The Science of Running and Immunity


by Dr. Reed Ferber, Ph.D. CAT(C)

The Covid-19 pandemic has affected all of us in many different and impactful ways.  We are now living in an unknown world where the idea of contracting the Covid virus is omnipresent and we are all thinking about how we can best protect ourselves and how we can deal with added stress.  One way that many runners manage their stress levels is by, well, running.  So in this article I thought I would talk about how running affects your immune system and how recent research can guide your workouts.

Overall, your immune system is remarkable at defending you against disease-causing microorganisms. Is it possible to boost your immune system and how much running/exercise is ideal in order to do so?

Overall, exercise immunology researchers agree that there is a ‘sweet-spot’ between the relationship of immunity response and exercise dose, defined as both the duration and intensity of exercise. Regular amounts of exercise reduces your risk to infection and boosts your immune system compared to inactivity.  This should be clear.  If you engage in some form of regular moderate exercise respiratory tract infections are generally reduced by 50%.  However, engaging in high-intensity exercise can put you at higher risk of contracting a respiratory tract infection since your immune response is now reduced.

Specifically, everyone has certain cells called T-lymphocytes, or T-cells, which are important for orchestrating an immune response to viral infections. Research has shown that high-intensity exercise can change T-cell activity and result in an inability to reach an adequate inflammatory reactive state.  In other words, heavy exercise reduces your immune system’s ability to mount an inflammatory response, which weakens your ability to defend against viruses, and increases your risk of infection.

The question now is what defines ‘moderate’ and what is ‘high intensity?’  There are several academic definitions but let’s keep things simple.  The Breath Test is a simple way to monitor the intensity of your running: if are unable to hold a conversation, and are out of breath, you’re working at a high intensity.  Another is by monitoring your heart rate.  You can calculate your maximum heart rate (using the equation 220 minus your age) and then determine your target heart rate zone. To work at a high intensity, you would train between 80-90% percent of your maximum heart rate.  Moderate exercise intensity is generally between 50% to 70% of your maximum heart rate.

Overall, runners need to run, especially in our current times.  I would suggest that you stick to 1 hour or less, at a conversation pace (if you are fortunate enough to be able to run with someone) and/or an average heart rate around 60% of your max heart rate. Occasional bouts of more high intensity periods are fine as research shows that it is sustained high intensity that negatively affects T-cell activity the most.

Next, considering that we have may more time to training and run, a critical factor is to stay injury-free so let’s review some solid research from a research group in Luxembourg who conducted a 22-week study where they followed 264 recreational runners.  Overall, 33% of the runners sustained an injury, which is what you would expect based on past research.  These authors reported that cross-training resulted in a 25% reduced risk of injury.  In other words, runners who spend more time in other sports (such as cycling or lifting weights), but still maintain their weekly mileage over fewer run-days, showed a reduced risk of injury.

Other factors and habits can also help to boost your immune system.  Eating healthy, getting lots of sleep, and washing your hands frequently are all important factors to consider.

I hope all of you and your families are safe and healthy.

Dr. Reed Ferber is the director of the Running Injury Clinic, a world leader in running-related research and 3D gait analysis technology. For more information, visit

Twitter: @DrFerber |

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