evidence: Four in 10 people who are clinically vulnerable generate lower levels of antibodies than healthy recipients after two shots of vaccine against SARS-CoV-2, a study has found.1 The Octave (Observational Cohort Trial T cells Antibodies and Vaccine Efficacy in SARS-CoV-2) trial is one of the largest in the world to have looked at the response to covid-19 vaccination in patients who are immunocompromised. It compared 600 patients, who had a weakened immune system because of their disease process or treatment, with the antibody response of healthy people from the Pitch (Protective Immunity from T Cells in Healthcare workers) study. [...] The findings, published as a preprint on the Lancet site,1 showed that 89% of immunocompromised patients seroconverted within four weeks of the second vaccine dose, as compared with 100% of healthy participants in the Pitch trial.