Training through winter supports your immune system. Here's the research.
The "immune open window" theory got the post-exercise dip wrong. A 2018 review explains why.
For a long time, the prevailing thinking was that a hard training session left you immunocompromised for a few hours afterwards. The so-called "open window" theory said that elevated training stress depressed immune function and made you more vulnerable to infection, especially during the colder months. It was the science quoted whenever someone justified backing off in winter.
A 2018 review in Frontiers in Immunology revisited the evidence for that claim and wasn't convinced.
What was being measured as immune suppression in the original studies was actually immune cell redistribution. After a training session, immune cells leave the bloodstream and move into peripheral tissues such as mucosal surfaces, the lungs, and the gut, where they patrol for pathogens. On a blood draw, that movement looks like a drop. In reality, the immune system is doing more, not less.
The fair caveat is that most of the original studies counted blood markers rather than tracking actual rates of illness. They also ignored the variables that genuinely affect immune function: sleep, travel, nutrition, and chronic stress. The 2018 paper is a better reading of older data, not a fresh trial.
The takeaway is straightforward. Regular training through the cold months supports immune function rather than running it down. Skipping the session because it's winter doesn't protect your health. It just sets the body back another week.
Keep training. The immune system can handle it.
ReferenceCampbell JP, Turner JE. Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan. Frontiers in Immunology, 2018. DOI: 10.3389/fimmu.2018.00648