Runners were warned of an immune “open window,” but crowds and travel may matter more

Key takeaways

• After the 1987 Los Angeles Marathon, 12.9% of finishers reported an infection in the following week, versus 2.2% of runners who skipped the race for other reasons, but none of those infections were confirmed by lab tests¹.
• In 2018, two University of Bath researchers argued that the drop in blood immune cells after hard exercise mostly reflects cells moving into tissues, and that crowds, travel and short sleep may explain race-week colds¹².
• A 2021 review that pooled studies of hundreds of thousands of adults linked regular activity with a 31% lower risk of community-acquired infections and a 37% lower risk of dying from one⁷.
• Among 48,440 Southern Californians with COVID-19 in 2020, 10.5% of consistently inactive patients were hospitalized, versus 3.2% of those who regularly met activity guidelines, and the gap held after adjusting for age and illnesses⁸.
• Exercise may give vaccines a small assist, especially in older adults, but the studies are small and their results mixed¹⁷¹⁰.

The runners who finished the 1987 Los Angeles Marathon had more than sore legs to report. When researchers led by D.C. Nieman surveyed them, 12.9% said they had come down with an infection in the week after the race¹.

Among runners who had planned to race but withdrew for reasons other than illness, only 2.2% did¹. By our math, that’s about 107 more sick runners per 1,000.

Findings like that built the “open window” idea. In the hours after a long, hard effort, the theory goes, the immune system lowers its guard and germs slip in³.

In 2018, two researchers at the University of Bath in England called that idea a myth¹². The debate they started changed how scientists read the evidence. It still isn’t fully settled⁴.

If you walk, run, swim or lift to stay well, the outcome matters to you. Below, we cover what a workout does to your immune cells, what the race studies really showed, and how regular movement relates to infections and vaccines.

The ‘open window’ rested on surveys and a dip in blood cells

Two kinds of evidence built the open-window idea. The first was surveys like the Los Angeles one, in which runners reported their own symptoms¹.

The second came from blood tests. During vigorous exercise lasting roughly 45 to 60 minutes or more, natural killer cells, front-line cells that attack infected cells, can rise up to tenfold in the blood. Killer T cells rise about 2.5-fold¹.

Then the numbers crash. One to two hours after the workout, lymphocytes, the family of white blood cells that includes T cells and natural killer cells, fall 30% to 60% below where they started¹. They usually return to normal within 24 hours¹.

To many scientists, that dip looked like a gap in the defenses. In a 2010 study, 10 elite male cyclists rode hard for two hours³. Their natural killer cell counts stayed low from four to eight hours afterward. Some white cells were worse at engulfing germs as late as the next day³.

The authors argued that changes like these may help explain why athletes in heavy endurance training report more colds³. Nieman and a colleague later summed up the 1980s research the same way: heavy exertion was tied to short-lived immune changes, higher inflammation markers and more upper respiratory infections⁵.

Two researchers argued the window was a myth

In 2018, John Campbell and James Turner of the University of Bath published a review in Frontiers in Immunology. Its title began “Debunking the Myth of Exercise-Induced Immune Suppression”¹². They made three main points.

The infections weren’t confirmed. None of the self-reported illnesses in the classic race studies were checked with lab tests¹. When one study ran lab tests on athletes who reported symptoms, only 11 of 37 episodes, about 30%, turned out to be infections¹.

That same study shows why the debate didn’t end there. Over five months, lab-confirmed infections struck 19% of elite athletes (6 of 32), compared with 10% of recreational athletes and 10% of people in the comparison group¹.

Distance and setting matter. Shorter races, from 5K runs to half marathons, didn’t raise reported colds¹. Among runners preparing for the 86.5-kilometer Comrades Marathon, symptoms peaked four weeks before the race, not after it¹.

Big events also pack people together. About 40% of people attending the Hajj, the pilgrimage to Mecca, report a respiratory infection, with no marathon involved¹. Air travel predicted illness symptoms in athletes¹. Long trips, public transport, disrupted sleep, poor food, cold and wet weather and nerves can all pile up around race day¹².

The missing cells aren’t dead. Fewer than 10% of the lymphocytes that vanish from the blood die¹. Campbell and Turner argued that stress hormones such as adrenaline push the rest out of the bloodstream and into tissues, which they read as stepped-up patrolling rather than weakness¹.

After a workout, immune cells may be on patrol

Think of your blood as a highway and your lungs, gut and bone marrow as the neighborhoods it serves. Rodent studies that tagged T cells with fluorescent markers found that after exercise, the cells left the blood and turned up in the gut, lungs and bone marrow¹. In mice, propranolol, a drug that blocks adrenaline’s signal, stopped this movement¹.

This is the catch. No one has directly shown the same redeployment in people, the review notes¹. It’s a strong hypothesis, built on animal tracking and the timing of human blood counts.

Some human evidence fits it. Natural killer cells released into the blood during exercise were better at killing myeloma and lymphoma cancer cells in lab dishes¹.

There’s a hint about the long run, too. One study compared 125 adults aged 55 to 79 who had kept cycling seriously through life with 75 inactive people their age¹. The cyclists had more “naive” T cells, fresh cells ready for germs they haven’t met, and more new graduates of the thymus, the gland where T cells train¹.

A single snapshot can’t prove cycling caused that difference. A study of young triathletes found lower thymus output, the opposite direction¹. Our article on immunity across the lifespan explains why the thymus shrinks with age.

Experts still disagree about elite athletes

In 2020, eight exercise immunologists, including Campbell, Turner and Nieman, laid out both sides in a formal debate in Exercise Immunology Review⁴.

They agreed on a lot. Regular, moderate exercise sessions of up to about 45 minutes help immune defense, especially in older adults and people with chronic diseases⁴. Infection is a real problem for top athletes: before major events, it is second only to injury as a reason for lost training days⁴.

They disagreed on whether athletes catch more respiratory infections than other people, and whether exercise itself is to blame⁴. Anxiety, broken sleep, travel, germ exposure, nutrition gaps and extreme heat or cold act on the same immune pathways, so their effects are hard to untangle⁴. The group’s verdict: infection risk has many causes, and exercise’s own role remains unresolved⁴.

The International Olympic Committee (IOC) reached a practical answer without settling the theory. Its 2016 expert consensus concluded that poorly managed load is a significant risk factor for illness in athletes⁶. “Load” meant more than miles: sudden jumps in training, crowded competition schedules, psychological pressure and travel all counted⁶.

Under-eating for your training is another route to more frequent illness, as our article on when exercise does harm explains.

Regular movement tracks with fewer serious infections

For people who aren’t elite athletes, the bigger question is whether everyday activity helps. The largest summary we found is a 2021 review in the journal Sports Medicine⁷.

It pooled six studies that followed 557,487 adults. Regularly active people had a 31% lower risk of community-acquired infections, the kind caught in daily life rather than in a hospital⁷. In four studies of 422,813 people, they had a 37% lower risk of dying from an infection⁷.

The review reported only relative risks, so we can’t translate those figures into cases per 1,000 people. Lab measures pointed the same way. Activity programs raised CD4 “helper” T cells by about 32 cells per microliter of blood, and raised IgA, an antibody in saliva that guards the mouth and throat⁷.

A 2023 study of more than 500,000 U.S. adults, summarized by CDC, found that people who met both the aerobic and muscle-strengthening guidelines were about half as likely to die of flu or pneumonia as people who met neither⁹. CDC still calls the immune benefit of activity an area of emerging research⁹.

COVID-19 offered a sharper test. At Kaiser Permanente in Southern California, nurses and medical assistants ask at each visit how many days a week a patient exercises moderately or harder, and for how long. The weekly total goes into the chart as an “exercise vital sign“⁸. A team led by Robert Sallis studied 48,440 adults diagnosed with COVID-19 in 2020 whose activity had been recorded at least three times in the two years before the pandemic⁸.

Of the consistently inactive patients, 10.5% were hospitalized and 2.4% died. Of those who consistently got 150 minutes or more a week, 3.2% were hospitalized and 0.4% died⁸. By our math, that’s about 73 more hospital stays and 20 more deaths per 1,000 patients, before accounting for age and health.

After adjusting for age, other demographics and known medical risks, inactive patients had about 2.3 times the odds of hospitalization and 2.5 times the odds of death⁸. In the authors’ comparison, inactivity was a stronger risk factor than smoking, obesity, diabetes, high blood pressure, heart disease or cancer. Older age and a past organ transplant were among the few that ranked higher⁸.

The limits matter. Activity was self-reported, and the study can’t rule out that people already in poorer health were both less active and sicker⁸. Kaiser Permanente’s community benefit funds paid for part of the work⁸.

Exercise may give a vaccine a small assist

Whether to get a particular vaccine is your decision, ideally made with a clinician you trust. If you do get one, can movement help it work? The evidence says a little, sometimes.

Pooled across six trials with 497 people, adding a regular exercise program raised vaccine antibody levels slightly⁷. Older adults may gain the most. Campbell and Turner counted at least eight studies in which active older adults, typically over 60, responded better to vaccines than inactive peers¹.

A single workout before a shot is less predictable. Six of eight trials found a boost, but the details vary¹. Young adults responded better to 5 of 11 pneumonia strains, but only when given a half dose of vaccine. At the full dose, exercise added nothing¹.

Women aged 55 to 75 who did moderate aerobic exercise before a shot responded better; men didn’t¹. A 45-minute walk didn’t help middle-aged women, and resistance exercise didn’t change flu antibodies in adults around age 73¹.

In 2022, Iowa State University researchers led by kinesiology professor Marian Kohut tested exercise after the shot¹⁰. Right after a flu shot or a first dose of the Pfizer-BioNTech COVID-19 vaccine, volunteers spent 90 minutes pedaling a stationary bike or walking briskly¹⁰¹¹. The goal was pace, not distance: a pulse of roughly 120 to 140¹⁰.

Over the next four weeks, they made more antibodies than people who sat or went about their day¹⁰. A 45-minute workout didn’t help. The exercise didn’t increase side effects after the COVID-19 shot¹¹. Kohut called the results preliminary, and the university’s report didn’t say how many people took part¹⁰.

All of these studies measured antibodies, not who later got sick¹⁷¹⁰. Our sleep article covers what research says about sleep around a shot, and our article on vaccine schedules explains who recommends what.

The immune-friendly training plan

This plan is our own design, built from the sources above. Find your situation and use the rating to decide how much weight to give it.

Your situation	What the evidence suggests	What to do	Rating
Staying active through cold and flu season	Regular activity is linked to fewer serious infections and infection deaths79	Aim for 150 minutes of moderate activity a week plus strength work on 2 days; short sessions count1213	Strong evidence of a link
Training for a big race	Race-week colds may owe as much to crowds, travel, nerves and short sleep as to the miles124	Guard sleep, eat well and keep up hand hygiene around the event1	Promising but early
Ramping up training	Sudden jumps in load raised illness risk in the IOC review6	Build distance and intensity gradually612	Promising but early
Heavy travel	Air travel predicted athletes’ illness symptoms1	Count travel days as part of your training load and plan sleep around them (our suggestion)16	Promising but early
Feeling sick	We found no human trials showing when exercise helps or hurts during an infection	Use our illness article for a step-by-step return, and rebuild gradually12	Not enough evidence
Getting a vaccine you’ve chosen	Active people, especially older adults, respond slightly better; 90 minutes afterward raised antibodies in one preliminary study1710	Keep your usual routine; easy movement afterward is reasonable if you feel well (our suggestion)11	Promising but early
Age 65 or older	Active older adults responded better to vaccines in several studies1	Follow the older-adult guideline, including balance work, as your health allows14	Promising but early

If you’re sick, our article “Ease back in after a cold, because with long COVID, pushing can backfire” covers when to rest, how to return and which heart warning signs mean stop and get checked. If you have a chronic condition or worrying symptoms, the federal guidelines say to work with a health care provider on which activities suit you¹².

How strong is the evidence?

Claim	Rating	What it rests on
Regular activity is linked to fewer infections and infection deaths	Strong evidence of a link	Pooled cohort studies of hundreds of thousands of people; cause not proven79
Inactive people fared worse with COVID-19	Promising but early	One large health-system study with self-reported activity8
One hard workout opens a window for infection	Not enough evidence	Self-reported symptoms, few lab-confirmed infections, experts still split134
The post-workout dip means cells are moving into tissues	Promising but early	Rodent tracking; not shown directly in people1
Crowds, travel and short sleep drive race-week illness	Promising but early	Mass-gathering and athlete travel data; hard to separate from exercise14
Sudden jumps in training load raise athletes’ illness risk	Promising but early	IOC expert consensus6
Regular exercise improves vaccine response	Promising but early	Small pooled effect, strongest in older adults17
Lifelong exercise slows immune aging	Not enough evidence	Snapshot studies with mixed results1

Questions readers ask

The bottom line

The “open window” grew from runners’ own reports of colds and a real, short-lived dip in blood immune cells. Those cells may be moving into tissues rather than disappearing, and the colds may owe a lot to crowds, travel and short sleep. Experts still debate whether hard exercise itself raises infection risk in elite athletes.

For everyone else, the picture is clearer. Regular activity tracks with fewer serious infections and may help vaccines work a bit better, especially later in life. Build up gradually, rebuild slowly after illness, and keep moving through cold and flu season.

Keep exploring: medicinal mushrooms immunity and stress and immune system.

Related reading

• Your immune system works more like a team than a volume knob, and some habits help it
• Ease back in after a cold, because with long COVID, pushing can backfire
• When exercise does harm, and why a famous too-much-jogging study rested on two deaths
• Your immune system makes you sleepy on purpose, and sleep returns the favor
• Exam stress slowed students’ vaccine response, and long-running stress weighs on immunity most

Sources

1. Campbell 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;9:648. frontiersin.org
2. University of Bath. “Research Debunks ‘Myth’ That Strenuous Exercise Suppresses the Immune System” (release on Campbell JP, Turner JE, Frontiers in Immunology, 2018). ScienceDaily, April 21, 2018. sciencedaily.com
3. Kakanis MW, Peake J, Brenu EW, et al. “The Open Window of Susceptibility to Infection After Acute Exercise in Healthy Young Male Elite Athletes.” Exercise Immunology Review, 2010;16:119-137 (Bond University record). research.bond.edu.au
4. Simpson RJ, Campbell JP, Gleeson M, et al. “Can Exercise Affect Immune Function to Increase Susceptibility to Infection?” Exercise Immunology Review, 2020;26:8-22 (University of Arizona record). experts.arizona.edu
5. Nieman DC, Wentz LM. “The Compelling Link Between Physical Activity and the Body’s Defense System.” Journal of Sport and Health Science, 2019;8(3):201-217 (DOAJ record). pubmed.ncbi.nlm.nih.gov
6. Schwellnus M, et al. “How Much Is Too Much? (Part 2) International Olympic Committee Consensus Statement on Load in Sport and Risk of Illness.” British Journal of Sports Medicine, 2016;50:1043-1052 (University of Pretoria repository). repository.up.ac.za
7. Chastin SFM, Abaraogu U, Bourgois JG, et al. “Effects of Regular Physical Activity on the Immune System, Vaccination and Risk of Community-Acquired Infectious Disease in the General Population: Systematic Review and Meta-Analysis.” Sports Medicine, 2021;51(8):1673-1686 (Edinburgh Napier University repository). napier-repository.worktribe.com
8. Sallis R, Young DR, Tartof SY, et al. “Physical Inactivity Is Associated With a Higher Risk for Severe COVID-19 Outcomes: A Study in 48 440 Adult Patients.” British Journal of Sports Medicine, 2021. pubmed.ncbi.nlm.nih.gov
9. Centers for Disease Control and Prevention. “Healthy Habits: Enhancing Immunity” (summarizing Webber BJ, Yun HC, Whitfield GP, British Journal of Sports Medicine, 2023;57:1231-1237). CDC, last reviewed April 7, 2026. cdc.gov
10. Iowa State University. “Exercise Post-Vaccine Bumps Up Antibodies, New Iowa State University Study Finds.” February 14, 2022. research.iastate.edu
11. Neuroscience News. “Exercise Post-Vaccine Bumps Up Antibodies” (reprint with abstract of Kohut M, et al., “Exercise After Influenza or COVID-19 Vaccination Increases Serum Antibody Without an Increase in Side Effects,” Brain, Behavior, and Immunity, 2022). neurosciencenews.com
12. U.S. Department of Health and Human Services. Physical Activity Guidelines for Americans, 2nd edition. HHS, 2018. odphp.health.gov
13. Centers for Disease Control and Prevention. “Adult Activity: An Overview.” CDC, last reviewed December 20, 2023. cdc.gov
14. Centers for Disease Control and Prevention. “Older Adult Activity: An Overview.” CDC, last reviewed December 4, 2025. cdc.gov