Inclusion Criteria:

• Signed informed consent
• Locally determined positive influenza test (by polymerase chain reaction [PCR] or other nucleic acid test, or by rapid antigen [Ag]) from a specimen obtained within 2 days prior to randomization
• Onset of illness no more than 7 days before randomization, defined as when the participant first experienced at least one respiratory symptom or fever
• Hospitalized (or in observation unit) for influenza, with anticipated hospitalization for more than 24 hours. Criteria for hospitalization will be up to the individual treating clinician.
• For women of child-bearing potential: willingness to abstain from sexual intercourse or use at least one form of hormonal or barrier contraception through Day 28 of the study
• Willingness to have blood and respiratory samples obtained and stored
• NEW score greater than or equal to 2 at screening (see the protocol for more information on this criterion)

Exclusion Criteria:

• Women who are pregnant or breast-feeding
• Strong clinical evidence (in the judgment of the site investigator) that the etiology of illness is primarily bacterial in origin
• Prior treatment with any investigational drug therapy within 30 days prior to screening
• History of allergic reaction to blood or plasma products (as judged by the site investigator)
• Known immunoglobulin A (IgA) deficiency
• A pre-existing condition or use of a medication that, in the opinion of the site investigator, may place the participant at a substantially increased risk of thrombosis (e.g., cryoglobulinemia, severe refractory hypertriglyceridemia, or clinically significant monoclonal gammopathy)
• Presence of any pre-existing illness that, in the opinion of the site investigator, would place the participant at an unreasonably increased risk through participation in this study
• Participants who, in the judgment of the site investigator, will be unlikely to comply with the requirements of this protocol
• Medical conditions for which receipt of a 500 mL volume of intravenous fluid may be dangerous to the participant (e.g., decompensated congestive heart failure)
• Receiving extracorporeal membrane oxygenation (ECMO)
• Suspicion that infection is due to an influenza strain or subtype other than A(H1N1)pdm09, H3N2, or influenza B (e.g., H5N1, H7N9)