Protective characteristics of COVID-19 convalescent and post-vaccination IgG antibodies

Soon after commencement of the SARS-CoV-2 disease outbreak of 2019 (COVID-19), it became evident that the receptor-binding domain of the viral spike protein is the target of neutralizing antibodies that comprise a critical element of protective immunity to the virus. This study addresses the relative lack of information regarding actual antibody concentrations in convalescent plasma samples from COVID-19 patients and extends these analyses to post-vaccination samples to estimate protective IgG antibody (Ab) levels. Both sample populations were similar and a protective Ab level of 7.5 {micro}g/ml was determined, based on 95% of the normal distribution of the post-vaccination population. The results of this study have implications for future vaccine development, projection of protective efficacy duration, and understanding of the immune response to SARS-CoV-2 infection. One-Sentence SummaryUsing two quantitative immunoassays, we have found similar IgG antibody responses to the SARS-CoV-2 spike protein in populations of COVID-19 survivors and vaccine recipients that indicate a protective antibody concentration.