A Phase II Clinical Study to Evaluate the Efficacy and Safety of YKYY018 for Inhalation Solution

Inclusion Criteria: 1. Participant must voluntarily consent to participate in this study and provide their written informed consent prior to start of any study-specific procedures. 2. Participant must be between 18 and 85 years of age (inclusive), no gender restrictions. 3. Diagnosed with RSV infection through antigen or nucleic acid testing within 24 hours prior to screening. 4. Participants must have at least one new onset of lower respiratory tract infection symptoms or worsening of existing lower respiratory tract symptoms (including cough), and the severity of any individual symptom is moderate or severe (RiiQ score ≥ 2), with an expected interval of no more than 72 hours between the onset of RSV infection symptoms and the first dose. 5. Female participants with fertility tested negative for blood pregnancy during the screening period. Male and female trial participants, as well as their reproductive age female partners, must start using effective contraception measures at least half a month before screening (3 months for oral contraceptives), agree to use effective contraception measures (abstinence, previous sterilization, hormone and/or barrier methods) during the study period and within one month after the end of the trial, and are not allowed to donate ovum/sperm during the period. 6. Participants are able to communicate effectively with the researcher and understand and comply with the requirements of this study. Exclusion Criteria: 1. At the time of screening, the patient had already been hospitalized for treatment, or it was determined by the researcher that hospitalization is expected within 24 hours after screening. 2. Diagnosis of influenza or novel coronavirus infection within 7 days before screening. 3. Suspected of merging with other respiratory pathogens that require anti infective drug treatment. 4. Based on imaging and the researcher's judgment, patients with active tuberculosis, untreated tuberculosis or incomplete tuberculosis history. 5. Planned to undergo bone marrow, stem cell, or other organ transplantation during the research period, or currently receiving immunomodulatory therapy (such as systemic glucocorticoids or immunosuppressants). 6. Accompanied by severe or unstable central nervous system, cardiovascular, respiratory system (such as requiring long-term oxygen therapy), liver, kidney, gastrointestinal, urinary system, endocrine system, or hematological system diseases, which researchers believe may confuse the research results or affect the safety of participants. 7. Individuals with a history of malignant tumors within the past 5 years (excluding non metastatic skin basal cell or squamous cell carcinoma that is currently stable and has received appropriate treatment or resection, as well as cervical cancer in situ), or those currently under evaluation for potential malignant tumors. 8. Unable to communicate or cooperate with medical staff due to neurological, psychiatric, or language disorders. 9. Any history of mechanical ventilation treatment within 30 days prior to screening (excluding general anesthesia for surgery). 10. Participants who cannot tolerate nebulized inhalation or have severe oral and/or maxillofacial deformities that affect the use of nebulized inhalers. 11. During the screening period, individuals with alanine aminotransferase (ALT), aspartate aminotransferase (AST) levels greater than 2 times the upper limit of normal (ULN), total bilirubin levels greater than 1.5 times ULN, or serum creatinine levels greater than 1.5 times ULN, or other laboratory test results that are clinically significant and may affect the evaluation of this trial, as determined by the researchers. 12. Participants have used within 3 days or 5 half lives (whichever is longer) before the first administration, or plan to use any research prohibited drugs during the study (prohibited drugs include interferon, ribavirin and drugs with antiviral effect/traditional Chinese patent medicines and simple preparations clearly listed in the drug manual). 13. Received RSV vaccine within 6 months prior to screening, or received any other vaccine (excluding inactivated vaccines) within 4 weeks prior to initial administration, or planned to receive the vaccine during the study. 14. Individuals who have participated in any other clinical trials and used investigational drugs or medical devices within one month or five half lives prior to screening (whichever is longer). 15. Known history of allergies or any severe allergic diseases to the investigational drug or its components. 16. Individuals who have undergone major surgeries within the previous month prior to screening; Or those who have not recovered after surgery during the screening period. 17. Participants who are known to be positive for human immunodeficiency virus (HIV-1 or HIV-2) antibodies. 18. History of drug abuse within the past 12 months prior to screening. 19. Despite guidance from the researcher, the participan is still unable to cooperate in completing the Respiratory Intensity and Impact Questionnaire (RiiQ) 20. Participants may not be able to complete the study for other reasons, or may be deemed unsuitable for inclusion by the researcher.