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Why your annual flu shot might work differently each year

NCT ID NCT04576377

Knowledge-focused Sponsor: The University of Hong Kong Source: ClinicalTrials.gov ↗

What the study statuses mean

This study's is highlighted.

Recruitment status, easiest to join first

Recruiting now
This trial is taking on new participants right now.
Not yet recruiting
Registered, but not yet taking participants.
By invitation only
Not open to general applications. Only people the study team invites can take part.
Paused
Paused for now. It may or may not start again.
Ongoing
Running, but no longer taking on new participants.
Completed This study
The trial has finished. Results may not be published yet.
Stopped early
Stopped early, before it reached the end. That can be for many reasons, including safety.
Cancelled
Cancelled before anyone took part.

Expanded access (not trials)

Expanded access
Not a trial. This treatment can be requested outside a study, case by case, for people who qualify.
Expanded access (paused)
Not a trial. The treatment can normally be requested outside a study, but is unavailable right now.
Expanded access (ended)
Not a trial. The treatment could once be requested outside a study, but no longer can.
Approved
The treatment has been approved, so it is available normally rather than through this programme.

When the status isn't known

Details not published
The full record has not been published yet, so there is little to show here.
Status unknown
This status has not been confirmed recently, so it may be out of date.

First seen Jun 25, 2026 · Last updated Jun 27, 2026 · Updated 1 time

Summary

This completed study in Hong Kong enrolled 977 adults aged 18-45 to understand how getting the flu vaccine year after year changes the body's immune response. Participants received either the Flublok vaccine or a saline placebo. Researchers measured antibody levels and looked for reasons why some vaccinated people still catch the flu. The goal is to improve future flu vaccination strategies.

What this could mean

Our plain-language read of the trial. This is informational only, not medical advice or a prediction.

Active substance
Flublok (recombinant quadrivalent influenza vaccine)
What this could lead to
If successful, this study could help predict who benefits most from annual flu shots and why some vaccinated people still get the flu.
What could go wrong
This is an observational knowledge-harvesting study, not a test of a new treatment. Results may not apply to older adults or other populations.

This is an AI summary of the original study and may miss details. Read our disclaimer.

Study facts

What this study's own registry entry says, in plain language.

Phase

Phase 4

Runs after approval, following long-term safety and how well the treatment works in everyday use.

Participants

977 people

The number who actually took part.

Started

Oct 2020

Finished

Dec 2025

Lead sponsor

Other sponsor

The registry's catch-all category, for sponsors it does not file as a company, a government agency, or a research network.

Who can take part

This study's own entry requirements. Only the study team can say for certain whether you qualify.

Ages

18 to 45 years

Sex

Anyone

Healthy volunteers

Accepted

You do not need to have the condition being studied to take part.

Show the full entry requirements

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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.

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Conditions

The condition(s) this trial relates to.

influenza prevention target

As listed by the trial registrant

The condition terms exactly as the trial's registrant entered them.

Influenza, human

Contacts and locations

Locations

  • The University of Hong Kong

    Hong Kong, 00000, Hong Kong

More trials for these conditions

Other studies related to the condition(s) this trial covers.