The lack of a global uniform surveillance case definition for RSV complicates the interpretation of surveillance data. A significant fraction (often >50%) of RSV-infected young children and elderly patients present without fever. Furthermore, RSV infection often presents with signs of apnea and / or sepsis in infants less than 6 months age.

In the WHO RSV surveillance pilot, the use of an extended SARI or an ARI case definition substantially increased the number of RSV infections detected. These definitions do not require fever to identify a suspect case. This is also important when surveillance data are used to estimate disease burden.

The RSV case definition is not intended to replace or modify the SARI or ILI definition for influenza surveillance. In practice, the physician or nurse in a sentinel hospital engaged in both RSV and influenza surveillance, screens patients with acute onset cough or shortness of breath and collects an age-appropriate respiratory specimen. Information on the presence or absence of fever is recorded in the specimen requisition form and sent to the laboratory along with the respiratory specimen. At the laboratory, all the specimens are tested for RSV, whereas those specimens from patients with fever are additionally tested for influenza.