Abstract: We included 11 trials involving 904 adults, published between 1990 and 2014. Eight studies were conducted in the USA, and one each in Canada, Spain and Turkey. Sample sizes ranged from 20 to 154 participants aged between 18 and 85 years old. The proportion of female participants varied between 52% and 100%. The duration of follow-up in the studies varied from seven days to 12 months. The exercise type most prescribed for the intervention was aerobic (walking in 70% of the studies, or bicycle riding or treadmill) at least five times a week. Duration was 30 to 45 minutes at moderate intensity. Participants were supervised in 90% of the studies. For four of the primary outcomes the results did not differ significantly and all were low-quality evidence (number of ARI episodes per person per year, rate ratio 0.91 (95% confidence interval (CI) 0.59 to 1.42); proportion of participants who experienced at least one ARI over the study period, risk ratio 0.76 (95% CI 0.57 to 1.01); severity of ARI symptoms, mean difference (MD) -110 (95% CI -324 to 104); and number of symptom days in the follow-up period, MD -2.1 days (95% CI -4.4 to 0.3)). However, one primary outcome, the number of symptom days per episode of illness, was reduced in those participants who exercised (MD -1.1 day, 95% CI -1.7 to -0.5, moderate-quality evidence). We found no significant differences for the secondary outcomes (laboratory parameters (blood lymphocytes, salivary secretory immunoglobulin and neutrophils); quality of life outcomes; cost-effectiveness and exercise-related injuries). There was good adherence to the intervention with no difference between the exercise and non-exercise groups. We rated the quality of evidence for the primary outcomes as low for most outcomes using the GRADE criteria: allocation concealment was not reported and there was a lack of blinding; in addition, there was imprecision (the CI is very wide because of a small number of participants) and inconsistency, which may be due to differences in study design. 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.

Plain-language summary: A search of the major databases to July 2014 found 11 trials involving 904 participants between the ages of 18 and 85 years old, which tested the effect of exercise on acute respiratory infection symptoms. Exercise was supervised and prescribed at least five times a week, with 30 to 45 minutes of moderate-intensity activities in most studies. 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.