This study evaluates the safety of the quadrivalent subunit influenza vaccine in individuals aged 3 years and older in Jiangsu Province, China. The primary objectives are to assess the occurrence of very rare adverse reactions (incidence <0.01%), investigate the incidence of cerebral infarction following vaccination, and evaluate the incidence of systemic adverse events among participants aged 9 to 17 years.

This is an open-label, single-arm, non-controlled, real-world safety study of the quadrivalent subunit influenza vaccine, designed to systematically track and evaluate the overall safety profile following vaccine administration.

The core objectives of this study are as follows:

1. To assess the occurrence of rare adverse reactions with an incidence rate of less than 0.01% after vaccination;
2. To investigate the incidence of cerebral infarction events following receipt of the study vaccine;
3. To evaluate the incidence of systemic adverse events after vaccination among participants aged 9 to 17 years.

A total of 3,105 eligible participants aged 3 years and older who voluntarily receive the vaccine will be enrolled. All enrolled participants will be administered a single 0.5 mL intramuscular dose of the quadrivalent influenza subunit vaccine.

The safety monitoring protocol is arranged as follows:

1. 30-minute immediate observation: On-site monitoring will be conducted at the vaccination clinic immediately after injection to detect immediate adverse reactions.
2. 28-day systematic safety follow-up: Participants will perform daily self-monitoring of adverse events within 28 days after vaccination. Investigators will conduct two on-site centralized follow-up visits on Day 7 and Day 28 post-vaccination respectively, to verify and collect adverse event data.
3. 6-month serious adverse event monitoring: The occurrence of serious adverse events will be monitored via spontaneous reporting by participants within 6 months after vaccination.

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.