy in preventing severe disease and hospitalization. The trial involved 32,449 participants.
The AstraZeneca/Oxford vaccine (AZD1222) was co-invented by the University of Oxford and its spin-off company Vaccitech. It uses a chimpanzee adenovirus vector (ChAdOx1) engineered to carry the gene for the SARS-CoV-2 spike protein. Once inside the body, cells infected by the adenovirus produce spike proteins, triggering the immune system to generate antibodies—including neutralizing antibodies—and activate T cells. If you later contract COVID-19, these antibodies and T cells work together to fight the virus.
The AstraZeneca/Oxford vaccine is one of the most globally anticipated COVID-19 vaccines due to its easy storage (standard refrigerator temperature, 2°C–8°C), low cost (under $10 for two doses), and high production capacity (100–200 million doses per month by 2021).
However, the vaccine has faced significant controversy since its clinical trials began. Starting March 7, several countries—including Austria, Denmark, France, Germany, Italy, Spain, Portugal, Sweden, and Slovenia—suspended use of the AstraZeneca/Oxford vaccine after more than 30 vaccinated individuals developed blood clots. These included cases of cerebral venous sinus thrombosis (CVST), which can be fatal.
On March 18, the European Medicines Agency (EMA) stated that after reviewing millions of cases, the AstraZeneca/Oxford vaccine does not increase the overall risk of blood clots.
This US phase 3 trial included a detailed assessment of thrombosis and CVST. Among 21,583 participants who received at least one dose, no increased risk of blood clots was observed. These results confirm the vaccine's safety.
Beyond blood clot concerns, the vaccine's effectiveness in older adults had also been questioned. In February, data from the final four clinical trials showed that less than 15% of participants were over 56, leading several countries to restrict its use in older populations. In this US phase 3 trial, about 20% of participants were aged 65 or older, and the vaccine showed 80% efficacy in this group. This result confirms the vaccine's protective effect in older adults.
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