The Chrono-Vax trial protocol: The effect of the time of day of influenza vaccination on immune responses in adults aged 60–85 years

Increasing evidence suggests that timing of vaccine administration affects immune responses, with some studies indicating that morning administration of the influenza vaccine elicits a stronger antibody response than afternoon vaccination in older adults. Existing trials focused on antibody responses, contrasting two time groups (morning versus afternoon), without assessing other immunological parameters, such as T-cell responses, which also play a crucial role in immunity to respiratory viruses and may contribute to the time-of-day dependent response to vaccination. Therefore, the Chrono-Vax trial aims to determine the effect of influenza vaccination timing on antibody and T-cell responses across a continuous time window from 09:00–17:00 to gain novel insights into an optimal administration time. Additionally, this study will investigate how chronotype influences the relationship between vaccination timing and vaccine-induced immune responses and examine the impact of vaccination timing on the incidence of influenza-like illness (ILI) and influenza infection up to six months post-vaccination. A total of 440 adults aged 60–85 years will be included in the trial and receive the seasonal influenza vaccine at a random time between 09:00 and 17:00. Blood samples will be collected at baseline and 28 days post-vaccination to determine vaccine-induced immune responses. ILI symptoms and influenza infection will be monitored by participants up to six months after vaccination using a diary and rapid diagnostic self-tests, respectively. As such, the Chrono-Vax trial aims to determine the optimal time of day for influenza vaccination, thereby contributing to more effective vaccination strategies and increased protection against severe disease in older adults.