Regular exercise can potentially help to protect you from infections such as COVID-19, as long as you don’t exercise too much. In one study, compared to people who did not exercise regularly, those who exercised had a reduced risk of becoming infected by COVID-19. They had a:
• 36 percent reduced risk of hospitalization from severe COVID-19
• 43 percent reduced risk of death from COVID-19
Those who followed guidelines recommending at least 150 minutes of moderate activity or 75 minutes of vigorous activity per week gained the most protection against COVID-19, but exercising regularly at less than that appeared also to help protect against infection (Am J of Preventive Medicine, Dec 14, 2022).

A review of 16 other studies of people who exercised during the COVID-19 pandemic found that exercising regularly was associated with reduced risk for becoming infected and also for suffering severe infections (Br J Sports Med, Aug 22, 2022;56(20):188–1193).

Your immune system helps to protect you from infections by producing cells and chemicals called cytokines that attack and kill invading germs (Ann Transplant, 2005;10(4):43-8). Exactly the same immune cells and cytokines are produced when you have an infection and when you have damaged tissues in your body, so you produce antibodies and cells to kill germs when you exercise. When you contract muscles repeatedly during exercise, you damage the Z-lines in muscle fibers, which is what causes delayed onset muscle soreness (DOMS) four to 24 hours after an intense workout. DOMS signals muscle damage that causes your immune cells to release the cytokines that help to kill germs (Int J Environ Res Public Health, 2021 Feb; 18(3): 1261). Your immune cells and chemicals are activated by the muscle cell damage caused by exercise, and we have extensive evidence that exercising after vaccinations for the flu or COVID-19 increases antibody response to these viruses (Brain, Behavior, and Immunity, May 2022;102:1-10). Elite athletes doing regular training show more pronounced induction of vaccine-specific T-cells and antibodies after tetravalent influenza vaccination, compared to controls (Brain Behav Immun, 2020 Jan;83:135-145).

Exercise can help to protect you from infections as long as you don’t exercise too much. Since exercise diverts your immune system to repair damaged Z-lines in muscle fibers, your immune system may also be distracted from protecting you during and for a short period after you finish exercising, so you may be at increased risk for infection for a few hours after you finish exercising (Med Sci Sports Exerc, July 2000;32(7 Suppl):S406-11). Extreme exercise and overtraining can hinder your immune system by causing so much Z-line muscle damage that your immune system is busy trying to repair muscle damage so it may not do an adequate job of protecting you from an infection. Marathon runners often report getting sick after races (Sports Med, 2007;37(4-5):412-5), and lab mice that run to exhaustion become more susceptible to the flu than sedentary animals (Brain Behav Immun, 2008 Nov;22(8):1152-5).

How to Use Exercise to Help Protect You from Infections
Almost all competitive athletes follow a stress and recover training program and so should you, even if you don’t compete in sports. To make muscles stronger, you need to exercise intensely enough to damage them, and to increase your ability to take in and use oxygen, you need to exercise hard enough to become short of breath. However, if you don’t follow your hard workouts with easy ones, you can suppress your immune system to increase risk for developing infections such as colds and increase your chances of injuring yourself.

In elite soccer players, taking intense workouts lowered salivary levels of Immunoglobulin A (IgA), an antibody that helps to prevent germs from entering your tissues and bloodstream (J of Strength and Condition Res, Sept 2016;30(9):2460–2469). The same reduction in salivary IgA occurred in football players after games (J Sports Sciences, August 2015). It has also been shown in ordinary exercisers (who are not competitive athletes) when they take intense workouts on consecutive days without allowing time for recovery (Frontiers in Physiology, June 28, 2016). IgA is found in tears, saliva, and secretions in your stomach, intestine, colon, bronchial tubes, and skin, as a first line of defense to prevent germs from entering your bloodstream. Having prolonged low levels of IgA can predict infections in the future. The researchers also found that consecutive intense workouts reduce levels of anti-inflammatory cytokines, which are proteins produced by white blood cells that act to dampen inflammation (“turn down your immune response”). These are the chemicals that control the strength of your immune response to invading germs.

Other studies show that consecutive days of intense exercise also reduce the number of white blood cells that defend you from invading germs. All of these changes increase your chances of developing an overtraining syndrome in which your muscles hurt and you feel exhausted all the time. Overtraining suppresses your immune system to increase your risk for infections, excessive muscle damage and delayed recovery from workouts.

How Muscles Heal after Intense Exercise
Muscles are made up of hundreds of thousands of individual fibers. To make muscles stronger, you have to damage them. Then when they heal, they become larger and stronger. You can tell that you are damaging a muscle during exercise by the soreness you feel in that muscle on the next day. If you take an easy workout when your muscles feel sore, your muscle can become stronger. If you take an intense workout on sore muscles, you can tear them and become injured.

The healing of muscles damaged by intense exercise is governed by your immune system:
• The exact same cells and cytokines that kill germs initiate the healing process.
• When you feel sore after intense exercise, your immune system goes into high gear to heal the damaged muscle tissue.
• If you do another intense workout when you feel sore and are trying to heal, your body senses that you are causing further damage, so it reduces the amount of white blood cells and proteins that initiate muscle healing.
• When you overtrain and have prolonged muscle soreness, your immune system can attack your healthy tissues in the same way that it attacks germs. This is called inflammation and can lead to the many “auto-immune” diseases such as chronic fatigue syndrome or inflammatory bowel disease. Inflammation can also lead to a heart attack because your overactive immunity can punch holes in arteries, which starts the formation of plaques.

Listen to Your Body
After intense exercise, you should expect to feel sore because of the muscle damage, which is good for you. If you do another intense workout when you are trying to heal, your body senses that you are causing further damage. Your body produces the same white blood cells and proteins that you use to kill germs and heal damaged muscles.
• Try to alternate harder workouts with easy recovery ones on consecutive days.
• If you are training properly, expect to feel sore every morning when you wake up. If your muscles feel better after a 5-10 minute warm up, take your planned workout.
• If you don’t feel better during your warm up, go home because continuing to exercise will only increase your chances of injuring yourself.

My Recommendations
Exercise helps to protect you from infections, but if you exercise too hard or too much, you can suppress your immune system and increase your risk for infections. Set up your exercise program so you exercise a little harder on one day, so on the next day, you feel mild delayed onset muscle soreness (DOMS). Then take off or go very easy for as many days as it takes for your muscles to feel fresh again. The number of recovery days depends on the intensity of the previous workout.

CAUTION: Intense or prolonged workouts can cause a heart attack in susceptible individuals. If you have any questions or concerns, check with your doctor.