Study to Assess the Efficacy of Baloxavir Marboxil Versus Placebo to Reduce Onward Transmission of Influenza A or B in Households

INCLUSION CRITERIA

Index Patients (IPs):

• Able to comply with the study protocol per investigator judgment.
• Diagnosed with acute influenza infection by investigator.
• Polymerase chain reaction [PCR] (+) or Rapid Influenza Diagnostic Test [RIDT] (+) for influenza A/B based on cobas® SARS-CoV-2 and influenza A/B or other point-of-care / local laboratory results.
• PCR (-) or antigen test (-) for SARS-CoV-2 based on cobas® SARS-CoV-2 and Influenza A/B test or other point-of-care / local laboratory result
• Presence of (a) fever (>=38.0 °C per tympanic or rectal thermometer; >=37.5 °C per axillary, oral or forehead/temporal thermometer) or (b) any influenza symptoms (cough, sore throat, nasal congestion, headache, feverishness or chills, muscle or joint pain, fatigue).
• The time interval between the onset of fever or influenza symptoms and the pre-dose examinations is 48 hours or less.
• IP lives in a household where: (1) No HHC is known to have been diagnosed with influenza or SARS-CoV-2 infection by a healthcare professional (HCP) in the past 4 weeks; (2) All HHCs are expected to meet the key HHC inclusion criteria; (3) >=1 HHCs are expected to participate in the full study who have not received the influenza vaccine within 6 months prior to screening.
• Women of childbearing potential: Agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures specified in the protocol

All HHCs (Part 1):

• PCR (-) or RIDT (-) based on cobas® SARS-CoV-2 and influenza A/B or other local point-of-care / local laboratory result.
• PCR (-) or antigen test (-) for SARS-CoV-2 based on cobas® SARS-CoV-2 and Influenza A/B or other POC / local laboratory result.
• HHC lives with no HHC who will be present in the home at any time during the study and who meets any HHC exclusion criteria.
• HHC lives with no HHC who does not meet HHC inclusion criteria (part 1).
• HHC lives in a household where ≥1 HHCs meet all of the following: Start screening within 24 hours after IP randomization; Have NOT received the influenza vaccine within 6 months prior to screening; and Fulfill full study HHC inclusion criteria part 2.

Full study HHCs (part 2) intended for full study must meet the following additional criteria for study entry:

• Agree to participate in the full study.
• Able to comply with the study protocol per investigator judgment
• No influenza symptoms within 7 days prior to screening. Alternatively, mild symptoms are permissible if determined by the investigator to be due to a preexisting condition.
• Temperature <38.0 °C (tympanic).
• Will reside in the index patient's house for at least 7 of the next 9 days and will be present for scheduled study visits.
• Willing and able to measure and record temperature, or have another household member perform the task on his or her behalf. Furthermore, a responsible adult will assume responsibility to oversee or perform this task on behalf of minors.
• In the 6 months prior to screening: a) Has not been diagnosed with influenza by a healthcare professional b) Has not received BXM, peramivir, laninamivir, oseltamivir, zanamivir, rimantadine, umifenovir, favipiravir or amantadine.
• Does not have a moderate or worse active infections OR infections requiring systemic (e.g., oral or intravenous) or otherwise internally administered (e.g., inhaled, intrathecal) antibiotic/antiviral/antifungal therapy, (topical therapies for mild external infections allowed).

EXCLUSION CRITERIA

IPs:

• IPs with severe influenza virus infection requiring inpatient treatment.
• IPs judged by the investigator to be at high risk for complications of influenza.
• IP is ≥12 years old and unable to swallow tablets (not applicable to IPs 5 to 11 year olds who will receive oral suspension).
• Women who are breastfeeding or have a positive pregnancy test in the pre-dose examinations.
• IPs with concurrent (non-influenza) infections requiring systemic antimicrobial and/or antiviral therapy at the