The goal is to evaluate the in-depth immunogenicity analysis (including B-cell and T-cell response) of coadministration of a omicron-containing COVID-19 vaccine and influenza vaccine among healthy adults during 2023-24 season.

This was an open-label, randomized clinical trial conducted at the International St. Mary's Hospital in Incheon, South Korea. This study included two study groups: Concomitant administration of omicron containing messenger ribonucleic acid (mRNA) COVID-19 booster and quadrivalent influenza vaccination (QIV) and separate administration of influenza vaccination followed by mRNA booster ≥4 weeks later

• immunogenicity analysis : Blood was drawn at baseline and follow-up visit 4 weeks, 3 months, 6 months, 10-12 months after immunization(For the COVID-19 vaccine, additional blood sampling will be conducted one week after vaccination).
• safety analysis : At 7 days after each vaccine dose, the participants were requested to record the occurrence, severity of solicited adverse events (AEs) through a standardized electronic questionnaire. Participants were also asked to record any unsolicited AEs during the 28 days after vaccination.

Inclusion Criteria:

• who agreed to receive both booster COVID-19 vaccine and influenza vaccine
• individuals who have received the COVID-19 vaccine three or more times and have passed at least 3 months after the last vaccination
• individuals with a history of a single SARS-CoV-2 infection during the Omicron outbreak period (January 2022-February 2023)

Exclusion Criteria:

• individuals with a contraindication to any of the vaccine compounds
• individuals with a history of influenza infection within the past 6 months