VITAL: Vaccination, Immunity, Time-restricted Eating, Aging and Lifestyle

Inclusion Criteria: * Male and female participants, enrolled in a 1:1 ratio * Age 60-85 years * Body mass index (BMI) 20-35 kg/m² * Capacity to give informed consent * Existing health insurance to allow evaluation and treatment of any incidental findings * Usual daily eating window \> 11 hours * First meal of the day before 10:00 AM * Willingness to receive seasonal influenza and COVID-19 vaccination and proof of scheduled appointment * Willingness and ability to follow a prescribed TRE dietary regimen (8-hour daily eating window; 16-hour fast without any caloric intake) * Appointment for simultaneous influenza and COVID-19 vaccination pre-arranged with primary care physician and coordinated with study team to align with TRE intervention Exclusion Criteria: * Any vaccination (especially influenza and/or COVID-19) within 6 months before the intervention start * Vaccinations not related to the study, administered during the study period from V0 to V4 * History of influenza infection within 6 months prior to initiation of the study intervention * History of severe adverse reactions to prior vaccinations * Use of pharmacological weight-loss agents (e.g., semaglutide) * Diabetes mellitus under ongoing pharmacological treatment * Symptoms of systemic inflammatory or autoimmune disease * Immunosuppression (including use of immunosuppressive drugs) * Severe hypertension (systolic \> 180 mmHg or diastolic \> 110 mmHg) * Diseases or functional disorders which, in the opinion of the study physician, preclude participation in the study * Participation in any fasting intervention (e.g., TRE, alternate-day fasting, 5:2, 18:6) within 6 months before enrollment * Participation in another diet or weight-loss program (e.g., intensive athletic training) * Night-shift or rotating-shift work * Severe, active, or unstable medical conditions requiring treatment * Postoperative recovery phase * Antibiotic therapy within 3 months before enrollment * Acute or chronic infections * Therapeutic or medically prescribed special diets * Vegan diet * Current smoker * Weight change \> 2 kg in the month before enrollment * Known substance, drug, or alcohol abuse * Anemia * Claustrophobia * Legal incapacity or any other circumstance that prevents full understanding of the nature, importance, and implications of the study