Study to Assess the Prophylactic Efficacy and Safety of Sebaloxavir Marboxil Tablets/Oral Suspension for Influenza Prevention in Household Contacts Aged ≥2 Years.

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).