Abbreviations: ILI, influenzalike illness; RSV, respiratory syncytial virus; SARI, severe acute respiratory infection.

^aPatients were classified according to ILI and/or SARI case definitions by retrospectively looking at their recorded symptoms in each season during the whole study period.

^bNumber of patients for whom symptoms were available.

^cPatients meeting ILI criteria (percentages calculated using numbers in row 1 as denominators).

^dNumbers of patients meeting ILI or SARI case definitions (percentage calculated using numbers in row 1 as denominators).

^eNumbers of patients who met SARI criteria but not ILI criteria and thus did not have a polymerase chain reaction (PCR) sample available, because the active surveillance network used to collect data (Valencia Hospital Surveillance Network for the Study of Influenza and Other Respiratory Viruses [VAHNSI]) was initially restricted to patients with ILI. Percentages were calculated using numbers in row 3 as denominators; this allowed us to estimate the percentage of “SARI-not-ILI” cases adding to the pool of ILI/SARI hospitalizations, which would be lost if the case definition were restricted to ILI only.

^fNumbers of patients with ILI with a laboratory (reverse-transcription PCR)–confirmed RSV infection. The positivity rates were calculated from the total numbers of ILI hospitalizations (row 2).

^gEstimated numbers of RSV cases based on patients meeting ILI/SARI criteria. Because PCR samples were not available in patients meeting the SARI-not-ILI definition (row 4), we assumed that the RSV positivity rate was constant between ILI and ILI/SARI. Applying this assumption we multiplied the ILI-RSV positivity rate (row 5) and the total number of ILI/SARI hospitalizations to obtain the final number of RSV cases among ILI/SARI hospitalizations.