Seasonal body rhythms may change vaccine strength

Seasonal rhythms – A large review of randomized vaccine trials suggests the strength of children’s immune responses can vary with the time of year—and may differ by latitude. Researchers are now left with an urgent question: should vaccination timing be adjusted, or would that r

For families counting down to the next round of childhood vaccines, the timing is usually fixed by schedules and school calendars—not by biology. But a growing body of research is pointing to a more uncomfortable possibility: the seasons may subtly change how the immune system responds.

In one study, researchers pooled data from 96 randomized-controlled trials involving around 48,000 children.. Those children had been vaccinated against 14 infections, including measles, polio, and chickenpox.. The trials took place in different countries at different times of year. giving the team a rare chance to compare seasonal and geographical differences in immunogenicity—the strength of the antibody response triggered by vaccination.

The findings were blunt.. “We found that there is indeed a seasonal immune response. ” said Laura Barrero Guevara at New York University. describing what stood out as the most exciting part: a latitudinal gradient.. In temperate regions, the stronger immune response showed up during winter, in both the northern and southern hemispheres.. The pattern matches what researchers would expect if day length—known as photoperiod—were shaping the body’s seasonal timing.

But as the focus moved closer to the equator, the immune picture became messier.. In the tropics. vaccine responses still fluctuated across the year. and for some vaccines—including rotavirus and polio—the seasonal swings were larger.. Yet peaks arrived at different times depending on the specific vaccine. breaking the clean “winter peak” pattern seen in temperate regions.

To reduce the chance that these effects were simply driven by recent exposure to infection. the researchers excluded children who already had antibodies against the pathogens prior to vaccination.. That choice makes the seasonal signal harder to dismiss—though it doesn’t solve the mystery of what causes it.

One team member. Matthieu Domenech de Cellès at the Max Planck Institute for Infection Biology in Berlin. explained why their initial hypothesis didn’t fully fit.. The team had considered a seasonal extension of circadian rhythms driven by photoperiodic cues.. But if that were the main driver. it would have implied a lower amplitude of seasonality in tropical regions than in temperate ones.. “This is not what we found. ” he said. adding that other mechanisms—or a combination of photoperiodism and other mechanisms—may be at play.

The idea that immune function changes with the seasons isn’t new. In 2020, Cathy Wyse at the University of Edinburgh—who wasn’t involved in the current study—reported seasonal fluctuations in inflammatory markers and several types of immune cell, with some peaking in winter and others in spring.

And beyond immune cells. another line of work has pointed to seasonal shifts in the body at the level of gene activity.. A study led by Manuel Irima at the Centre for Genomic Regulation in Barcelona found seasonal patterns in gene expression across multiple human tissues. including hormone-producing regions of the brain and testes. as well as many immune-related genes.. Irima said he thinks the new results may be related to those observations. even if he doesn’t believe researchers are close to a mechanistic explanation.

Wyse framed the most important takeaway slightly differently.. “The really exciting finding of this paper is not about vaccination,” she said.. “It is that human immune function [is] different across the seasons.” To her. that suggests humans may carry an inbuilt seasonal timing system.. Animals, birds, and fish use one—she argued—but humans have been harder to demonstrate.

That system is thought to reside in the hypothalamus. the same brain region that houses the suprachiasmatic nucleus. which coordinates circadian rhythms.. Animals living nearer the equator also have the machinery. Wyse said. but their annual rhythms are often weaker because day length changes less across the year.. Their biology may then become more tied to other environmental cues such as food availability or the start of rainy seasons.

The seasonal reach may extend well beyond immunity.. Earlier this year. Timothy Hearn at the University of Cambridge and David Whitmore at University College London reported that births in the UK followed striking seasonal patterns for much of the 20th century.. Historically. births peaked in spring. but the pattern shifted abruptly in the mid-1970s after the widespread availability of the contraceptive pill.

Hearn added that while evidence for seasonal biology in humans is growing harder to dismiss. it remains difficult to untangle whether the rhythms reflect an intrinsic biological calendar or simply the overlapping effects of changing environments.. “Season is a term that catches a bundle of correlated environmental exposures. ” he said. “and is further complicated by accompanying changes in infections. diet. activity. sleep and social behaviour.”

Even if the immune response to vaccination really does rise and fall with the seasons. a critical gap remains: antibody strength does not automatically translate into meaningful differences in vaccine effectiveness.. Wyse stressed that families shouldn’t interpret the findings as permission to adjust vaccination schedules.

“If you think. ‘OK. well. I’m going to be vaccinated in the winter because that’s better. ’ so you put off getting vaccinated for a month. that actually might be a higher risk than waiting for a tiny improvement. even if there was one. ” she said.. “Time will tell whether there is any clinical benefit to vaccination at different times.”

For now, her message is clear: “At the moment, there is not enough evidence for that.”

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