This study evaluates the safety of the quadrivalent subunit influenza vaccine following its post-marketing widespread use in individuals aged 3 years and older, aiming to assess the occurrence of very rare adverse reactions (with an incidence rate of <0.01%).

This is a large-scale, multicenter, open-label, single-arm, non-controlled real-world safety observational study following influenza vaccination. It is designed to assess the occurrence of very rare adverse reactions (incidence <0.01%) following vaccination with a quadrivalent influenza subunit vaccine and to demonstrate its superior safety profile.

A total of 42,881 participants aged 3 years and above were enrolled. Each received a single 0.5 ml dose of the quadrivalent influenza subunit vaccine. Safety monitoring included immediate observation for 30 minutes post-vaccination and systematic surveillance over the subsequent 28 days. Data on adverse events (AEs) and serious adverse events (SAEs) were collected during two time intervals: from 30 minutes to 7 days (inclusive) and from 8 to 28 days (inclusive) after vaccination. Any newly identified adverse reactions during the study period were also recorded.

Inclusion Criteria

1. Healthy individuals aged 3 years or above.
2. The participant (and/or their guardian/legal representative) has provided informed consent, voluntarily agreed to participate, signed the Informed Consent Form, and is able to comply with the requirements of this study protocol.

Exclusion Criteria

1. Individuals with a known allergy to any component of the vaccine, including eggs, excipients, formaldehyde, or Triton N-101.
2. Individuals suffering from an acute illness, severe chronic disease, an acute exacerbation of a chronic disease, common cold, or fever.
3. Individuals with uncontrolled epilepsy or other progressive neurological disorders, or a history of Guillain-Barré syndrome.
4. Any other condition considered by the investigator as inappropriate for participation in this study.

Healthy individuals aged 3 years or above