Exercise and Immunity in Cold and Flu Season

IMMUNE HEALTH · ACTIVE LIVING

When colds start making the rounds at work and at home, it is natural to wonder whether your workouts are shoring up your defenses or wearing them down. The research offers a reassuring answer, along with a few sensible caveats.

The link between exercise and immunity has been studied for decades, and the picture has grown clearer and more encouraging. This guide explains what regular activity appears to do for your immune system, why an old worry about hard workouts has largely been set aside, how sleep, stress and vaccines fit in, and how to decide whether to train when you feel a cold coming on. It is written for the months ahead, when respiratory bugs peak.

What the Research Says About Exercise and Immunity

The broadest summary of the evidence comes from a 2021 systematic review and meta-analysis by Chastin and colleagues in Sports Medicine. Pooling six large studies with more than 557,000 adults, the authors found that people with higher habitual physical activity had a 31 percent lower risk of community-acquired infectious disease. Across four studies covering more than 422,000 people, they also found a 37 percent lower risk of dying from infectious disease.

Those figures come from observational research, so they show an association rather than proof that exercise alone made the difference. Still, the same review pooled controlled trials and found measurable shifts in immune markers among people who joined exercise programs, along with a modestly stronger antibody response to vaccination. The authors concluded that regular moderate-to-vigorous activity is associated with lower infection risk and a sturdier early line of immune defense.

Harvard Health Publishing frames it more simply: like a healthy diet, exercise contributes to general good health and therefore to a healthy immune system. In other words, no single workout supercharges your defenses overnight. The benefit appears to come from being active week after week, season after season.

What Happens in Your Body During and After a Workout

Exercise mobilizes immune cells. During a session, some of them, especially natural killer cells, can rise up to tenfold in the bloodstream, according to researchers at the University of Bath who reviewed the field in Frontiers in Immunology in 2018.

After the session ends, some immune cell counts fall, occasionally below where they started, for several hours. For years that dip was read as a sign of weakened immunity. The Bath authors, John Campbell and James Turner, argued that these cells are not being destroyed. Instead, they appear to move into tissues such as the lungs, where infections are more likely to begin, which amounts to heightened surveillance rather than a stand-down.

The Bath team concluded that exercise most likely improves immune competency across the lifespan rather than undermining it, a useful thought on a gray morning when skipping the walk feels tempting.

The “Open Window” Idea, Revisited

A long-standing theory held that hard or prolonged exercise opens a temporary window of lowered immunity, during which infections can take hold more easily. Much of the support traced back to 1980s surveys of marathon runners, who reported more cold symptoms after big races.

In their review, Campbell and Turner concluded that “it is a misconception to label any form of acute exercise as immunosuppressive.” They pointed to other explanations for illness around big events: large crowds, long-distance travel that disrupts sleep, an inadequate diet, getting cold and wet, and psychological stress.

The practical lesson is reassuring. A challenging session is nothing to fear during cold season. What deserves attention is everything around it: sleep, eating enough, stress and time spent in packed rooms when bugs are circulating.

Why Staying Active Matters More as the Immune System Ages

Immune function changes over the decades. Harvard Health notes that as we age, the immune response becomes less capable, which contributes to more infections, and it ties part of that shift to a decline in T cells. Researchers call this gradual change immunosenescence.

Here the evidence is still developing, but it points in a hopeful direction. The Bath review suggested that regular physical activity “might limit or delay ageing of the immune system.” That word “might” matters, because this is a promising possibility rather than a settled finding. Even so, it adds one more reason to keep moving through your 40s, 50s and beyond, alongside the better-established benefits for your heart, muscles and bones.

If you want a concrete target, the Physical Activity Guidelines for Americans call for at least 150 minutes a week of moderate-intensity aerobic activity plus muscle-strengthening activity on at least two days. Our weekly blueprint for how much exercise you need shows how to fit those pieces into a real schedule.

Sleep, Stress and Vaccines Complete the Picture

Exercise works best as one part of a wider routine. Harvard’s list of immune-friendly habits includes not smoking, eating plenty of fruits and vegetables, exercising regularly, getting adequate sleep, keeping stress in check, staying current on vaccines and maintaining a healthy weight. It also cautions that, so far, there is no evidence that products marketed for immunity actually bolster it, and that megadoses of single vitamins do not help.

Sleep deserves special attention. In a 2012 study published in the journal SLEEP, researchers followed 125 healthy adults aged 40 to 60 through a hepatitis B vaccine series. Participants who averaged fewer than six hours a night were 11.5 times more likely to be left unprotected by the vaccine than those who slept more than seven hours, while sleep quality made no measurable difference. If training is crowding out your rest, our guide to exercise and sleep explains how to make the two work together.

Stress matters too, both on its own and through the way it chips away at sleep and recovery. Harvard notes that scientists are still working out exactly how psychological stress affects immune responses, so treat it as a sensible priority rather than a proven lever. Our piece on stress and workout recovery offers practical ways to keep a demanding season from undoing your progress.

Finally, the CDC recommends that everyone 6 months and older get a flu vaccine every season, with rare exceptions, and it describes September and October as generally good times to be vaccinated. The agency also stresses that people with certain chronic health conditions should get a flu shot. Your doctor or pharmacist can tell you which vaccines make sense for you this year.

Should You Exercise When You Have a Cold?

Mayo Clinic offers a simple test often called the above-the-neck rule. If your symptoms are limited to a runny nose, nasal congestion, sneezing or a minor sore throat, exercise is generally fine, though you should reduce the intensity and length of the session. Walking instead of running is the classic example.

Symptoms below the neck are a different story. Mayo advises skipping exercise with chest congestion, a hacking cough or an upset stomach, and not exercising at all with a fever, fatigue or widespread muscle aches. If you have COVID-19 or another contagious illness, avoid working out around other people.

What you notice	A sensible response
Runny nose, congestion, sneezing or a minor sore throat	Light activity is generally fine; make the session shorter and easier
Chest congestion or a hacking cough	Skip exercise until the symptoms ease
Upset stomach	Rest and focus on fluids
Fever, fatigue or widespread muscle aches	No exercise; let your body recover
Any contagious illness	Stay away from classes and shared gym spaces

When you start to feel better, ease back in gradually. Mayo points out that a few days off will not erase your fitness, and that checking with your doctor is wise whenever you are unsure. Seek medical care promptly for chest pain, trouble breathing or a fever that will not settle.

A Practical Week for Cold and Flu Season

None of this calls for a special immune program. The aim is a steady, moderate routine you can keep through shorter days and busy calendars. Here is one way to arrange a week that lands near the federal targets without draining your reserves.

Day	Activity	Why it helps
Monday	Brisk walk, 30 minutes	Daylight and easy aerobic work to start the week
Tuesday	Full-body strength session	Covers one of your two weekly strength days
Wednesday	Brisk walk or bike ride, 30 minutes	Moderate effort you can hold a conversation through
Thursday	Rest or gentle mobility	A good night to protect an early bedtime
Friday	Full-body strength session plus a 15-minute walk	Completes your strength work for the week
Saturday	Longer walk, hike or ride, 45 to 60 minutes	The week’s longest aerobic session
Sunday	Easy walk of 20 to 30 minutes, or rest	Keeps you moving without adding fatigue

If you are new to exercise, start with less and build over several weeks. And if you have heart disease, diabetes, kidney disease, a chronic lung condition, new symptoms, a recent injury or surgery, or you are pregnant, talk with your doctor before starting or stepping up your training. Our guide to walking for health is a friendly place to begin.