Your body clock has seasonal rhythms and it matters for vaccines

The time of year that a child gets vaccinated against polio could affect the strength of their immune response Many people have the sense that their health ebbs and flows with the seasons. Now, research suggests that our response to vaccines – and our physiology more generally – varies across the year. Although humans aren’t usually considered seasonal creatures, many plants and animals follow biological calendars that influence when they flower, breed, migrate or hibernate.

Over the past decade, a growing number of studies have indicated that humans, too, may experience subtle seasonal shifts in immune activity, hormone levels and gene expression. “The really exciting finding of this paper is not about vaccination – it is that human immune function [is] different across the seasons,” says Cathy Wyse at the University of Edinburgh, UK, who wasn’t involved in the study. “This suggests that humans might have inbuilt seasonal timing, as is seen in animals, birds and across biology.” With research suggesting that our response to influenza vaccines follows 24-hour circadian patterns, Laura Barrero Guevara at New York University and her colleagues were inspired to investigate the seasonality of vaccine outcomes more broadly. The team pooled data from 96 randomised-controlled trials involving around 48,000 children who had been vaccinated against 14 infections, including measles, polio and chickenpox. These were conducted in different countries at different times of year, which enabled the researchers to compare seasonal and geographical differences in immunogenicity, the strength of the antibody response triggered by vaccination. “We found that there is indeed a seasonal immune response,” says Barrero Guevara. “I think the most exciting part was seeing this latitudinal gradient. In temperate regions, the stronger immune response was during the winter, both in the northern and southern hemispheres, which is what you’d expect if it was being influenced by seasonal changes in day length, or photoperiod.” Closer to the equator, the immune system appeared to follow a less predictable seasonal pattern. There were still strong annual fluctuations in vaccine responses in the tropics, with larger seasonal swings for some vaccines , including rotavirus and polio. However, unlike the relatively consistent winter-linked peaks observed in temperate regions, peak responses in the tropics occurred at different times depending on the vaccine. The researchers excluded children who already had antibodies against the pathogens prior to vaccination, making it unlikely that recent exposure to those infections explained the findings. But it is still unclear what is driving them.

Key points

• Over the past decade, a growing number of studies have indicated that humans, too, may experience subtle seasonal shifts in immune activity, hormone levels and gene expression.
• “The really exciting finding of this paper is not about vaccination – it is that human immune function [is] different across the seasons,” says Cathy Wyse at the University of Edinburgh, UK, who wa…
• “This suggests that humans might have inbuilt seasonal timing, as is seen in animals, birds and across biology.” With research suggesting that our response to influenza vaccines follows 24-hour cir…
• The team pooled data from 96 randomised-controlled trials involving around 48,000 children who had been vaccinated against 14 infections, including measles, polio and chickenpox.
• These were conducted in different countries at different times of year, which enabled the researchers to compare seasonal and geographical differences in immunogenicity, the strength of the antibod…