The vaccine, AZD1222, harnesses ChAdOx1, a replication-deficient simian adenoviral vector platform against COVID-19. The simian adenoviral vaccine platform, being actively developed by both the University of Oxford and Vaccitech Limited, has been tested safely in clinical trials for numerous disease indications and is now being applied to COVID-19. Through genetic modification, the viral vector cannot reproduce in the body but contains the genetic code for the SARS-COV-2 coronavirus Spike protein, enabling it to produce the protein after intramuscular injection. The Spike protein is located on the surface of the coronavirus and has a central role in how the virus infects human cells, making it an ideal vaccine target. When the Spike protein is detected by the immune system, antibodies and cellular immune responses are formed against it which are known to combat viral infection. 

AZD1222 is one the few vaccine technologies to have already demonstrated an ability to induce a strong antibody and cellular immune response against another coronavirus, Middle East Respiratory Syndrome (MERS), in previous animal studies and human clinical trials, conducted by Oxford University with commercial rights assigned to Vaccitech Limited. Simian adenoviral vector vaccines have also been safely administered to many thousands of people worldwide. In addition to world-leading vaccine characteristics, the technology is one of the most advanced in development. Clinical trials by Oxford University have begun in April with an adaptive design leading to rapid efficacy trials. The vaccine is now available in several countries, such as the UK. AstraZeneca has applied for conditional market approval with EMA on January 12, 2021.