This is a Phase 3, randomized, parallel-group, comparator-controlled, observer-blind, multicenter study of immunogenicity and safety in approximately 7700 male and female adults aged 50 years and older (approximately equally split between two age groups: 50-64 years; 65 years and older), who are healthy or have stable comorbidities that increase their risk of complications from influenza infection. Three lots of aQIVc will be evaluated for consistency and pooled for the comparison with the 2 control vaccines.

Subjects will be randomly assigned to receive 1 of 3 lots of aQIVc, QIVr, or aQIV in a 1:1:1:2:2 ratio (for a 3:2:2 ratio for aQIVc, QIVr, and aQIV).

The study will have a treatment period (Day 1 to Day 29) and a follow-up period (Day 30 up to Day 181); a subset of 770 subjects will be followed up up to Day 365.

Main Inclusion Criteria:

• Individuals, aged 50 years and older, who are healthy or have stable comorbidities that increase their risk of complications from influenza infection
• Individuals who can comply with all study procedures

Main Exclusion Criteria:

• Progressive, unstable, or uncontrolled clinical conditions
• Known hypersensitivity or allergy to any study vaccine component
• Known history of Guillain-Barré syndrome or other demyelinating disease
• Condition representing a contraindication to vaccination or blood draw
• Abnormal function of immune system due to known disorder or medication.
• Influenza vaccination within 180 days prior to informed consent.

SEQIRUS supports the release of anonymized subject-level and study-level data in compliance with regulatory requirements, including Clinical Documents which are part of the Common Technical Document (CTD) modules submitted to regulatory agencies for public release.

Summary results disclosure is either in document form (e.g., International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) E3 Clinical Study Report synopsis) or structured data form (such as summary results in ClinicalTrials.gov (United States) or eudract.ema.europa.eu (EU Clinical Trial Registry [EU CTR])).

SEQIRUS discloses results from clinical studies within 12 months of last patient last visit (LPLV) unless otherwise mandated by local laws or regulations.

SEQIRUS will consider requests from qualified scientific and medical researchers to disclose protocols, anonymized subject-level data and study-level data when there is medical, scientific and/or public health interest to ensure the safe use of a Seqirus product licensed on or after 1 January 2014 in the United States (US) and/or the European Union (EU). This applies to Seqirus-sponsored interventional studies initiated after 27 September 2007 and ongoing as of 26 December 2007, that have been included as part of a US or EU submission package which received approval in US and EU on or after 1 January 2014 and have been accepted for publication.