The aims of this vaccine trial are: (1) to measure humoral and selected cellular immune responses to repeated influenza vaccination with Flublok, including these responses' associations with age, birth year, and prior vaccination history; (2) to identify the characteristics of study participants who are vaccinated but still become infected with influenza virus ("vaccine failures") and participants who have poor immune responses to vaccination; and (3) to predict how influenza vaccinations and infections shape immunity.

Background: Influenza vaccine is the most frequently used vaccine in the United States and globally. Influenza vaccination provides variable protection against influenza virus infection from year to year, with multiple factors contributing to variation in vaccine effectiveness. First, viral evolution necessitates regular updates to vaccine strains, and the degree of match between vaccine and circulating strains affects vaccine protection. A more serious issue, which motivates this study, is that repeated influenza vaccination may lead to "focusing" of immune responses to older strains, potentially reducing protection against recent strains.

Aims and objectives: The aims of this trial are: (1) to measure humoral and selected cellular immune responses to repeated influenza vaccination, including these responses' associations with age, birth year, and prior vaccination history; (2) to identify the characteristics of study participants who are vaccinated but still become infected with influenza virus ("vaccine failures"); and (3) to predict how influenza vaccinations and infections shape immunity.

Study design: *DRIVE I* A 4-year immunogenicity study with a randomized controlled design including 447 adults who are 18-45 years of age. Participants will be randomized to 5 groups in equal proportions, where the groups receive Flublok (Sanofi Pasteur) vaccine (V) or saline placebo (P) in years 1-4: group 1: V+V+V+V; group 2: P+V+V+V; group 3: P+P+V+V; group 4: P+P+P+V; group 5: P+P+P+P.

*DRIVE II* A 3-year and one-month immunogenicity study with a randomized controlled design among 530 adults who are 18-45 years of age. Participants will be randomized into 4 groups in equal proportions, where the groups will receive Flublok (Sanofi Pasteur) vaccine (V) or saline placebo (P) in year 1-4: group 1: V+V+V+V; group 2: P+V+V+V; group 3: P+P+V+V; group 4: P+P+P+V.

*DRIVE I & DRIVE II* All participants will receive influenza vaccination at the end of the final year. We will collect blood samples and nasal strip samples before vaccination and various timepoints after vaccination. Whole blood samples will be collected from a subset for later PBMC analysis. We will actively monitor participants for acute respiratory illnesses throughout the follow-up period, and collect and test respiratory swabs and blood samples to identify respiratory virus infections and acute immune responses to infection.

Number of Subjects: DRIVE I: 447 enrolled in autumn and winter 2020/21. DRIVE II: 530 enrolled in autumn and winter 2021/22.

Main outcome measures: The primary outcome measures are the humoral immune responses at day 30 after vaccination measured by hemagglutinin inhibition and microneutralization assays. The investigators will also study a number of secondary outcomes, including the persistence of immune responses 91, 182, 273 and 365 days after vaccination, and the immune responses to natural laboratory-confirmed influenza virus infections, as well as immunity and immune responses to other respiratory viruses including COVID-19 (SARS-CoV- 2).

Potential implications: Our study will provide novel insight into the effects of repeat influenza vaccination and infection on the strength and breadth of immune responses to influenza, the mechanisms underlying heterogeneity in vaccine response and vaccine failure, and biological factors that could explain variation in influenza vaccine effectiveness.

Inclusion Criteria:

• Aged 18-45 years at enrolment.
• Capable of providing informed consent.
• Resident in Hong Kong in the coming 2 years.

Exclusion Criteria:

• Vaccinated against influenza in the past 24 months.
• Included in one of the priority groups to receive influenza vaccination in Hong Kong (priority groups include pregnant women, long-stay residents of institutions for persons with disability, persons with chronic medical problems (chronic cardiovascular, lung, metabolic or kidney diseases, obesity (body mass index 30 or above) and chronic neurological condition), healthcare workers or persons working in poultry, pig farming or pig slaughtering industry).
• With diagnosed medical conditions related to their immune system.
• Currently taking medication for any condition that impairs immune system.
• Individuals who report medical conditions not suitable to receive inactivated influenza vaccines, such as: Severe allergic reaction (e.g., anaphylaxis) after previous dose of any influenza vaccine; or to a vaccine component; moderate or severe acute illness with or without fever after any previous influenza vaccination; or a history of Guillain-Barré syndrome (GBS) within 6 weeks of previous influenza vaccination.
• Individuals, who report medical conditions not suitable to receive intramuscular injection, such as bleeding disorders; habitually taking anticoagulants (with the exception of antiplatelets such as aspirin).
• Individuals who have any medical conditions not suitable to receive inactivated influenza vaccines as determined by a clinician.

We will release anonymized individual patient data along with publication of study results.

On publication of study results

Freely available on a public repository such as Dryad or Github