The purpose of this study is to evaluate the antibody response to the influenza vaccine (AdimFlu-S), as measured by hemagglutination inhibition (HAI) at 4 weeks post immunization in pregnant women in compliance with the requirements of the current European Union (EU) recommendations for the evaluation of the immunogenicity for a new formulation of a licensed flu vaccine (CPMP/BWP/214/96). Besides, the vaccine safety will also be evaluated.

Inclusion Criteria:

1. Pregnant female aged ≥ 18 years old.
2. Subject is pregnant for at least 3 months, inclusive.
3. Subject is willing and able to adhere to visit schedules and all study requirements.
4. Subject has read and signed the study-specific informed consent.

Exclusion Criteria:

1. Subject with previous complicated pregnancy or preterm delivery, spontaneous or medical abortion;
2. Subject with history or concurrent high risk of dangerous gestation such as gestational diabetes mellitus (GDM), pregnant induced hypertension, or preeclampsia;
3. Subject received any influenza vaccine within the previous 6 months;
4. Subject has a history of hypersensitivity to eggs or egg protein or similar pharmacological effects to study vaccine;
5. Subject or her family has the history of Guillain-Barré Syndrome;
6. Subject has current upper respiratory illness (URI), including the common cold or nasal congestion within 72 hours;
7. Subject with influenza-like illness as defined by the presence of fever (temperature over 38ºC) and at least two of the following four symptoms: headache, muscle/joint aches and pains (e.g. myalgia/arthralgia), sore throat and cough;
8. Subject receive any treatment with an investigational drug or device, or participation in a clinical study, within 3 months before consent;
9. Subject has immunodeficiency or is under immunosuppressive treatment.
10. Subject received any vaccine within 1 week prior to study vaccination or expected to receive one within 1 week after study vaccination;
11. Subject received any blood products, including immunoglobulin, in the past 3 months before consent;
12. Subject has underlying condition in the investigators' opinion may interfere with evaluation of the vaccine;