Background
Viral acute respiratory illnesses (ARIs) are the most common acute illnesses in elite athletes. However, the occurrence, aetiology, and clinical manifestations of viral ARIs in athletes remain unclear.

Methods
Twenty-four elite cross-country skiers and 22 elite orienteers were followed for 12 months. Thirty-two normally exercising, healthy young adults were recruited as controls. Occurrences of ARI symptoms were collected weekly with a digital questionnaire. Nasal swabs for respiratory viruses were collected at the onset of symptoms and once monthly when asymptomatic.

Results
A significantly higher incidence density (per person per year) of ARI during the 12-month follow-up period was detected in the skiers compared to the controls (mean (SD) 3.39 (2.13) vs. 2.11 (1.98), respectively, p = 0.037) whereas the differences between the skiers and orienteers (mean (SD) 2.39 (1.07)) and between the orienteers and controls did not reach statistical significance (p = 0.053 and 0.506, respectively). The COVID-19 pandemic prevention measures and lockdown dramatically eliminated the occurrence of ARIs in all study groups. ARI episodes were shorter and milder in the orienteers (not studied in the skiers) compared to the controls (p = 0.001 and p = 0.001). A combination of international flights and participation in a competition was associated with a significant risk of an ARI episode in the skiers (p = 0.048). Rhinoviruses (54.1%) and seasonal coronaviruses (21.6%) were the most common viruses detected in all study groups.

Conclusion
The incidence of ARIs was higher among the skiers compared to the orienteers and the controls. However, ARI episodes were shorter and milder in the orienteers compared to the controls.