The immune response to childhood vaccines is seasonal

Vaccination programs worldwide have effectively reduced the burden of childhood diseases, yet immune responses remain highly heterogeneous among individuals 1,2. While host characteristics such as age and sex are established determinants of vaccine immunogenicity, the timing of vaccination, specifically the calendar season of vaccination, remains largely underexplored 3. Although circadian rhythms are known to regulate daily immune function 4, evidence for long-term circannual patterns has been limited by the difficulty of collecting year-round vaccination data across diverse populations. Here, we show that the season of vaccination systematically shapes the immune response across a broad range of pediatric vaccines. By leveraging data from 96 randomized control trials worldwide, including over 48,000 children vaccinated against 14 pathogens, we demonstrate that immunogenicity after vaccination follows a pronounced latitudinal gradient, typically peaking during colder months in temperate regions and exhibiting distinct variability in the tropics. These findings suggest that the circadian human immune response might extend to a circannual scale, potentially synchronized by environmental cues. Incorporating the season of vaccination into the design of clinical trials and public health campaigns may optimize vaccine performance and enhance seroprotection.