ChAdOx1S

ChAdOx1S is a replication-deficient viral-vectored vaccine based on the chimpanzee adenovirus ChAdOx1. It is engineered to express the spike (S) glycoprotein of SARS-CoV-2 and is part of the ChAdOx1-S vaccine family developed by the University of Oxford’s Jenner Institute and licensed to manufacturers under various branding. The S designation refers to the spike protein that the immune system is intended to target.

Mechanism of action: Following intramuscular administration, the ChAdOx1 vector enters host cells and expresses the SARS-CoV-2 spike protein. This expression elicits humoral and cellular immune responses, including neutralizing antibodies and CD4+ and CD8+ T-cell responses, with the aim of preventing symptomatic and severe coronavirus disease.

Clinical development and efficacy: In clinical trials, efficacy estimates varied with dosing regimens and populations. Two-dose regimens generally produced substantial protection against symptomatic COVID-19, with aggregate efficacy reported around 60–70% in some analyses, and higher efficacy observed in certain dosing schedules or subgroups. Real-world effectiveness against severe disease was consistently demonstrated in many settings, though results differed with viral variants.

Safety and regulatory status: Common adverse events include local and systemic reactions such as injection-site pain, fatigue, and headache. Rare but serious adverse events, including thrombotic events with thrombocytopenia, have been reported in association with adenoviral-vector vaccines and prompted precautionary guidance in some jurisdictions. Regulatory use and recommendations have varied by country, with ongoing pharmacovigilance and variant-related effectiveness monitoring.