This is a randomized, double-blind, multicenter, parallel-group, placebo-controlled study designed to evaluate the clinical efficacy of BXM for the reduction of direct transmission of influenza A/B from OwH IPs to their HHCs. For this household study, IPs with influenza will be randomized to receive BXM or placebo, and their HHCs will be repeatedly tested for influenza virus and assessed for influenza symptoms during the next 9 days. The total number of new HHC infections (symptomatic and asymptomatic) and total number of new HHC infections associated with symptoms will be key measures used to evaluate the “virological transmission” and “symptomatic transmission” endpoints. Approximately 1,130 IPs with influenza and approximately 2,030 evaluable HHCs will participate in the study. Index Patient: Screening and Randomization Eligible IPs must be aged from ³ 5 to £ 64 years old, have influenza symptom onset within 48 hours, test positive for influenza A/B, and be OwH (i.e., not at high risk for complications of influenza). Screening assessments informing IP eligibility include physical examination, vital signs, height and weight, medical history and concomitant therapies, urine pregnancy testing, and respiratory sampling for influenza testing. With regard to the household, IPs should be determined at screening to live with 1 or more HHCs who have not received an influenza vaccine in the past 6 months (“unvaccinated HHCs”), are likely to fulfill all HHC eligibility criteria, and are expected to participate in the “full study” (i.e., participate in all study assessments). IPs who meet the eligibility criteria will be randomized in a 1:1 ratio to receive a single dose of either BXM or placebo within 2 hours of randomization. The dose and formulation of BXM is based on weight and age. Index Patient: Post-Randomization Respiratory samples and AEs will be collected. IPs who are ³ 12 years old will also complete questionnaires describing their health status and absence from work or school. IPs who are < 12 years old will be asked to complete a questionnaire on the palatability and acceptability of the study drug oral suspension. Household Contacts: Screening and Enrollment All HHCs present in the home must have their screening visit start within 24 hours of IP randomization. If any HHC tests positive for influenza A/B, then all HHCs fail screening. If 1 or more unvaccinated HHCs meet all HHC eligibility criteria and agree to participate in the full study, it is allowable for additional HHCs to not participate beyond the screening visit (even if Baloxavir Marboxil—F. Hoffmann-La Roche Ltd 18/Protocol MV40618, Version 4 they meet all HHC eligibility criteria). There is no maximum number of vaccinated and unvaccinated HHCs that can participate in the full study. HHCs who agree to participate in the full study (“full study HHCs”) must meet the full study criteria (HHC inclusion criteria 7 to 14), including that they must reside in the household for 7 of the 9 study days, and must not have any influenza symptoms at screening (mild symptoms determined by the investigator to be due to a preexisting condition are allowed). Screening assessments informing HHC eligibility include influenza symptoms, medical history, concomitant medications, and respiratory sampling for influenza testing. Full Study Household Contacts: Post-Screening Only full study HHCs participate in visits and assessments post-screening. Full study HHCs are monitored for new or worsening influenza symptoms, and will also maintain a daily temperature diary. HHCs will be instructed to telephone the site if they develop influenza symptoms or fever so that a scheduled or unscheduled visit occurring within 24 hours can be arranged. Respiratory samples, adverse events due to study procedures, and concomitant medications will be collected. Study Visits The IP screening visit must occur at the study site. All other visits for IPs and HHCs may be conducted at home or at the study site. Home visits may be performed by home nurses or by site staff (if local practices are in place). All assessments except respiratory swab collection may be conducted remotely (e.g., via telephone or computer). SARS-CoV-2 Testing Given overlapping influenza and coronavirus disease 2019 symptoms, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) testing will be conducted at screening and as needed during study conduct. If any subject (IP or HHC) tests positive for SARS-CoV-2, then all subjects in the household should be discontinued.