COVID-19 Booster and IIV Schedule in Immunocompromised Hosts

The goal of this pragmatic embedded open-label, 2 x 2 factorial phase II randomized controlled trial is to evaluate strategies to improve COVID-19 booster and influenza vaccine immunogenicity in people living with immunocompromising conditions (PLIC). The main questions it aims to answer are: 1. Is co-administration of seasonal inactivated influenza vaccine (IIV) with the most up-to-date recommended COVID-19 booster dose non-inferior in inducing a 1-month peak protective humoral response against COVID-19, compared to a strategy of sequential administration of COVID-19 booster dose followed by seasonal IIV given one month later? 2. Is the administration of the most up-to-date recommended COVID-19 booster doses at 3-month intervals superior at maintaining a longer term protective humoral immune response, compared to booster doses administered at 6-month intervals? Researchers will compare (1) COVID-19 and Influenza vaccines administered at Day 0 + COVID-19 Booster at a 3-month interval, (2) COVID-19 vaccine administered at Day 0 and Influenza vaccine administered at Day 28 + COVID-19 Booster at a 3-month interval, (3) COVID-19 and Influenza vaccines administered at Day 0 + COVID-19 Booster at a 6-month interval, and (4) COVID-19 vaccine administered at Day 0 and Influenza vaccine administered at Day 28 + COVID-19 Booster at a 6-month interval to see if median neutralization capacity of patient sera is non-inferior in the co- vs. sequential administration arms at 1-month after the initial COVID-19 booster and superior in the 3-month interval arms vs. the 6-month interval arms at 12 months after the initial COVID-19 booster. These outcomes will also be compared at 2-months for question 1 and 6-months for question 2. People living with immunocompromising conditions who take part in the trial will have blood samples drawn to verify immune response, be monitored for changes in clinical events and therapies, and complete questionnaires to verify adverse effects, quality of life and economic impact.