Timing and sequence of vaccination against COVID-19 and Influenza (TACTIC)

The current COVID-19 pandemic has a global impact on morbidity, mortality and society. Vaccinations provide the most effective approach to prevention and several vaccines have been developed with great speed and are now approved for use against COVID-19. The vaccination campaign is also in full swing in the Netherlands, with the elderly being the first to have access to the vaccines. Although it is still uncertain how long these vaccines will provide protection, it is believed that annual renewed vaccination for the winter season will be necessary. Importantly, the timing of the vaccination in the autumn of 2021 will coincide with the time at which the annual influenza vaccination takes place in the vulnerable population. Viral interference on the one hand, and vaccine interference on the other hand, is a well-known phenomenon in which two viral infections or two vaccines influence the immunological response and the outcome of each other. Because of the vaccinations for both COVID-19 and influenza that will take place later this year, it is therefore crucial to investigate the effects of the combined vaccination against both viruses to ensure their effectiveness. It is not clear which specific sequence of these two vaccines provides better protection. Both from a public health perspective (what is the optimal vaccination strategy for next autumn) and from a scientific point of view (unraveling the potential immunological interplay between the two vaccines), the study we propose here is of great importance. In addition to effectiveness and safety, immunological mechanisms will also be investigated as endpoints. We will investigate the humoral (antibodies), cellular (T-cell responses) and innate (cytokine production) immune responses after different schedules and combinations of COVID-19 and influenza vaccines, in order to identify the optimal schedule of administration.