Acute respiratory infections are the leading cause of acute diseases and are the third largest cause of mortality in the world. They are responsible for at least 6% of worldwide disability and have a high annual economic burden. They can be caused by several infectious agents, such as viruses or bacteria, and are classified as either upper respiratory tract infections, involving the area from the nostrils to the vocal cords or as lower respiratory tract infections, involving the area under the vocal cords which includes the lungs. According to the World Health Organization, acute respiratory infections are defined as infections of the respiratory tract lasting less than 30 days and the clinical manifestations range from common cold to pneumonia. The clinical course of acute respiratory infections might be unpredictable, especially in developing countries. Treatments includes rest, analgesics and, if indicated, antibiotics. Prevention strategies for acute respiratory infections are important, and they range from the promotion of healthy habits, such as regular physical activity, and, if available, vaccination, to the cessation of unhealthy habits such as smoking. In particular, a well-planned and structured exercise program might be very useful in improving individuals' health status and in the management of acute respiratory infections by reducing their occurrence, severity and duration.

The aim of this Cochrane systematic review was to evaluate the effectiveness of exercise for altering the occurrence, severity, or duration of acute respiratory infections in the general population. The intervention was exercise compared with no exercise or usual care. Primary outcomes evaluated were number of acute respiratory infection episodes/year, proportion of participants who experienced at least one acute respiratory infection over the study period, severity of acute respiratory infection symptoms, number of symptom days in the 12 weeks of follow-up period and number of symptom days per episode of illness. Secondary outcomes were laboratory-assessed immune parameters, quality of life, costs to the patient, exercise-related injuries and adherence to the intervention group.

This is an update of a review published in 2015. Current review identified 14 studies involving 1377 adults, including subjects with chronic respiratory conditions, aged from 18 to 85 years. None of the included studies compared exercise to usual care. Characteristics of most of the included studies were aerobic exercise (walking, bicycling, treadmill, or a combination of these) prescribed at least three times a week, with 30 to 45 minutes of moderate-intensity activities. Duration of follow-up was of minimum 1 week-to maximum 36 weeks (median 12 weeks).

The authors found low-quality evidence that exercise reduce the number of episodes of acute respiratory infections, and the proportion of participants who experienced at least one episode over the study period. The authors also found low-quality evidence that exercise reduce severity of acute respiratory infection symptoms, the number of symptom days in the 12 weeks of follow-up period, and the number of symptom days per episode of illness. For the secondary outcomes of the study, the authors found a moderate to low quality evidence that exercise makes no difference in terms of laboratory-assessed immune parameters; moderate quality of evidence that exercise makes no difference in terms of quality of life and costs to the patient, and low quality evidence that exercise makes little or no difference on exercise-related injuries and adherence to the intervention group.

The level of evidence for all outcome measures was downgraded to low due to limitations in study design, implementation and imprecision of results.

Further randomized controlled trials including an appropriate patient selection, blinding of outcome assessors, reporting of all outcomes analyzed, and registration of study protocols, are needed. Moreover, future studies should include comparison of exercise versus usual care, which have not been reported to date in the literature.

Comment by Sara Liguori