SARS-CoV-2 spike protein ELISA calculations and summary data

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 and binding affinities in convalescent plasma (CP) samples from COVID-19 patients and extends these analyses to post-vaccination (PV) samples to estimate protective IgG antibody (Ab) levels.  A direct enzyme-linked immunosorbent assay (ELISA) was used to measure IgG anti-spike protein (SP) antibodies (Abs) relative to human chimeric spike S1 Ab standards.  Microplate wells were coated with recombinant SP.  Affinities of Ab binding to SP were determined by previously described methods.  Binding affinities were also determined in an RBD-specific sandwich ELISA.  Two indices of protective immunity were determined as permutations of Ab molar concentration divided by affinity as dissociation constant (KD). The range and geometric means of Ab concentrations in 21 CP and 21 PV samples were similar and a protective Ab level of 7.5 µg/ml was determined for the latter population, based on 95% of the normal distribution of the PV population.  A population (n = 21) of plasma samples from individuals receiving only one vaccination with the BNT162b2 or mRNA-1273 vaccines (PtV) exhibited a geometric mean Ab concentration significantly (p < 0.03) lower than the PV 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.

Methods
Convalescent plasma (CP) samples were obtained from the University of Texas Health Science Center at Houston (UTHealth)/Memorial Hermann COVID-19 Convalescent Plasma Program under the direction of Dr. Henry E. Wang.  Convalescent plasma (CP) donors previously tested positive for COVID-19, were symptom-free for >14 days, tested negative for COVID-19 antigen prior to donation, and tested positive for CP antibodies.  Donor plasma was collected by the Gulf Coast Regional Blood Center before August 2020, which created a general stock of CP units that were distributed among therapeutic CP programs in the greater Houston area. Each plasma sample derived from a single donor.

Post-vaccination (PV) plasma samples were obtained from Dr. Luis Ostrosky’s laboratory at the Division of Infectious Diseases, Department of Internal Medicine, McGovern School of Medicine of UTHealth.  Two groups of 21 samples, each collected 15-29 days after administration of the first or second dose of the Pfizer or Moderna COVID-19 vaccine between January 4 and February 5, 2021, were studied.  Analysis of de-identified convalescent and post-vaccination plasma samples was approved by the UTHealth Committee for Protection of Human Subjects.