Childhood vaccines are significantly more effective when administered during the colder months of the year.

The human immune system was always thought to respond to a vaccine with the same strength regardless of the date. A massive analysis of 48,000 children reveals that the body's internal clock is synchronized with the seasons. People living in temperate zones show a much higher immune response in the winter than in the summer. This annual rhythm is likely driven by changes in day length and environmental temperature. Adjusting the timing of vaccination campaigns could save thousands of lives by ensuring children get the maximum possible protection.