Vaccines are highly effective at protecting people from COVID-19. We want to find out whether they work equally well in people with obesity and diabetes, who are at higher risk of serious COVID-19 infection. To address this question, we are asking people who come to clinics at Addenbrookes Hospital, Cambridge and Kings College Hospital, London to help us by donating a blood sample before and after they receive a booster vaccine. We will use state-of-the art techniques to look at the immune response in detail and compare their results to those from over a thousand volunteers with a normal weight. If people are having weight loss treatments including surgery, we would like to see if their treatment has an impact on the immune response. Our findings will inform UK policy on vaccination in people who are overweight and people who have diabetes.