Comparative Immunogenicity of Concomitant vs Sequential mRNA COVID-19 and Influenza Vaccinations

This study is a prospective, randomized comparative immunogenicity study in an enrolled cohort. During this study, eligible participants will be randomly assigned to receive an approved quadrivalent cell culture-based influenza vaccine (ccIIV4, Seqirus) and an approved mRNA COVID-19 vaccine (Moderna) either concomitantly or sequentially, 28 days apart. Participants (aged 6-11 years and 18-64 years) will be enrolled in the 2023-2024 influenza season. Demographic and health data (including influenza and COVID-19 vaccination and infection history) will be collected upon enrollment. Enrolled participants will be randomized to one of the following interventions (2:1:1) (i) concomitant administration of the mRNA COVID-19 vaccine (Moderna) and quadrivalent influenza vaccine (ccIIV4, Seqirus); (ii)sequential administration of the quadrivalent influenza vaccine (ccIIV4, Seqirus) at Visit 1 (day 0) and the mRNA COVID-19 vaccine(Moderna) at Visit 2 (day 28); (iii) sequential administration of the mRNA COVID-19 vaccine (Moderna) at Visit 1 (day 0) followed by the quadrivalent influenza vaccine (ccIIV4, Seqirus) at Visit 2 (day 28). Participants will not be blinded to vaccine group. Whole blood samples to isolate sera for immune assays will be collected prior to vaccination administration at Visit 1 (day 0), Visit 2 (day 28) Visit 3 (day 56; post-vaccination 2) and Visit 4 (day180; end of local flu circulation). Blood samples to isolate PBMC and plasma will be collected from a subset of 250 participants (200 adults and 50 children). If participants exhibit ARI during the study period, the participants may be asked to present for collection of a nasal swab for viral testing for acute influenza or SARS-CoV-2 infection (within 10 days after symptom onset), and blood specimen to isolate sera for immune assays. For participants with confirmed acute infection, the participants may be asked to present for collection of a convalescent-phase blood specimen approximately 28 days after acute visit for isolation of sera, PBMC and plasma.