The ChAdOx-1-S vaccine is a viral vector-based COVID-19 vaccine. It uses a modified chimpanzee adenovirus to deliver the genetic code for the spike protein antigen of SARS-CoV-2, prompting an immune response. The vaccine has shown approximately 70% efficacy in preventing COVID-19 after two doses, with efficacy increasing to around 90% with a longer interval between doses. It provides protection against various strains, including Alpha and Delta. Common side effects include injection site pain, fatigue, and mild flu-like symptoms, with rare cases of blood clotting disorders such as thrombosis with thrombocytopenia syndrome (TTS). The ChAdOx-1-S vaccine is considered safe and effective, with its benefits outweighing the potential risks.