Fluad vs. Fluzone High-Dose Vaccine Effectiveness Among Adults ≥65 Years

Relative Vaccine Effectiveness of Adjuvanted Inactivated Influenza Vaccine (aIIV) vs High-Dose Inactivated Influenza Vaccine (HD-IIV) for Prevention of Lab-Confirmed Influenza Among Adults ≥65 Years of Age in the United States

This study will evaluate the relative vaccine effectiveness of quadrivalent adjuvanted inactivated influenza vaccine (aIIV4) versus quadrivalent high-dose inactivated influenza vaccine (HD-IIV4) in preventing polymerase chain reaction (PCR)-confirmed influenza and influenza-related outcomes in adults ≥65 years of age during the 2023/24, 2024/25, and 2026/27 influenza seasons. The study is an observational study conducted at Kaiser Permanente Northern California (KPNC), an integrated health care system in the United States.

Inclusion Criteria:

• Age ≥65 years at the time of influenza vaccination
• KPNC member at the time of vaccination
• Receive either aIIV4 or HD-IIV4 at a KPNC facility during the 2023/24 and/or 2024/25 influenza seasons

Exclusion Criteria:

• Age <65 years at the time of influenza vaccination
• Received either aIIV4 or HD-IIV4 in the inpatient setting
• Receive more than one influenza vaccination during a single season period (i.e., 2023/24 or 2024/25 influenza seasons)
• Receive an influenza vaccination outside of KPNC