RecruitingBiological interventionUnderstanding Immunity to the Flu Vaccine in COVID-19 PatientsThe purpose of this study is to measure immunity to the flu vaccine over time in patients who have had COVID-19 and may have other medical conditions including obesity, type 2 diabetes, chronic fatigue, or long-term COVID-19 symptoms. Adults and children (age 9 to 64) who had been diagnosed with COVID-19 as well as controls without COVID-19 will be invited to participate in this study.