The purpose of this study is to evaluate the safety and immunogenicity of different formulations of vaccines encoding the RSV monovalent antigen or the Flu hemagglutinin subtype 5 (H5) antigen in healthy participants aged 18 to 49 years. The total duration of study participation for each participant varies by stage and treatment arm. Stage 1: * For Arm 1, Arm 2, and Arm 3 the duration of study participation will be approximately 7 months for each participant. * For Arm 4, Arm 5, and Arm 6 the duration of study participation will be approximately 6 months for each participant. Stage 2: For all arms, the duration of study participation will be approximately 7 months for each participant.
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Presence of any unsolicited systemic Adverse Events (AEs) reported within 30 minutes after vaccination
Timeframe: On Day 1
Presence of solicited injection site reactions (ie, injection site pain, injection site erythema, injection site swelling, pre-listed in participant's diary and in the eCRF (electronic case report form)) occurring through 7 days after each vaccination
Timeframe: Day 1 through day 8
Presence of solicited systemic reactions (ie, fever, headache, and only new or worsened fatigue, myalgia, arthralgia, and chills pre-listed in the participant's diary and in the eCRF) occurring through 7 days after each vaccination
Timeframe: Day 1 through day 8
Presence of unsolicited AEs reported through 20 days after the first vaccination and after the second vaccination for IMPs with the Flu H5 antigen
Timeframe: Day 1 through day 21
Presence of unsolicited AEs reported through 28 days after vaccination for IMPs with the RSV antigen
Timeframe: Day 1 through day 29
Presence of SAEs (Serious Adverse Events) and AESIs (Adverse Events of Special Interest) throughout the study (ie, through 6 months after vaccination)
Timeframe: SAE: Screening day through day 202 (IMPs containing Flu H5 antigen), Screening day through day 181 (IMPs containing RSV antigen); AESI: Day 1 through day 202 (IMPs containing Flu H5 antigen), Day 1 through day 181 (IMPs containing RSV antigen)
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Presence of out-of-range biological test results (including shift from baseline values) through 7 days after each vaccination
Timeframe: Screening through day 8
Presence of AEs leading to study discontinuation throughout the study (ie, through 6 months after vaccination)
Timeframe: Screening through day 202 (IMPs containing Flu H5 antigen), Screening through day 181 (IMPs containing RSV antigen)
RSV A and RSV B serum nAb (neutralizing antibodies) titers at pre-vaccination and 28 days post-vaccination in all participants receiving IMPs with the RSV antigen
Timeframe: Day 1 and day 29
Flu H5 antibody (Ab) titer measured by hemagglutination inhibition (HAI) at pre-vaccination and 21 and 42 days post-vaccination in all participants receiving IMPs with the Flu H5 antigen
Timeframe: Day 1, day 22 and day 43
Hemagglutination Inhibition (HAI) geometric mean titer ratio at 21 and 42 days post-vaccination relative to pre-vaccination for all participants receiving IMPs with the Flu H5 antigen
Timeframe: Day 1, day 22 and day 43
Seroconversion rate based on Hemagglutination inhibition (HAI) antibody titers at pre-vaccination and 21 and 42 days post-vaccination in all participants receiving IMPs with the Flu H5 antigen
Timeframe: Day 1, day 22 and day 43
Proportion of participants with Hemagglutination Inhibition (HAI) antibody titer ≥40 at pre-vaccination and 21 and 42 days post-vaccination in all participants receiving IMPs with the Flu H5 antigen
Timeframe: Day 1, day 22, and day 43
Proportion of participants with detectable HAI antibody titer ≥10 at pre-vaccination and 21 and 42 days post-vaccination in all participants receiving IMPs with the Flu H5 antigen
Timeframe: Day 1, day 22, and day 43