Inclusion Criteria
Subjects must meet all of the following inclusion criteria to be eligible for participation in this
study:
1. At the time of randomization, requires supplemental oxygen ≥2 LPM due to hypoxemia.
Hypoxemia can be defined by meeting at least one of the following criteria:
  - SpO2 <92% on or off supplemental oxygen
  - Respiratory failure necessitating mechanical or non-invasive ventilation (CPAP or Bi-PAP)
  - Written declaration from Investigator that subject is clinically hypoxemic and removal of oxygen supplementation would not be considered clinically appropriate for the purpose of measuring SpO2 while on room air
   Note: Documentation of hypoxemia prior to or during the most recent oxygen supplementation must be available in source.
   Note: Subjects who also require invasive mechanical ventilation (MV) or non-invasive positive pressure ventilation (CPAP or bi-PAP) are eligible, although ventilator support is not mandatory
2. Immunocompromised, as defined by one or more of the following:
  - Received an autologous or allogeneic hematopoietic stem cell transplantation (HSCT) at any time in the past
  - Received a solid organ transplant at any time in the past
  - Has been or is currently being treated with chemotherapy for hematologic malignancies (e.g., leukemia, myeloma, lymphoma) and/or solid tumor malignancies (e.g., lung, breast, brain cancer) at any time in the past
  - Has an immunodeficiency due to congenital abnormality (only applicable to subjects age < 18 years old) or pre-term birth (only applicable to subjects age ≤ 2 years old)
3. Has, within 3 days prior to randomization, a confirmed LRTI with a sialic acid dependent respiratory virus (SAD-RV, see definition). This can be confirmed by:
  - bronchoalveolar lavage (BAL) positive for SAD-RV
  - lung biopsy positive for SAD-RV
  - chest imaging with a new or worsening finding of pulmonary infiltrate, bronchiolitis, or pneumonitis temporally associated with an upper respiratory tract sample (e.g., tracheal aspirate, sputum, nasopharyngeal swab (NPS), nasopharyngeal wash) positive for SADRV
   Note: These requirements can be fulfilled by results from samples and/or chest imaging that are collected/performed locally as per standard of care within 3 days prior to randomization.
   Note: For all subjects, after obtaining informed consent but within 3 days prior to randomization, a separate nasopharyngeal swab sample will be collected and sent to thecentral laboratory for confirmation of a SAD-RV. The results from the central lab analysis are not required to initiate the subject on study treatment.
4. If female, subject must meet one of the following conditions:
  - Not be of childbearing potential, defined as one or more of the following conditions:
     - Premenarchal
     - Postmenopausal for at least 1 year
     - Surgically sterile due to bilateral tubal ligation, bilateral oophorectomy, or hysterectomy;
  - Be of childbearing potential and meet all of the following criteria:
     - Has a negative urine or serum pregnancy test (beta-human chorionic gonadotropin) at screening
     - Agrees to practice an acceptable method of contraception (or meets criteria for waiving contraception) from screening until at least 30 days after last dose of study medication (see Section 8.10.1 for details)
Additional contraception requirements may need to be followed according to local regulations and/or requirements.
5. Non-vasectomized males are required to practice effective birth control methods (see Section 8.10.1) from screening until at least 30 days after last dose of study medication
6. Capable of understanding and complying with procedures as outlined in the protocol as judged by the Investigator and able to sign informed consent form prior to the initiation of any screening or study-specific procedures. Subjects must have the ability to return to the hospital to comply with required procedures if they are discharged during the study.
   Note: For subjects, including minors and patients with medical incapacity or impaired consciousness such that they are not able to give fully informed voluntary consent, the subjects' legal representative (parent, guardian or surrogate) must sign an institutional review board (IRB)/independent ethical committee (IEC)-approved informed consent document prior to the initiation of any screening or study-specific procedures. Minor subjects must also sign an IRB/IEC-approved assent form unless not required per local and institution regulations
 

 Exclusion Criteria
Subjects who meet any of the following exclusion criteria are not to be enrolled in this study:
1. Subjects may not be on hospice care or, in the opinion of the investigator, have a low chance of survival during the first 10 days of treatment
2. Subjects with Alanine Aminotransferase (ALT), Aspartate Aminotransferase (AST), or Alkaline Phosphatase (ALP) ≥3x ULN and Total Bilirubin (TBILI) ≥2x ULN
Note: Subjects with ALT/AST/ALP ≥ 3x ULN AND TB ≥2x ULN that have been chronically stable (for >1 year on more than one assessments) due to known liver pathology including malignancy (primary or metastasis), chronic medications, transplantation, or chronic infection will not be excluded
3. Female subjects breastfeeding or planning to breastfeed at any time through 30 days after the last dose of study drug
4. Subjects taking any other investigational drug used to treat pulmonary infection.
5. Psychiatric or cognitive illness or recreational drug/alcohol use that, in the opinion of the principal investigator, would affect subject safety and/or compliance
6. Subjects with known hypersensitivity to DAS181 and/or any of its components

7. Subjects with severe sepsis due to either their baseline SAD-RV infection or a concurrent viral, bacterial, or fungal infection and meet at least one of the following criteria:
   - Has evidence of vital organ failure outside of the lung (e.g., liver, kidney)
   - Requires vasopressors to maintain blood pressure