Duration of Protection against Mild and Severe Disease by Covid-19 Vaccines

Background:Vaccines against severe acute respiratory syndrome coronavirus 2(SARS-CoV-2),the virus that causes coronavirus disease 2019(Covid-19),have been used since December 2020 in the United Kingdom.Real-world data have shown the vaccines to be highly effective against Covid-19 and related severe disease and death.Vaccine effectiveness may wane over time since the receipt of the second dose of the ChAdOx1-S(ChAdOx1 nCoV-19)and BNT162b2 vaccines.