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COVID-19 Risk, Exercise and Immunity

coronavirus covid-19 cycling endurance exercise immune function immune system immunity risk running walking Feb 10, 2021
Person Running

 Why are so many athletes contracting COVID-19? This was the question my editor asked me a couple of weeks ago. In response, I explained that after exercising, our immunity is decreased. So, our risk of infection is greater following a workout. Consequently, if we are exposed to the coronavirus when in this weakened immune state, it would be much easier for us to contract COVID-19 and become ill. Turns out, I may have been wrong! Or maybe not . . . You’ll have to keep reading to find out.

Endurance Exercise Intensity and Immunity

There are two strong camps of supporters on either side of the debate about exercise and immune function. The biggest disagreement is around high intensity, long-duration exercise, like marathons and ultra-marathons. 

To illustrate, look at the J-curve that’s shown in the graph. This is a model that experts continue to use in order to describe the relationship between exercise intensity and Upper Respiratory Tract Infection (URTI) risk. As depicted, URTI risk decreases with exercise up to 60% intensity. And URTIs rise sharply with heavy exertion above 60% intensity. Just to make the graph clear, many studies categorize moderate exercise as 30-45 minutes of walking, five days a week. 

J-Curve of Exercise Load and URTI Risk

Image linked from: https://www.sciencedirect.com/science/article/pii/S2095254618301005?via%3Dihub#bib0095

Unfortunately, heavy exertion is not so clearly defined, nor is it sports specific. There is a huge difference between a one-hour run and a 100-mile run over 24-hours or a multi-day endurance event. Hence, part of the reason why there is so much controversy around the issue.  

Elite Athletes and Infections

Interestingly, despite supporting the J-curve model, the International Olympic Committee warned that the right side of the graph may not apply to the highest-level elite athletes. Malm supports this claim in his article when he states that since athletes are in top condition, their immune systems can fight infections better. He suggests an S-Curve with infections going down for elite athletes. 

Clearly, researchers have a lot more to study, including how other stressors (such as sleep, nutrition, travel, and anxiety) impact the immune system. Nieman and Wentz summarize the situation well in their 2019 review article:

“The direct connection between exercise-induced immune changes and infection risk has not yet been established, and will require long-term studies with large cohorts. More research is needed to more clearly demonstrate the linkage between heavy exertion, illness symptoms, and pathogen-based illnesses, and the relative importance of associated factors such as travel, pathogen exposure, exercise-induced immune perturbations, sleep disruption, mental stress, and nutrition support.”

Moderate Intensity Exercise Builds Immunity

However, many experts actually do agree on how moderate intensity exercise benefits immune health. Moderate exercise boosts immunity with the following mechanisms:

  • Reduces stress hormones
  • Lowers inflammation
  • Possibly diversifies the gut microbiota (a large portion of the immune system is in the gut)
  • Accelerates active immune cell exchanges between the circulation and tissues
  • Increases antioxidants that improve immune system function
  • Delays immunosenescence, immune dysregulation that happens with aging
  • The list goes on and on.

Your Exercise Routine and COVID-19

Here are some exercise tips to follow:

Get Started or Restarted

Regardless if you are exercising or not right now, this is the time to move! Any exercise is better than no exercise. Pick something you enjoy and make it fun! 

Be Consistent

Exercise has a cumulative effect. In other words, you need to move every day. Most of the exercise and immunity studies assume you are doing regular endurance exercise for 45 minutes, 5 days a week. The beneficial effects on the immune system become stronger the more frequently you exercise.

Progress Gradually

Generally, you should increase by no more than 10% per week. As an example, if you are walking for 10 minutes now, you would increase to 11 minutes next week. This progressive approach allows the body to adapt without becoming overwhelmed.

Limit Exposure

Follow all the recommendations about mask wearing, hand cleaning and social distancing. In addition, just in case you pushed it a bit too hard and may have reduced immunity, consider skipping a visit to any public areas such as stores, gas stations, etc. after your workout.  In other words, do your errands before your exercise session. 

Boost Immunity Through Healthy Habits

  • Watch your nutrition! Find a plan that works for you and makes you feel good. You can refer to the USDA nutrition guidelines that were updated in December 2020. 
  • Reduce stress! There are many ways to reduce your fear, anxiety and worry. Start with these blogs:   

5 Lessons from the Mindfulness-Based Stress Reduction (MBSR) Course

Start Meditating to Reduce Stress: 3 Easy Practices

So, there’s my long-winded answer to the question, Why are so many athletes contracting COVID-19? It appears that I was right and possibly wrong in my off-the-cuff response. As science continues to evolve, the correct answer is always the same. It Depends.  

Stay Healthy - Be Safe - Keep Moving!



Simpson RJ, Campbell JP, Gleeson M, et al. Can exercise affect immune function to increase susceptibility to infection?. Exerc Immunol Rev. 2020;26:8-22.

Campbell JP, Turner JE. Debunking the myth of exercise-induced immune suppression: Redefining the impact of exercise on immunological health across the lifespan. Front Immunol. 2018;9:648. doi:10.3389/fimmu.2018.00648

Quinn, E. (2020, August 20) Can Too Much Exercise Decrease Your Immunity? verywellfit retrieved from: https://www.verywellfit.com/exercise-and-immunity-3120439#citation-5

Nieman DC, Wentz LM. The compelling link between physical activity and the body's defense system. J Sport Health Sci. 2019;8(3):201-217. doi:0.1016/j.jshs.2018.09.009

Malm C. Susceptibility to infections in elite athletes: the S-curve. Scand J Med Sci Sports (2006) 16(1):4–6. doi:10.1111/j.1600-0838.2005.00499

de Sousa CV, Sales MM, Rosa TS, Lewis JE, de Andrade RV, Simões HG. The Antioxidant Effect of Exercise: A Systematic Review and Meta-Analysis. Sports Med. 2017 Feb;47(2):277-293. doi: 10.1007/s40279-016-0566-1. PMID: 27260682

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