We are a team working across England, Scotland and Wales studying the immune response in healthcare workers, in collaboration with the SIREN study and PITCH consortium. The COVID-19 pandemic saw a rapid application of cutting-edge vaccine technology to a novel and evolving disease. These vaccines can produce a profound immune response, yet infections after vaccination are reported. By studying healthcare workers, a highly vaccinated population with increased exposure to COVID-19, we aim to investigate what factors predispose someone suffering a COVID-19 infection after vaccination or prior infection.