Aging/immunology; COVID-19/immunology; COVID-19/prevention & control; SARS-CoV-2/immunology; Adaptive Immunity/immunology; Adolescent; Adult; Angiotensin-Converting Enzyme 2/analysis; Angiotensin-Converting Enzyme 2/metabolism; Antibodies, Viral/analysis; Antibodies, Viral/immunology; COVID-19/diagnosis; COVID-19/virology; COVID-19 Testing; Child; Common Cold/immunology; Common Cold/transmission; Common Cold/virology; Coronavirus Nucleocapsid Proteins/immunology; Humans; Immunity, Innate/immunology; Immunologic Memory/immunology; Phosphoproteins/immunology; Receptors, Coronavirus/analysis; Receptors, Coronavirus/metabolism; Rhinovirus/immunology; SARS-CoV-2/isolation & purification; SARS-CoV-2/pathogenicity; Spike Glycoprotein, Coronavirus/immunology; Systemic Inflammatory Response Syndrome/immunology; Systemic Inflammatory Response Syndrome/virology; T-Lymphocytes/immunology; Time Factors; Young Adult