evidence: Covid-19: 40% of patients with weakened immune system mount lower response to vaccines [...] 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 have been shared with the UK’s Joint Committee on Vaccination and Immunisation, which is poised to decide on provision of booster vaccines in early September. Some countries that are already administering booster doses have prioritised people with chronic diseases or who are taking immunosuppressive therapies.2