A third dose of inactivated SARS-CoV-2 vaccine induces robust antibody responses in people with inadequate response to two-dose vaccination

Mass application of SARS-CoV-2 vaccines is a major approach to reduce SARS-CoV-2 transmission and COVID-19 symptoms worldwide. The inactivated SARS-CoV-2 vaccine is one of the most widely administered vaccine types in many countries, with over 2 billion doses administered as of 31 August 2021. How-ever, clinical trials estimate 65%–85% protection from detectable symptoms for inactivated SARS-CoV-2 vaccines [1–4], with 15%–35% of people remaining in-sufficiently protected after two-dose im-munization.Immuneprotectionforthese‘non-responders’iscriticalforpopulationimmunityasestablishedbymassvaccina-tion.Athirddoseofinactivated(Coro-naVac)vaccinehasbeenshowntoboostantibodyresponsebybetween3–5foldsinthegeneralpopulation[5].Whetherathirddosecaninduceanadequatean-tibodyresponsein‘non-responders’re-mainsunclear.Herewereportatrialofathirddoseinacohortof105vacci-natedparticipants(SupplementaryFig.S1)withsuboptimalantibodyresponses(non-responders)aftertwodoses(43.8%weremen;averageage43