This clinical study will be performed in subjects aged 12-60 years old enrolled in NCT01310374 who received 2010 seasonal influenza vaccine to evaluate the persistence of the immune response of influenza vaccine up to 24 months after vaccination.

Subjects were previously vaccinated at 12 to 60 years of age with 2010 seasonal influenza vaccine.The persistence phase starts 24 months after the primary vaccination and blood samples will be taken at 6, 12, 18 and 24 months after primary vaccination. Subjects whose antibody titer less than 1:40 at 6 months after primary vaccination will receive a booster dose of influenza vaccine.

Inclusion Criteria:

• aged 12-60 years old
• male or non-pregnant female
• volunteers received 2010 seasonal influenza vaccine
• clinically healthy as determined by: medical history inquiring and physical examination
• provide written informed consents before joining the trial

Exclusion Criteria:

• without history of 2010 seasonal influenza vaccine administration,
• allergic to any ingredient of vaccine,
• autoimmune disease or immunodeficiency,
• severe adverse reaction after vaccination, such as allergies, hives, breathing difficulties, angioedema or abdominal pain,
• severe acute and chronic diseases
• axillary temperature over 37.0℃ at the time of vaccination.