To evaluate the safety of administering 1 or 2 doses of the quadrivalent influenza virus split vaccine (MDCK cells) (0.5ml/dose) in individuals aged 6 months and above.

This is a single center, randomized, double-blind, controlled phase I clinical trial. Plan to include 300 participants aged 6 months and above. The experiment was divided into 5 age groups, with 60 cases each aged 6-35 months, 3-8 years, 9-17 years, 18-59 years, and ≥ 60 years old. Each age group was randomly assigned to the experimental group, control group, or placebo group in a 1:1:1 ratio. Among them, the participants aged 6 to 35 months and 3 to 8 years old will receive 2 doses throughout the entire process, with an interval of 28 days between each dose. Participants aged 9-17, 18-59, and ≥ 60 years old are vaccinated with one dose throughout the entire process.

Inclusion Criteria:

• Healthy individuals aged 6 months or more on the day of enrollment who can provide legal identification documents.
• Volunteers and/or their legal guardians (or entrusted persons) have informed consent, voluntarily participate, and sign an informed consent form.
• Volunteers and/or their legal guardians (or entrusted persons) have the ability (non-illiterate) to understand the research procedures, could use thermometers, scales, and fill out diary cards as required, and can comply with the requirements of the clinical trial protocol to complete the clinical study.

Exclusion Criteria:

（Exclusion criteria for the first dose）

• On the day of enrollment, the underarm temperature of individuals ≤ 14 years old is ≥ 37.5 ℃, and on the day of enrollment, the underarm temperature of individuals>14 years old is ≥ 37.3 ℃.
• Women of childbearing age (from menarche to menopause for ≤ 1 year) who have tested positive for urinary pregnancy test, and those who are breastfeeding, pregnant, or planning to conceive within 6 months.
• Have had or are currently suffering from influenza within the past 3 months (confirmed by clinical, serological, or microbiological methods).
• Individuals aged 6 months to 8 years have previously received any influenza vaccine (including registered or experimental) or have planned to receive any influenza vaccine during the study period. Individuals aged 9 and above have received any influenza vaccine (including registered or experimental) within the past 6 months or have planned to receive any influenza vaccine within the past 6 months.

Has a history of allergic reactions to any component of the research vaccine, including consuming eggs or using gentamicin sulfate.

• Has any history of severe allergies to vaccines or drugs in the past. Population aged 6 to 35 months: premature birth (delivered before the 37th week of pregnancy), low birth weight (birth weight<2500g) infants.
• Population aged 6 to 35 months: those with a history of difficult childbirth, suffocation rescue, and neurological damage.
• Congenital malformations (clinically significant according to researchers) or developmental disorders, genetic defects, and severe malnutrition.
• Population aged 3-8 years and 6-35 months: individuals with abnormal growth and development, severe eczema.
• Suffering from acute illness or being in the acute phase of chronic illness within 3 days before vaccination.
• Used antipyretic and analgesic drugs or anti allergic drugs within 3 days before vaccination.
• Suffering from serious chronic diseases, serious cardiovascular diseases, hypertension beyond drug control (systolic pressure ≥ 140mmHg or diastolic pressure ≥ 90mmHg, only applicable to people aged 18 and above), diabetes with complications, liver and kidney diseases, malignant tumors, etc.
• Have a history of receiving attenuated live vaccines within 14 days (including the 14th day) before vaccination and have a history of receiving other vaccines within 7 days (including the 7th day).
• Individuals who receive immunotherapy or immunosuppressive therapy within 3 months (continuous oral or intravenous for more than 14 days).
• Suffering from congenital or acquired immunodeficiency, human immunodeficiency virus infection, lymphoma, leukemia, or other autoimmune diseases.
• Asthma history, unstable in the past two years requiring emergency treatment, hospitalization, intubation, oral or intravenous corticosteroids.
• Have received blood or blood related products within the past 6 months.
• Individuals with progressive neurological disorders have a history of seizures, epilepsy, encephalopathy, Guillain Barre syndrome, psychiatric disorders, transverse myelitis, or family history.
• Have a history of abnormal coagulation function (such as coagulation factor deficiency, coagulation related diseases), or use anticoagulants within 3 weeks before vaccination.
• Non splenic, functional non splenic, splenectomy, or partial resection of other important organs.
• Plan to relocate before the end of the study or leave the local area for a long time during the scheduled study visit period.
• Being or planning to participate in other clinical trials in the near future.
• Abnormal laboratory testing indicators, except for minor abnormalities judged by doctors to have no clinical significance.
• The researcher determines any situation that is not suitable for participation in this clinical trial.

（Exclusion criteria for the second dose）

• On the day of vaccination, the axillary body temperature is ≥ 37.5 ℃.
• Fever (axillary temperature ≥ 38.5 ℃) within 3 days before vaccination, or use of antipyretic and analgesic drugs or anti allergic drugs within 3 days before vaccination.
• Suffering from various acute or chronic diseases with acute attacks within 3 days before vaccination.
• Insufficient interval between vaccination of other vaccines (with a history of receiving attenuated live vaccines within 14 days (including the 14th day) before vaccination, and a history of receiving other vaccines within 7 days (including the 7th day).
• Other situations where researchers believe vaccination should be postponed.