With the current COVID-19 pandemic creating global disruption, uncertainty and with many countries enforcing society lockdowns, it is making physical activity and exercise more onerous. However, one of the few exceptions to the lockdown rules is exercise, so lets examine the evidence on exercise and immune function.

It is well known that regular bouts of exercise lasting up to 45 minutes of moderate to vigorous exercise is beneficial for immune defence, particularly in older adults and those with chronic diseases. This type of exercise is beneficial for the normal functioning of the immune system and is likely to help lower the risk of respiratory infections/illnesses. However, there is debate within the scientific community whether acute bouts of vigorous intensity exercise leads to a period of immune suppression post exercise.

There has been a long held concept in exercise immunology developed in the 1980s and 1990s called the “open window” hypothesis which proposes a J curve relationship between exercise intensity and infection risk. Which is supported by the belief that athletes who engage in high volume endurance training experience a greater incidence of Upper Respiratory Tract Infections (URTI) compared to those who are less active. Until recently this concept has remained relatively unchallenged.

The “open window” hypothesis suggests that following a prolonged (>1.5hr) and vigorous acute bout of exercise or following chronic intense training (>1.5hr on most days) there is an “open window” which results in an increased risk of opportunistic infections such as URTI’s. The three principles underpinning this concept are:

1). Infection risk increases after prolonged vigorous aerobic exercise

2). Acute bouts of vigorous exercise can lead to temporary reductions in salivary immunoglobulins resulting in higher risk of opportunistic infections

3). A period of post exercise reduction in peripheral blood immune cells resulting in a period of immune suppression.

J Curve – relationship between the risk of infection and level of exercise intensity

Recently though there has been emerging evidence suggesting this concept may be outdated. There is evidence, albeit small that indicates international athletes suffer from less URTI than national athletes. This raises the likelihood that infection susceptibility is more likely multifactorial including genetics, sleep, stress, nutrition, travel, circadian misalignment and increased exposure risks due to close proximity of crowds rather than being directly attributed to acute or chronic bouts of vigorous training. This also indicates that international athletes are potentially better supported, have access to better education helping them to improve their life-style behaviours over national athletes resulting in lower risks of infection.

Secondary to this, evidence supports the opposite of the three principles upholding the “open window” concept. With no changes seen in mucosal immunity which has previously been flagged as an indication of immune suppression. The reduction in blood immune cells (primarily lymphocytes) 1 to 2 hours post exercise reflects a transient and time dependent redistribution of immune cells to peripheral tissues resulting in a heighten state of immune surveillance and regulation leading to enhanced antibacterial and antiviral immunity, not suppression of the immune system.

Further research is needed to confirm or refute the “open window” concept however, it currently appears that the infection risk post vigorous exercise is more likely to be associated with a multitude of other factors rather than purely post exercise immune suppression. So for athletes and non-athletes the message remain the same during COVID-19, regular moderate to vigorous exercise is beneficial to enhance immune function to reduce the risk of bacterial and viral infections including URTI’s. And remember that good hygiene practices (washing hands regularly, not touching your face), physical distancing, getting good quality sleep, reducing stress levels and eating healthy wholefoods are the keys keeping your immune system in peak condition.

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