To compare the immunogenicity, cellular immune response, and safety between adjuvanted (aIIV4) and non-adjuvanted (IIV4) seasonal influenza vaccines in the Taiwanese elderly population with chronic medical conditions.

Annual influenza vaccination is the most important and cost-effective intervention in reducing the impact of influenza, and a key component of the WHO response and preparedness efforts for influenza of pandemic potential. Reduced effectiveness of conventional egg-based vaccination was observed among risk group, such as elderly and people on with chronic medical conditions, which may result from their impaired immunogenicity. WHO, as well as ACIP of the USA, Europe, and Australia suggested adjuvanted influenza vaccines as one of alternatives for those people, which could decrease the disease burden significantly in several analyses. An adjuvanted commercial vaccine has been approved in Taiwan recently, but the local data of safety and effectiveness analysis is limited. This 1-year randomized control trial is designed to estimate the safety and effectiveness of adjuvanted influenza vaccine among Asian elderly people when compared to non-adjuvanted vaccines.

Inclusion Criteria:

• Adults aged 65 years or above
• Presence of at least 1 chronic medical condition associated with increased risk of complicated influenza, including hypertension, chronic obstructive pulmonary disease (group A, B, E), asthma, congestive heart failure (ACC/AHA stage A, B, C, D), coronary arterial disease, diabetes mellitus, chronic liver disease (Child-Pugh score A, B, C) or chronic kidney disease (stage 1~4), neuromuscular disorders, hematological or solid organ malignancies, recipient of hematopoietic stem cell transplantation or solid organ transplantation.

Exclusion Criteria:

• Contraindications to influenza vaccination including history of severe adverse event after vaccination, such as anaphylaxis or Guillain-Barré Syndrome, or a severe bleeding tendency
• Prior receipt of influenza vaccination during the current flu season.
• Determination by the Principal Investigator (PI) that the participant is ineligible for enrollment based on clinical assessment.