Outpatient COVID-19 convalescent plasma recipient antibody thresholds correlated to reduced hospitalizations within a randomized trial

Abstract

Background: The COVID-19 convalescent plasma (CCP) viral specific antibody levels that translate into recipient post-transfusion antibody levels sufficient to prevent disease progression is not defined.

Methods: This secondary analysis correlated donor and recipient antibody levels to hospitalization risk among unvaccinated, seronegative CCP recipients within the outpatient, double blind, randomized clinical trial that compared CCP to control plasma. The majority of COVID-19 CCP arm hospitalizations (15/17, 88%) occurred in this unvaccinated, seronegative subgroup. A functional cutoff to delineate recipient high versus low post-transfusion antibody levels was established by two methods: 1) analyzing virus neutralization-equivalent anti-S-RBD IgG responses in donors or 2) receiver operating characteristic (ROC) analysis.

Results: SARS-CoV-2 anti-S-RBD IgG antibody was diluted by a factor of 21.3 into post-transfusion seronegative recipients from matched donor units. Viral specific antibody delivered approximated 1.2 mg. The high antibody recipients transfused early (symptom onset within 5 days) had no hospitalizations. A CCP recipient analysis for antibody thresholds correlated to reduced hospitalizations found a significant association with Fisher's exact test between early and high antibodies versus all other CCP recipients (or control plasma) with antibody cutoffs established by both methods-donor virus neutralization-based cutoff: (0/85; 0% versus 15/276; 5.6%) p=0.03 or ROC based cutoff: (0/94; 0% versus 15/267; 5.4%) p=0.01.

Conclusion: In unvaccinated, seronegative CCP recipients, early transfusion of plasma units corresponding to the upper 30% of all study donors reduced outpatient hospitalizations. These high antibody level plasma units, given early, should be reserved for therapeutic use.Trial registration: NCT04373460.

Funding: Defense Health Agency and others.

Keywords: COVID-19; convalescent plasma; hospitalization; outpatients; randomized controlled trial; viral load.

Figure 1.

CONSORT diagram depicting enrollment, allocation, and analysis flow of recipients.

Figure 2.. Utilizing CCP donor neutralizing antibody and anti-S-RBD levels to establish a functional cutoff associated with hospitalization protection among their respective screened seronegative recipients.

(A) A 1:40 dilutional titer for virus neutralizing antibody (nAb) was previously identified as a correlate of protection in influenza studies. Here, we use the 1:40 dilutional titer for nAb to identify the upper limit level of donor anti-S-RBD IgG 2728 AUC associated with protection from hospitalization. 1:10 dilutional titer is the limit of detection for nAb. (B) Ratio of matched donor anti-S-RBD IgG AUC to that of their respective CCP seronegative recipients that was used to infer the functional cutoff in recipients was determined to be 21.3 (C) Anti-S-RBD IgG AUC levels among donors and post-transfusion recipients segregated by screen vaccination status and serostatus compared by Kruskall-Wallis with Dunn’s post-hoc corrections, *p<0.033, **p<0.002, ***p<0.001. Unvaccinated subsequently hospitalized (red dots) post transfusion recipients in screen seronegative (n=15) and screen seropositive (n=2). (D) Screen seronegative, unvaccinated recipient D0 (post-transfusion) antibody (n=361) segregated by recipient days from symptom onset to transfusion and high (>128 AUC) or low (≤ 128 AUC) levels. Recipient high and low cutoffs were calculated by using a 21.3-fold drop from the donor cutoff determined by the upper value of the 95% confidence interval for the geometric mean of donor anti-S-RBD AUC at a 1:40 nAb titer associated with protection. Subsequently hospitalized (red dot) and non-hospitalized (blue dot) recipients are shown. (E) Firth logistic regression model adjusted for age, sex, BMI, and variant to compare the predicted probabilities of hospitalization across early vs. late and high (>128 AUC) vs. low (≤ 128 AUC) categories of screen seronegative, unvaccinated CCP recipients. The horizontal dashed line represents a predicted probability of 0%. P-values for the predicted probability of each category are shown with p<0.05 considered significant.

Figure 3.. CCP recipient D0 post-transfusion and matched donor antibody levels stratified by duration from symptom onset to transfusion using cutoffs established by the ROC and maximum antibody threshold method.

(A) RCDC curves were used to compare the maximum antibody thresholds, as represented by the red dashed lines, to delineate high and low antibody levels for early or late CCP recipients. Early recipients are delineated at antilog of 2.06 anti-S-RBD AUC (115 AUC) while late recipients are delineated at antilog of 2.58 anti-S-RBD AUC (380.2 AUC). (B) Screen seronegative, unvaccinated recipient D0 post-transfusion antibody (n=361) segregated by recipient days from symptom onset to transfusion based on high versus low cutoffs established by the ROC and maximum antibody threshold method. Subsequently hospitalized (red) and non-hospitalized (blue) recipients are shown. (C) Firth logistic regression model adjusted for age, sex, BMI, and variant to compare the predicted probabilities of hospitalization across early vs. late and high vs. low categories of screen seronegative, unvaccinated CCP recipients based on the ROC and maximum antibody threshold method. The horizontal dashed line represents a predicted probability of 0%. P-values for the predicted probability of each category are shown with p<0.05 considered significant.

Figure 4.. Antibody levels over three months post-transfusion by hospitalization status and treatment group for screen seronegative, unvaccinated recipients.

Log₁₀-transformed antibody levels up to 90 days post-transfusion were segregated by treatment and hospitalization status of recipients using a linear mixed effects regression model, adjusted for variant, age, sex, and BMI. CCP recipients have greater AUC levels on D0, but by D14, the hospitalized recipients have greater AUC levels than non-hospitalized. The average days from transfusion to hospitalization was 3.05 days, with all post-transfusion hospitalizations occurring between D0 and D14. The dashed line represents the log-transformed cutoff (1.924) for seropositivity. This diagnostic threshold is equivalent to the anti-S-RBD IgG log₁₀-transformed 180 titer.