Cold-water immersion acutely increases inflammation without clearly improving immune function.
Cold-water immersion produces time-dependent stress and inflammatory responses, but current trials do not show a clear acute improvement in immune function. A 2025 systematic review and meta-analysis included 11 randomized studies involving 3,177 healthy adults. Cold exposure consisted of baths or showers at 15°C (59°F) or colder for at least 30 seconds.
Inflammatory markers increased immediately after cold-water immersion and remained elevated one hour later. By contrast, the meta-analysis found no significant effect on the immune-function measurements assessed immediately or one hour after exposure. Stress was lower 12 hours afterward, but not at the other measured time points.
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The review's main longer-term illness signal was a 29 percent reduction in sickness absence among people assigned to cold showers. That result came from a single large trial that found no reduction in the number of days participants actually felt ill.
The evidence base remains limited by the small number of randomized trials, small samples in most experiments, substantial differences among protocols, and limited participant diversity. Cold-water exposure clearly acts as a physiological stressor, but claims that it prevents colds or broadly 'boosts' immunity are not yet supported.