The purpose of this study is to evaluate the efficacy, safety, pharmacokinetics (PK), neutralizing antibody and antidrug antibody (ADA) response for TNM001 in infants entering their first RSV season.

This study adopts an adaptive seamless dose selection design and consists of two parts: Part 1 is a phase 2b dose ranging trial which will support to determine the dose for Part 2, the phase 3 trial. The study population includes early and mid-term preterm infants [gestational age (GA)﹤35 weeks 0 day] and late preterm infants or full-term infants (≥35 weeks 0 day GA), with or without Congenital Heart Disease (CHD) or premature infants Chronic Lung Disease (CLD). A total of approximately 2250 infants will be randomized 2:1 to receive either TNM001 or placebo. All subjects will be followed for 240 days after dosing. This study will be conducted in appropriately 50 sites in China.

Inclusion Criteria:

• 1. Early and mid-term preterm infants (<35 weeks 0 day GA) and late preterm infants or full-term infants (≥35 weeks 0 day GA) under 1 year of age, with or without Congenital Heart Disease (CHD) or premature infants Chronic Lung Disease (CLD)，who are entering their first RSV season at the time of screening.

Exclusion Criteria:

• 1. Any fever (> 38.0°C) or acute illness within 7 days prior to randomization
• 2. History of RSV infection or active RSV infection prior to, or at the time of, randomization
• 3. Drug medication prior to randomization or expected to be treated by medicines during the study period.
• 4. Currently receiving or expected to receive immunosuppressive therapy during the study period.
• 5. Renal impairment or hepatic dysfunction
• 6. Nervous system disease or neuromuscular disease
• 7. Prior history of a suspected or actual acute life-threatening event
• 8. Known immunodeficiency including HIV, mother with HIV infection unless the child's infection has been excluded.
• 9. Known allergy history of immunoglobulin products, receipt or expected to receive immunoglobulins or blood products during the study period.
• 10.Receipt of RSV vaccine or mAb