Influenza infection is an important public health priority, with seasonal outbreaks and pandemics causing considerable global morbidity and mortality. The PK, pharmacodynamics (PD), safety and efficacy of IV zanamivir have been evaluated in adults, adolescents and infants more than or equal to (>=) 6 months of age with hospitalized influenza in the IV zanamivir global development program. However, antiviral treatment of neonates and infants under 6 months of age hospitalized with influenza infection remains a medical unmet need. Given the immaturity of the immune system at this age, there are no licensed influenza vaccines for children aged less than six months old. As a requirement of the Pediatric Investigation Plan European Union (EU), GlaxoSmithKline (GSK) will be conducting this open-label, multi-center, single arm, post-marketing authorization study to evaluate the PK and collect safety and tolerability information of IV zanamivir in hospitalized neonates and infants under 6 months of age with confirmed complicated influenza infection. The total duration of study participation for each participant will be up to 24 days with a study treatment period up to 10 days and 14 days of post-treatment follow up. However, for a given participant, the initial 5-day treatment course may be extended for up to 5 additional days if clinical symptoms, participant characteristics or virological tests as assessed by the investigator warrant further treatment. DECTOVA is a trademark of GlaxoSmithKline group of companies.

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

Qualified researchers may request access to anonymized individual patient-level data (IPD) and related study documents of the eligible studies via the Data Sharing Portal. Details on GSK's data sharing criteria can be found at: https://www.gsk.com/en-gb/innovation/trials/data-transparency/

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or terminated asset(s) across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension may be granted, when justified, for up to 6 months.