Higher risk of upper respiratory tract infection post marathon running: when physical exercise becomes a threat to the immune system

Background: Several studies have reported that marathon runners have a higher risk of upper respiratory tract infections (URTI) post marathon than non-exercising controls. However, other studies did not find a higher risk of URTI in the same participants before and after a marathon, precluding a conclusive consensus. Besides the between-subjects effects, another important confounding factor in these results is the different pre and post follow-up time to track URTI. Objectives: Identify by meta-analysis whether a marathon Running increases the risk of URTI, adjusting the follow-up time to track URTI. Data sources: We searched for articles using MEDLINE (PubMed), Embase, Scopus, Web of Science, the Cochrane Library, and EBSCOhost, combining the marathon and respiratory infection descriptor synonyms, on 1st December 2022. Eligibility criteria: The PICOS framework included human population, comparison between pre and post marathon running, of URTI symptoms (assessed from one to 4 weeks), in noncontrolled intervention studies. Data Synthesis: Because follow-up was longer before the marathon in many studies, we adjusted the number of subjects with infections before marathon to the equivalent post-marathon follow-up duration. There was 18% higher incidence of URTI post-marathon (OR 1.18 95%CI [1.05-1.33], p= 0.005) in a very consistent meta-analysis (I2 = 0%, p = 0.69), with no risk of publication bias (Egger test p-value = 0.82) for the 7 studies included. The main issues with quality of the studies were bias in measuring the outcome, bias in classification of intervention (participation in the marathon) and time-varying confounding (corrected for analysis), and therefore the quality of evidence was moderate (GRADE approach = 3). Limitations: The need for follow-up time adjustment is a limitation, since the number of URTI recorded could be different if the original studies had used the same follow-up time pre and post marathon. The subjectivity of the URTI assessments is another limitation in this field. Conclusions: There is an increased risk of URTI post marathon running and research on this topic to understand mechanisms might support runners to find efficient interventions to reduce this risk. Protocol: Protocol registration on in the International Prospective Register of Systematic Reviews (PROSPERO): CRD42022380991.