Characteristics of observational studies included in the qualitative analysis.

Legend: N = number, LOS = Length of stay, ICU = admission to the intensive care unit, ILI: influenza-like illness, CAP: community acquired pneumonia; IC: immunocompromised; C-R: cardio-respiratory ARI: acute respiratory infections; NPAs: Nasopharyngeal aspirates; NPs: nasopharyngeal swabs; Ops: oropharyngeal swabs; TS: throat swabs: RT-PCR: real-time PCR. Studies, which included commercial assays (RT-PCR) were labeled as RT-PCR kit, with the mention of the manufacturer.