To evaluate Serious Adverse events related to vaccination of Ad26.COV2S1 or NVX-CoV2372 during the entire study.;To evaluate the immunogenicity of Ad26.COV2S1 and NVX-CoV2372 vaccines at day 28; To evaluate reactogenicity (solicited adverse events (AEs) Ad26.COV2S1 and NVX-CoV2372 following boosting.

To evaluate Serious Adverse events related to vaccination of Ad26.COV2S1 or NVX-CoV2372 during the entire study.;To evaluate the immunogenicity of Ad26.COV2S1 and NVX-CoV2372 vaccines at day 28; To evaluate reactogenicity (solicited adverse events (AEs) Ad26.COV2S1 and NVX-CoV2372 following boosting.