Maybe something, but not what is claimed: the Dutch RCT (3,018 people) found 30 to 90 seconds of cold at shower's end cut sickness absence from work 29% - while illness days themselves did not significantly change.
People got sick at the same rate but worked through it. That is a vitality effect, not an immune upgrade: no evidence cold exposure raises immune function enough to prevent infection. Skip the cold shock with heart conditions. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Do cold showers boost your immune system?
The honest answer is "maybe something, but not what the influencers claim" - and unusually, there is a real randomized trial to argue with. The Dutch cold-shower RCT (3,018 people, 30 to 90 seconds of cold at the end of a normal shower for 30 days) found a 29% reduction in sickness absence from work in the cold-shower groups - but no significant reduction in illness days themselves. Read that split carefully: people got sick at the same rate but went to work anyway, or felt well enough to. That is an interesting vitality effect - not an immune-system upgrade. There is no evidence cold showers raise white-cell counts in a way that prevents infection, and the "boosts immunity" claim outruns the data. What cold showers plausibly do: train the stress response (the controlled shock is a daily hormetic dose), improve alertness reliably, and feel like discipline, which has its own effects. They are safe for most healthy people - skip the cold shock with heart conditions, where the sudden cardiovascular jolt is a real risk. If you enjoy them, enjoy them; just bank the benefit as alertness and grit, not an infection shield.