The Moscow and Berlin marathons are behind us, and by midweek some finishers will start feeling the familiar signs: a scratchy throat, a blocked nose, aching all over. Runners have long called this the post-marathon “immune dip”. The scientific literature even had an “open window” theory — that after long, hard exercise immunity “crashes” for a few hours or days. Let's see what is left of that theory.

Where the idea came from

Los Angeles, the late 1980s. David Nieman and colleagues (Journal of Sports Medicine and Physical Fitness, 1990) sent questionnaires to Los Angeles Marathon entrants and received responses from 2311 runners.

Among finishers who had not been ill before the race, 12.9% (236 of 1828) reported a cold in the week after it.
Among equally experienced runners who had entered but did not start, the figure was 2.2% (3 of 134).
The odds ratio, as calculated by the authors, was 5.9. It looks like an almost sixfold increase in risk.
Separately, those who ran 97 km a week or more had 2 times higher odds of getting ill in the two months before the race than those who ran less than 32 km.

At the same time, immunologists saw that the number of lymphocytes in the blood drops sharply right after prolonged exercise. And so the theory took shape: hard exercise → a temporary “hole” in immunity → infection.

What the meta-analysis says

The early studies had a problem: illnesses before and after the race were counted over different periods. For example, two months before the marathon and one or two weeks after it.

André Sardeli and colleagues from the universities of Campinas (Brazil), Birmingham (UK) and Illinois (USA) (Exercise Immunology Review, 2024) pooled 7 studies and aligned the observation periods before and after the race.

The risk of an upper respiratory tract infection after a marathon was 18% higher than before it: odds ratio 1.18 (95% CI 1.05–1.33).
The results of the studies were remarkably consistent (I² = 0%), with no signs of publication bias.
The quality of evidence was moderate: symptoms were mostly self-reported, without lab tests.

The upshot: the increase in risk is real, but it is not “sixfold” — more like “one fifth higher”.

Was there ever an “open window”?

In 2018 John Campbell and James Turner (Frontiers in Immunology) published a review with a telling title: “Debunking the Myth of Exercise-Induced Immune Suppression”. Their arguments:

Lymphocytes do not die after exercise; they redistribute — leaving the blood for the tissues, including the lungs and mucous membranes, where immune defence is actually needed. For the immune system this is more of a mobilisation than a crash.
A marathon means crowds. The expo, the start village, public transport, flights, hotels. A marathon runner's chance of meeting a virus in those days is objectively higher than that of someone sitting at home.
Poor sleep and stress before the race lower your defences on their own.

One more detail — not every post-race “cold” is an infection. Spence and colleagues (Medicine & Science in Sports & Exercise, 2007) took swabs from athletes with cold symptoms and found a pathogen in only about a third of cases. The rest may be airway irritation from long periods of mouth-breathing cold or dry air, allergies or inflammation.

Limitations

Almost all the data are self-reports: “a scratchy throat” does not equal “a viral infection”.
The meta-analysis included only 7 studies, and the comparison was “before versus after” in the same people, with no group that did not run a marathon.
It is impossible to separate the contribution of the exercise itself from that of crowds, flights and poor sleep.

How to apply this

Your main protection comes in the week before the race, not after it. Hands, crowds, transport. Do not linger at the expo longer than you need to.
Sleep. Poor sleep before the race is one of the few factors you control.
Eat carbohydrates during the race. Carbohydrates during prolonged exercise reduce the release of stress hormones such as cortisol, and a cortisol surge is one explanation for the temporary shifts in immune markers after a race. There is little direct evidence yet that this means fewer colds, but you need the carbohydrates for performance anyway — how to plan your marathon fuelling.
Do not try to make up for lost time. If you catch a cold in the last days of your taper, an extra hard session will not help — how the taper works.
After the finish — rest, not a “recovery” 10K on day two. Keep the first week to light activity.
The “above the neck” rule: if your symptoms are only in the nose and throat, an easy run is acceptable. Fever, body aches, a cough “in the chest” — no running until you have recovered: exercising with a viral infection carries a risk of myocarditis.

The bottom line

The classic study: colds in 12.9% of Los Angeles Marathon finishers versus 2.2% of those who did not run.
A meta-analysis of 7 studies adjusted for the length of follow-up: the risk is 18% higher — a real but modest effect.
The “open window” theory is being challenged: lymphocytes redistribute into the tissues, and the main factors are crowds, flights, poor sleep and stress.
The best prevention is hygiene and sleep in the week before the race, carbohydrates during it and an easy week afterwards.

Sources: Sardeli A.V., de Araujo R.B., Woods J.A., Lord J.M., Chacon-Mikahil M.P.T. “Higher risk of upper respiratory tract infection post marathon running: when physical exercise becomes a threat to the immune system”. Exercise Immunology Review, 2024;30:6–13. PMID: 39094181 · Nieman D.C., Johanssen L.M., Lee J.W., Arabatzis K. “Infectious episodes in runners before and after the Los Angeles Marathon”. Journal of Sports Medicine and Physical Fitness, 1990;30(3):316–328. PMID: 2266764 · Campbell J.P., Turner J.E. “Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan”. Frontiers in Immunology, 2018;9:648. DOI: 10.3389/fimmu.2018.00648 · Spence L., Brown W.J., Pyne D.B. et al. “Incidence, Etiology, and Symptomatology of Upper Respiratory Illness in Elite Athletes”. Medicine & Science in Sports & Exercise, 2007;39(4):577–586. DOI: 10.1249/mss.0b013e31802e851a