Naturally the immune system of the human body is a deeply intricate network of cells and molecules designed to keep us (the host) free from infection and disease. Physical exercise has been known to have a deep impact on the “normal” functioning of the immune system. Having higher age and gender adjusted scores for cardio-respiratory fitness and performing regular moderate to intense exercise has been shown to improve immune responses to vaccination, minimize chronic low-grade inflammation, and improve numerous immune markers in many disease states including cancer, HIV, cardiovascular disease, diabetes, cognitive impairment and obesity. The continuing COVID-19 pandemic has raised several questions regarding how exercise can protect us from infection by boosting immunity. This is becoming increasingly significant since many of us have restricted access to the gyms and parks where we would usually carry out exercise and physical activity regimens. Compounding this problem are the known negative effects of social isolation and restriction on immunity. Glucocorticoids such as cortisol are elevated during periods of isolation and confinement and can inhibit many critical functions of our immune system. When we are stressed, our T-cells ability to multiply in response to infectious agents is noticeably reduced, as well as the ability of certain effector lymphocytes like Natural Killer Cells (NK-cells) and killer T-Cells (CD8+ T-cells) to recognize and kill cells in our body that have become cancerous or have been infected with viruses. It is also vitally important that our immune cells maintain their ability to redeploy so that they may patrol vulnerable areas in or body (e.g., the upper respiratory tract and the lungs) to prevent viruses and other pathogens from gaining a foothold. This process is also important to minimize the impact of the virus and to expedite viral resolution should we become infected.
Each exercise session, mainly those of whole-body dynamic cardio-respiratory exercise, instantly mobilizes literally billions of immune cells, especially those cell types that are capable of carrying out effector functions like recognizing and killing virus-infected cells. First the mobilized cells enter the blood compartment from marginated vascular pools, the spleen and the bone marrow before trafficking to secondary lymphoid organs and tissues, specific to the lungs and the gut where increased immune defense might be required. Those immune cells mobilized with exercise are prepared and looking for a fight. They are frequently recirculated between blood and tissue, causing increase host immune surveillance, which, in theory, makes us more impervious to infection and more well equipped to handle any infectious agent that has secured a foothold. During exercise we also release various proteins that can help us maintain immunity, particularly muscle derived cytokines such as (Interleukin-6) IL-6, Interleukin-7 (IL-7) and Interleukin-15 (IL-15). Cytokine IL-6 has been shown to coordinate immune cell trafficking toward areas of infection, while IL-7 can promote the construction of new T-cells from the thymus and IL-15 helps to maintain the peripheral T-cell and NK-cell compartments, all of which work together to increase our resistance to infection. For older adult’s exercise is especially important as they are more susceptible to infection in general and have also been identified as a particularly exposed population in the COVID-19 epidemic.
That being said, it is of critical importance that we make conscious effort to maintain if not increase our activity levels within recommended guidelines. Physical activity has not only a positive direct effect on the cells and molecules in our immune system, but it has also been known to counteract the adverse effects of quarantine and isolation stress on several aspects of immunity. Even though there is no scientific data currently regarding the effects of exercise on coronaviruses, there is evidence that exercise can protect us (the host) from various other viral infections including but not limited to influenza, rhinovirus (another cause of the common cold) and herpes viruses such as Epstein-Barr (EBV), Varicella-zoster (VZV) and herpes-simplex-virus-1 (HSV-1). Studies have shown that moderate-intensity fitness training during a live influenza infection seemed to protect mice from death. It also fostered a positive immune cell composition and cytokine shift in the lungs that was associated with prolonged survival. Understanding how exercise can lessen the negative effects of stress to maintain immune function, particularly during prolonged periods of quarantine and isolation will continue to be an area of research. Many studies have been done showing that astronauts who had greater pre-flight cardiorespiratory fitness and skeletal muscle endurance before a six-month mission were not only less likely to reactivate Epstein-Barr and Varicella-Zoster during their mission however copies of Epstein-Barr viral DNA were also minimized indicating that their ability to infect others is also reduced. Those with lower pre-flight fitness levels seemed to return especially those with the lowest levels of cardio-respiratory conditioning seemed to be more likely to have reactivated a virus during the mission. Since viral reactivation is a global indicator that our immune system has been diminished, which, in this context, could be largely due to the stressors associated with quarantine and isolation. This research indicates that exercise, along with the effects it can have on cells and molecules in our immune system, may be an effective stress-induced countermeasure to help maintain positive immune function and lower the risk of infection.
Currently, the greatest risk of COVID-19 infection is exposure. It is paramount that we find creative ways to exercise while maintaining social distancing and proper hygienic countermeasures. While exercise may not prevent us from becoming infected if exposed, it is likely that keeping active will boost our immune system to help minimize the deleterious effects of the virus, ameliorate our symptoms, expedite our recovery times and lower the likelihood that we can infect others with whom we come into contact. This is merely my intuition, but I do expect a large body of exercise immunology research to follow after this pandemic so that we can provide more specific exercise recommendations as they pertain to infection risk and control in both healthy and medical populations.