The number of acute respiratory infections per person per year and the severity of these symptoms were similar in the exercising and non-exercising groups. Similarly, the number of people experiencing at least one acute respiratory infection and the number of symptom days in the follow-up period were similar among people who did or did not exercise. One analysis of four trials suggested that the number of days of illness per episode of infection might be reduced by exercise.
The quality of the trials was poor, which means that there might be benefit or even harm attributable to exercise.
We need further studies with fewer potential biases to understand whether exercise is able to reduce the occurrence, severity or duration of acute respiratory infections.
We cannot determine whether exercise is effective at altering the occurrence, severity or duration of acute respiratory infections. One analysis of four trials suggests that the number of days of illness per episode of infection might be reduced by exercise. The small size of the studies, risk of bias and heterogeneous populations trialled all contribute to the uncertainty. Larger studies, with less risk of bias from patient selection, blinding of outcomes assessors, reporting of all outcomes measured and with registration of study protocols, are required to settle the question.