The loss of productivity due to influenza virus (flu) infections (3 to 5 million cases globally), which can sometimes amount to as much as five working days, is a major socioeconomic issue. This study compares baloxavir marboxil’s cost-effectiveness with that of oseltamivir for influenza treatment in adult outpatient settings in Japan from the healthcare-payer perspective. A decision tree model was constructed with probabilities from CAPSTONE 1 for low-risk patients and CAPSTONE 2 for high-risk patients. Direct costs included medical costs and drug prices. Medical costs were obtained from the Japanese Medical Fee Index (2019 version). Effectiveness outcomes (quality-adjusted life-year [QALY]) were measured using the EQ-5D-5L questionnaire for adult patients who previously experienced flu infection. The time horizon was 14 days. Deterministic and probabilistic sensitivity analyses were performed to examine the robustness of the results. The base case analysis revealed oseltamivir dominated baloxavir marboxil for low-risk patients. However, the incremental cost-effectiveness ratio (ICER) for high-risk patients with body weight < 80 kg was 3,178,163 yen/QALY (US$ 30,905/QALY), which was less than the willingness to pay 5,000,000 yen/QALY (US$ 48,621/QALY). 

Moreover, the ICER for high-risk patients with influenza and body weight ≥ 80 kg was 10,709,187 yen/QALY (US$ 104,139/QALY). The study revealed baloxavir marboxil was more cost-effective than oseltamivir for high-risk patients with body weight < 80 kg. As the concept of a formulary in hospitals has gradually become popular, healthcare providers may find the insights from this study helpful in developing a novel formulary for decision-makers to consider.