An Intravenous (IV) Zanamivir Pharmacokinetics (PK) Study in Hospitalized Neonates and Infants With Influenza Infection

Inclusion Criteria: * Neonates and infants who are aged less than 6 months (corrected age) at the time of the informed consent signed by legally acceptable representative (LAR) of minors. Preterm neonates and infants will be eligible for inclusion but must have reached PMA of at least 28 weeks. * Participants who are hospitalized with influenza infection, confirmed by a positive rapid molecular diagnostic test for influenza, or a local quantitative Reverse transcriptase-polymerase chain reaction (RT-PCR) test and who must have a potential for improvement Participants with negative rapid molecular test result suspected of having influenza can be enrolled following confirmatory testing by quantitative RT-PCR. * Participants with a high risk of altered oral drug absorption, represented by multi-organ dysfunction (dysfunction of at least 2 organs, as defined by the treating physician). (applicable only for Netherlands) * Body weight \>=1 kilograms (kg). * No gender restriction. * LAR of minors are willing and able to give written informed consent to participate in the study (or included as permitted by local regulatory authorities, Independent Ethics Committees \[IECs\] or local laws). Exclusion criteria: * Participants who are known or suspected to be hypersensitive to any component of the study medication. * Participants with a disease process which is likely to be irreversible. * Liver function: * Participants who meet the following criteria at Baseline: 1. Alanine transaminase (ALT) \>=3 times upper limit of normal (ULN) with bilirubin \>=2 times ULN 2. or isolated bilirubin \>=2 times ULN and \>50 percent (%) direct bilirubin 3. or ALT \>=5 times ULN Inclusion of participants with liver function tests that fall outside these criteria must be discussed and agreed with the medical monitor. * Current or chronic history of liver disease or known hepatic or biliary abnormalities (with the exception of benign conditions such as Gilbert's syndrome). Inclusion of participants with neonatal hyperbilirubinemia may be considered if appropriately managed according to local guidelines and must be discussed with the medical monitor (Not-applicable for Great Britain). * Participants who require concurrent therapy with another anti influenza drug. * Participants who have participated in a study using an investigational drug within 30 days prior to Baseline. * Child in care (CiC), as defined below: * A child who has been placed under the control or protection of an agency, organization, institution or entity by the courts, the government or a government body, acting in accordance with powers conferred on them by law or regulation. * The definition of a CiC can include a child cared for by foster parents or living in a care home or institution, provided that the arrangement falls within the definition above. The definition of a CiC does not include a child who is adopted or has an appointed legal guardian. * Participants undergoing treatment by Extracorporeal membrane oxygenation (ECMO) or hemofiltration. * Participants who are positive for severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2) as determined by a diagnostic test, at screening