1. COVID-19 Vaccinations in the United States,2021
2. U.S. COVID-19 vaccine tracker: See Your State’s Progress,2021
3. Herd Immunity and COVID-19 (Coronavirus): what You Need to Know,2021
4. Delta coronavirus variant: scientists brace for impact;Callaway;Nature (London),2021
5. SARS-CoV-2 variant classifications and definitions;Center for Disease Control and Prevention,2021