This study is a randomized, double-blind, placebo-controlled phase III clinical trial. The trial participants include index participants (IP) and household contacts (HHC). A total of 536 index participants (the first person in the household diagnosed with influenza) and 858 household contacts (family members living with the index participant) are planned to be enrolled. After the index participants and their household contacts have signed informed consent form and passed screening eligibility, randomization will be performed at the household level in a 1:1 ratio, stratifying by the time from onset of the index participant's initial influenza symptoms to signing of informed consent form (\<24 hours, ≥24 hours), index participant's viral subtype (influenza A, influenza B), and household contact's age (\<12 years, ≥12 years). Household contacts will be assigned to either the Sebaloxavir marboxil group or the placebo group. The study objective is to compare the prophylactic efficacy and safety of Sebaloxavir marboxil tablets/oral suspension versus placebo in household contacts (HHCs).
1. The index participant (aged ≥18 years), or the index participant (aged <18 years) together with their legal guardian, understands and voluntarily provides written informed consent form (ICF). The index participant must be ≥1 year of age at the time of ICF signing, with no gender restriction.
2. The index participant must be the first case of influenza virus infection in the household during the current influenza season.
3. The index participant must have a positive result from a rapid influenza diagnostic test (RIDT) or polymerase chain reaction (PCR) assay.
4. At the time of HHC randomization, the interval from the onset of the index participant's initial influenza symptoms to informed consent must be ≤48 hours. Symptom onset is defined as: first axillary temperature ≥37.5°C, or the presence of at least one other influenza symptom.
1. Participants who are diagnosed with bacterial or other non-influenza virus infections at screening requiring systemic antibacterial or antiviral treatment;
2. Participants with suspected hypersensitivity to the active ingredients or excipients of oseltamivir phosphate oral suspension/capsules.
1. Aged ≥2 years at the time of randomization, with no gender restriction;
2. Has been living in the same household with the index participant for at least 48 hours at the time of randomization;