2. Clinical Influenza with Negative Rapid Antigen Test (RAT). Subjects with a negative RAT test may be enrolled once the site has been approved by the Sponsor to enroll such subjects, based on documentation of an outbreak of influenza in the community. An influenza outbreak may be documented in the catchment area of the hospital via one of the following methods: 1) local confirmation of influenza A or B infection in the current influenza season by a) the institution's local laboratory, or b) the local public health system, or c) the national public health system, or d) a laboratory of a recognized multinational influenza surveillance scheme such as the European Influenza Surveillance Network (EISN); 2) prior enrollment of a RAT positive subject into this study at the same institution in the current influenza season.