Immune Suppression After Marathon: What the Open Window Theory Actually Shows
Table of Contents
Key Takeaways: Immune Suppression After Marathon
- The “open window” theory suggests that prolonged endurance events like marathons may create a temporary period of altered immune function, but modern research debates whether this represents true immune suppression or normal immune redistribution and recovery.
- Early studies found higher rates of self-reported upper respiratory symptoms after marathon and ultramarathon events, but later research suggests that not all post-race symptoms represent true infections.
- Marathon running clearly causes measurable short-term immune changes, including neutrophilia, lymphocyte redistribution, cytokine responses, cortisol changes, and temporary alterations in mucosal immunity.
- These post-marathon immune changes should not automatically be viewed as harmful. Many responses may reflect normal physiological adaptation, inflammation, and tissue repair after extreme physical stress.
- The biggest practical contributors to illness around race events may include multiple factors: training load, recovery, sleep disruption, travel, crowd exposure, and individual health status — not only the “open window” itself.
- For most healthy athletes, concerns about temporary immune changes after a marathon should not discourage endurance exercise. The focus should remain on sensible recovery, adequate sleep, nutrition, and gradually returning to training.
Introduction: Immune Suppression After Marathon
In clinical practice, patients often try to explain why they became ill. Sometimes the suspected trigger is cold weather, sometimes air travel, and sometimes returning to training or work too soon. In many cases, this is partly speculation; in other cases, the context may genuinely matter.
Marathon runners are one group where the question is more than just casual guessing. For decades, runners have been warned about a possible “danger zone” in the days after a race — a period when the immune system may be temporarily altered and susceptibility to infection has been debated. This idea, often called the “open window” theory, has influenced advice on post-race nutrition, supplements, and recovery.
But the modern picture is more nuanced than the simple claim that “marathons suppress immunity.” In my view, the more useful clinical question is not whether every sore throat after a marathon proves immune failure, but what the open window theory actually shows, what it does not show, and how runners should interpret post-race symptoms without overreacting.
The Evidence for Immune Suppression After Marathon: Where the Open Window Theory Comes From
The open window hypothesis originates largely from the work of David Nieman and colleagues in the late 1980s and 1990s. In one of the most frequently cited studies in the field, of 1,828 Los Angeles Marathon participants who had not reported an infectious episode in the two months before the race, 236 (12.9%) reported an infectious episode in the week after the race compared to only 3 of 134 (2.2%) similarly experienced runners who had signed up but did not participate — an odds ratio of 5.9 (95% CI 1.9–18.8) [1].
A separate ultramarathon study, reviewed by Campbell and Turner [2], found a comparable pattern: roughly one-third of runners completing a 56 km ultramarathon self-reported upper respiratory tract infection (URTI) symptoms within two weeks, compared to about half that rate in home-based controls.
Epidemiological data summarized in the same review suggests that shorter running events such as 5 km, 10 km, and half-marathons did not appear to increase self-reported URTI incidence to the same extent [2], suggesting the effect — if real — depends on race duration, and that immune suppression after a marathon is a distinct phenomenon from the response to shorter training sessions.
This led to the “J-shaped curve” model of exercise and infection risk, and — combined with an observed rise and subsequent fall in circulating immune cells after prolonged exercise — the term “open window,” commonly described as lasting between 3 and 72 hours, depending on the immune measure [5].
This is also clinically relevant because the relationship between exercise and infection risk is not simply “more exercise equals more illness.” The same body of work behind the J-shaped curve suggested that people doing moderate amounts of exercise may report fewer upper respiratory tract infections than sedentary individuals, while very prolonged or heavy exertion may create a different short-term situation [5].
In practice, this matters when advising patients after an infection. I often find that people are unsure whether they need to avoid exercise “just in case.” Once the clear signs of infection have settled, a cautious return to movement can often be more helpful than prolonged inactivity. The point is not to rush back into maximal training, but to avoid turning normal recovery into unnecessary deconditioning. For many patients, the harms of staying inactive for too long may be more relevant than the theoretical risk of returning to light activity too early.
Assessing Immune Suppression After Marathon: What Happens in the Blood
Regardless of how the infection-risk debate is ultimately resolved, the acute physiological changes underlying immune suppression after marathon completion are well documented and reproducible.
Leukocytosis and neutrophilia. Marathon completion reliably produces race-induced leukocytosis attributable to neutrophilia, alongside peripheral neutrophilia accompanied by an increase in band neutrophils and monocytosis [10]. In one marathon cohort, plasma IL-6 responses correlated strongly with the increase in band neutrophil count, consistent with a possible role for IL-6 in mediating bone marrow release of neutrophils [10] — though a strong correlation does not, on its own, establish that IL-6 is the sole or dominant driver of this response.
Lymphopenia. In parallel, marathon runners show a race-induced significant decrease in the number of lymphocytes, memory helper T cells, naive/memory/activated cytotoxic T cells, natural killer (NK) cells, NKT cells, and B1 cells, alongside an increase in activated helper T cells and regulatory T cells [9]. Across the wider literature, the classic biphasic response to a vigorous, sustained bout of exercise (lasting roughly 45–60 minutes or more) is a dramatic lymphocytosis during exercise — with NK cells rising up to 10-fold and CD8+ T cells around 2.5-fold — followed by a nadir approximately 1–2 hours after exercise, when lymphocyte counts fall below pre-exercise levels before normalising within 24 hours [2]. This lymphocyte nadir is the physiological signature most often cited as evidence of immune suppression after a marathon.
Cortisol and cytokines. A marathon triggers a substantial acute-phase cytokine response. Post-race increases occur in suPAR, CD163, white blood cells, the pro-inflammatory cytokines IL-6 and IL-8, and the anti-inflammatory cytokine IL-10 [7]. Among these, the increases in suPAR, CD163, IL-8, and IL-10 specifically are more pronounced after a full marathon than a half-marathon [7], and the 3-hour post-race increases in these markers correlated significantly with concurrent changes in TNF-α and cortisol [7]. IL-6 in particular stays elevated longer than other markers: kinetics data show prolonged elevation of serum IL-6 for at least 24 hours after a marathon, unlike the faster normalisation seen after shorter races [8].
Salivary IgA. Mucosal antibody secretion also falls transiently. In a study of 98 runners across two competitive marathon races, salivary IgA secretion rate fell by 25% from pre-race levels by 1.5 hours post-race [6].
For context on the practical, clinically relevant side of what a post-race blood panel looks like — the transaminase, CK, and hematology shifts alongside these immune changes — see the post-marathon blood work article.
From a clinical perspective, I would be careful not to interpret these blood changes only as “damage” or “immune failure.” A marathon is a major physiological stressor. It involves prolonged muscle work, tissue strain, metabolic stress, and a normal inflammatory repair response afterwards. In that sense, many of the changes seen in the blood after a marathon can also be understood as part of the body’s recovery and repair process.
This is important for patients and athletes because the practical message should not become exaggerated. I would not advise someone to avoid marathon running simply because there may be a short-term increase in illness susceptibility after the race. The broader health effects of regular training and an active lifestyle are usually far more important than the temporary immune-marker changes seen after a single demanding event. In practice, I would focus on the whole picture: recovery, sleep, nutrition, symptoms, training history, and whether the athlete is returning gradually rather than forcing hard training immediately after the race.
Is Immune Suppression After Marathon Real? The Scientific Controversy
This is where exercise immunology has genuinely moved since the 1990s, and it is worth presenting both sides, because the field itself is actively divided on whether “immune suppression after marathon” is even the right way to describe what’s happening.
The case for reinterpreting the open window. A widely cited 2018 review argued that the classical pillars of the hypothesis are weaker than commonly assumed. The authors pointed out several problems with the original epidemiological studies: in one study using nasopharyngeal and throat swabs, of 37 self-reported URTI episodes among athletes, only 11 (30%) had a positive laboratory diagnosis [2], raising the possibility that many “infections” reported after marathons are non-infectious symptoms — for example allergy, asthma, non-specific mucosal inflammation, or airway trauma from increased ventilation or cold-air exposure — rather than genuine pathogen-driven illness.
The same review also reframed the post-exercise lymphopenia itself. Rather than reflecting immune suppression, it argues this represents a heightened state of immune surveillance and regulation, driven by a preferential mobilisation of lymphocytes to peripheral tissues such as the gut and lungs — where pathogens are more likely to be encountered — rather than a loss of immune competency [2]. Supporting this reinterpretation, the review notes that typically fewer than 10% of lymphocytes undergo post-exercise apoptosis, which is a relatively small contribution given the 30–60% fall in circulating lymphocyte numbers [2]. It also highlights that the salivary IgA evidence is inconsistent: a field study of Comrades Marathon (86.5 km) ultrarunners found salivary IgA levels in the 4 weeks before and 2 weeks after the race were unrelated to the incidence of self-reported URTI [2].
The case for retaining the concept. This reinterpretation is itself disputed within the field. A 2020 consensus debate article was explicitly framed around the question: “Can exercise affect immune function to increase susceptibility to infection?” The authors described this as a fundamental and contentious issue in exercise immunology that remains unresolved and requires further research to determine whether exercise-induced immune changes alter infection risk [3]. Elsewhere, it has been noted that both innate and acquired immunity are often reported to decrease transiently in the hours after heavy exertion, typically by 15–70%, with prolonged heavy training sessions decreasing immune function and potentially providing an “open window” for opportunistic infections [4]. In other words, real, measurable immune suppression after a marathon is not a settled question — it is an active scientific debate between researchers who interpret the same lymphopenia and cytokine data through different lenses.
What both sides agree on. Importantly, there is more common ground than the framing suggests. Both camps agree that infection burden among high-level endurance athletes is a genuine practical problem, and that confounding factors around mass-participation events likely explain much of the epidemiological signal historically attributed to post-marathon immune suppression. The 2018 review points out that around 40% of individuals attending a large crowded gathering like the Hajj self-report a URTI, and that air travel is a significant predictor of illness symptoms in athletes [2], implicating hypobaric hypoxia, sleep disruption, fatigue, altered diet, dehydration, and psychological stress rather than the exercise bout itself. This matters practically: a marathon runner’s post-race illness risk may have as much to do with travel, crowd exposure, and sleep disruption around race weekend as with true immune suppression after the marathon itself.
My practical reading of this debate is fairly conservative. Whether the post-marathon immune effect is interpreted as true suppression, redistribution, or something in between, I do not see it as a factor that usually changes everyday clinical decision-making for an otherwise healthy athlete.
Patients will continue to speculate about why they became ill after a race: the weather, the flight, the hotel, the crowd, poor sleep, or the race itself. Some of those factors may genuinely matter. But in most practical situations, I would be cautious about placing too much weight on the “open window” alone. For the individual runner, the more useful approach is usually to look at the whole context — symptoms, recovery, sleep, travel, training load, and gradual return — rather than treating a marathon as a uniquely dangerous immune event.
Conclusion: Immune Suppression After Marathon
The open window theory is best understood as a useful but limited framework. Marathon running can clearly produce short-term changes in immune cells, inflammatory markers, cortisol, and salivary IgA. Older epidemiological studies also suggested that some runners report more upper respiratory symptoms after very prolonged races. But these findings do not mean that every post-race sore throat is caused by clinically meaningful immune suppression.
From a practical clinical perspective, I would treat the “open window” as one possible part of the picture, not the whole explanation. Travel, crowd exposure, poor sleep, dehydration, heavy training load, and normal post-race tissue repair may all matter. For an otherwise healthy runner, the main message is not to fear marathon running, but to respect recovery: sleep, eat, rehydrate, monitor symptoms, and return to harder training gradually.
In my view, the overall health value of regular training remains strongly positive. The open window theory may help explain why some athletes feel vulnerable after extreme endurance events, but it should not turn a temporary physiological response into unnecessary anxiety. Most clinical decisions still come down to the basics: how the athlete feels, whether infection symptoms are present, and whether recovery is progressing normally.
Bibliography
[1] https://pubmed.ncbi.nlm.nih.gov/2266764/
[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC5911985/
[3] https://pubmed.ncbi.nlm.nih.gov/32139352/
[4] https://pubmed.ncbi.nlm.nih.gov/29637836/
[5] https://pmc.ncbi.nlm.nih.gov/articles/PMC7176256/
[6] https://pubmed.ncbi.nlm.nih.gov/11774070/
[7] https://pmc.ncbi.nlm.nih.gov/articles/PMC5005625/
[8] https://pmc.ncbi.nlm.nih.gov/articles/PMC8893166/
