Convalescent plasma for hospitalized patients with COVID-19: an open-label, randomized controlled trial

Patients

This trial was stopped at the planned interim analysis because the conditional power estimate was 1.6% (below the stopping criterion of 20%). Between 14 May 2020 and 29 January 2021, 940 patients were randomized (2:1) to convalescent plasma or standard of care in 72 hospital sites in Canada, the United States and Brazil (Fig. 1 and Supplementary Table 1). Two patients randomized to plasma withdrew consent before treatment. Demographics of the baseline study population (n = 938) were balanced between groups for all study populations (Table 1 and Supplementary Tables 2 and 3). The median age was 69 years, with 59% male and 41% female, and the median time from the onset of any COVID-19 symptom was 8 d (interquartile range (IQR), 5–10 d). Most patients (n = 766, 81.7%) were receiving systemic corticosteroids at the time of enrolment. Seventeen patients were lost to follow-up between discharge and day 30, precluding assessment of the primary outcome.

Primary outcome

In the intention-to-treat population (n = 921), intubation or death occurred in 199 (32.4%) of 614 patients in the convalescent plasma group and 86 (28.0%) of 307 patients in the standard of care group (RR = 1.16, 95% CI 0.94–1.43, P = 0.18) (Fig. 2a). The time to intubation or death was not significantly different between groups (Fig. 2b). In the per-protocol analysis (n = 851), intubation or death occurred in 167 (30.5%) of 548 patients in the convalescent plasma group and 85 (28.1%) of 303 patients in the standard of care group (RR = 1.09, 95% CI 0.87–1.35, P = 0.46) (Supplementary Table 4).

Secondary efficacy outcomes and subgroup analyses

Secondary outcomes for the intention-to-treat population are shown in Fig. 2a. There were no differences in mortality or intubation or other secondary efficacy outcomes. Similarly, in the per-protocol analysis, there were no differences in the secondary efficacy outcomes (Supplementary Table 4 and Extended Data Figs. 1–3). No significant differences were observed in most subgroups, including time from diagnosis to randomization <3 d for both the intention-to-treat (Fig. 3) and per-protocol (Extended Data Fig. 4) populations. The subgroup of patients served by blood supplier 3 (Fig. 3) and the post hoc subgroup of patients who were not receiving corticosteroids (Extended Data Figs. 5 and 6) had worse outcomes with convalescent plasma compared to standard of care.

Safety

Serious adverse events occurred in 205 (33.4%) of 614 patients in the convalescent plasma arm compared to 81 (26.4%) of 307 patients in the standard of care arm for the intention-to-treat population (RR = 1.27, 95% CI 1.02–1.57, P = 0.034; Fig. 2a and Supplementary Tables 4–6). Most of these events were worsening hypoxemia and respiratory failure. Transfusion-related complications were recorded in 35 (5.7%) of 614 patients in the convalescent plasma group (Supplementary Tables 7 and 8). Of the 35 reactions, four were life-threatening (two transfusion-associated circulatory overload, one possible transfusion-related acute lung injury and one transfusion-associated dyspnea), and none was fatal. Thirteen of the 35 reactions were classified as transfusion-associated dyspnea. Two patients underwent serological investigation for transfusion-related acute lung injury (both negative).

Effect-modifying role of antibodies in convalescent plasma

The distributions of antibodies in convalescent plasma units varied by blood supplier (Fig. 4, Supplementary Table 9 and Extended Data Fig. 7); therefore, antibody analyses controlled for supplier to address possible confounding. Transfusion of convalescent plasma with average (log-transformed) levels of antibody-dependent cellular cytotoxicity (ADCC) yielded an OR of 1.16 (95% CI 0.85–1.57) for the primary outcome relative to standard of care. Each one-unit increase in the standardized log-transformed ADCC was associated with a 24% reduction in the OR of the treatment effect (OR = 0.76, 95% CI 0.62–0.92) (Fig. 4 and Supplementary Table 10). This effect-modifying role was also significant for the neutralization test (OR = 0.77, 95% CI 0.63–0.94) but not for anti-receptor-binding domain (RBD) enzyme-linked immunosorbent assay (ELISA) (OR = 0.84, 95% CI 0.69–1.03) or IgG against the full transmembrane spike (OR = 1.01, 95% CI 0.82–1.23).

When all four serologic markers were included in the multivariate model, each one-unit increase in the standardized log-transformed anti-spike IgG marker was associated with a 53% increase in the OR for the deleterious effect of convalescent plasma on the primary outcome (OR = 1.53, 95% CI 1.14–2.05); increases in ADCC and neutralization independently improved the effect of CCP (OR = 0.66, 95% CI 0.50–0.87 and OR = 0.74, 95% CI 0.57–0.95, respectively), whereas levels of anti-RBD antibodies had no effect-modifying role (OR = 1.02, 95% CI 0.76–1.38) (Supplementary Table 10). There was no evidence of significant interaction among the four serologic measures in the general additive model (Fig. 4 and Extended Data Fig. 8).

Meta-analysis

Of the 15 other reported randomized trials, 11 used only high-titer plasma^5,7,10–18, and four applied less stringent plasma selection criteria, allowing for variable plasma titers^6,19–21. Including the results from CONCOR-1, a total of 15,301 patients participated in trials using high-titer plasma, and 968 participated in trials applying less stringent criteria. The summary estimates for the RR of mortality in high-titer plasma trials was 0.97 (95% CI 0.92–1.02) compared to 1.25 (95% CI 0.92–1.69) in trials using unselected convalescent plasma (Fig. 5).

Trial design and oversight

CONCOR-1 was an investigator-initiated, multi-center, open-label, randomized controlled trial conducted at 72 hospital sites in Canada, the United States and Brazil³⁴. Eligible patients were randomly assigned to receive either convalescent plasma or standard of care. The study was approved by Clinical Trials Ontario (research ethics board of record: Sunnybrook Health Sciences Centre), project no. 2159; the Quebec Ministry of Health and Social Services multicenter ethics review (research ethics board of record: Comité d’éthique de la recherche du CHU Sainte-Justine), project no. MP-21-2020-2863; the Weil Cornell Medicine General Institutional Review Board, protocol no. 20-04021981; the Comissão Nacional de Ética em Pesquisa, approval no. 4.305.792; the Héma-Québec Research Ethics Board; the Canadian Blood Services Research Ethics Board; Research Ethics BC (research ethics board of record: the University of British Columbia Clinical Research Ethics Board); the Conjoint Health Research Ethics Board; the University of Alberta Health Research Ethics Board (Biomedical Committee); the Saskatchewan Health Authority Research Ethics Board; the University of Saskatchewan Biomedical Research Ethics Board; the University of Manitoba Biomedical Research Board; the Queensway Carleton Hospital Research Ethics Board; the Scarborough Health Network Research Ethics Board; the Windsor Regional Hospital Research Ethics Board; and the Bureau de l’Éthique of Vitalité Health Network. Regulatory authorization was obtained from Health Canada (control no. 238201) and the U.S. FDA (IND 22075). The trial was registered at ClinicalTrials.gov (NCT04348656). An independent data safety monitoring committee performed trial oversight and made recommendations after review of safety reports planned at every 100 patients and at the planned interim analysis based on the first 600 patients. External monitoring was performed at all sites to assess protocol adherence, reporting of adverse events and accuracy of data entry. Full details of the study design, conduct, oversight and analyses are provided in the protocol and statistical analysis plan, which are available online.

Participants

Eligible participants were (1) ≥16 years of age in Canada or ≥18 years of age in the United States and Brazil; (2) admitted to the hospital ward with confirmed COVID-19; (3) required supplemental oxygen; and (4) a 500-ml of ABO-compatible COVID-19 convalescent plasma (CCP) was available. The availability of ABO-compatible convalescent plasma from donors who had recovered from COVID-19 infection was an eligibility requirement. Exclusion criteria were (1) more than 12 d from the onset of respiratory symptoms; (2) imminent or current intubation; (3) a contraindication to plasma transfusion; or (4) a plan for no active treatment. Consent was obtained from all donors and participants (or their legally authorized representative).

Randomization and intervention

Patients were randomized in a 2:1 ratio to receive convalescent plasma or standard of care using a secure, concealed, computer-generated, web-accessed randomization sequence (REDCap v11.0.1)³⁵. Randomization was stratified by site and age (<60 and ≥60 years) with allocation made with permuted blocks of size 3 or 6. Patients randomized to convalescent plasma received one or two units of apheresis plasma amounting to approximately 500 ml from one or two donors. The plasma was stored frozen and was thawed as per standard blood bank procedures and infused within 24 h of randomization. Patients were monitored by clinical staff for transfusion-related adverse events as per local procedures. Individuals assigned to standard of care received usual medical care as per routine practices at each site. The investigational product was prepared by Canadian Blood Services and Héma-Québec (Canada), the New York Blood Center (United States)³⁶ and Hemorio (Brazil). Each supplier had different criteria for qualifying convalescent plasma units that were based on the presence of either viral neutralizing antibodies, measured by the plaque reduction neutralization assay and expressed as the concentration of serum that reduced the number of virus-induced plaques by 50% (PRNT50)^37,38 using a threshold titer of >1:160 or antibodies against the RBD of the SARS-CoV-2 spike protein using a threshold titer of >1:100. Female donors with previous pregnancies were excluded from donation, unless they tested negative for HLA antibodies. In addition, a retained sample from every plasma donation was tested at reference laboratories after the transfusion for (1) anti-RBD antibodies (IgM, IgA and IgG) by ELISA;^39,40 (2) viral neutralization by the PRNT50 assay using live virus;^37,38 (3) IgG antibodies binding to the full-length trimeric transmembrane SARS-CoV-2 spike protein expressed on 293T cells by flow cytometry;⁴¹ and (4) Fc-mediated function by ADCC assay against the full spike protein expressed on CEM.NKr cells (see supplement for complete description)^42,43. For each plasma unit, the absolute antibody content was defined as the product of the unit volume and the concentration of the antibody (or functional capacity) in the plasma. These calculations were used to estimate the total antibody content from the transfusion of two units.

Trial outcomes

The primary outcome was the composite of intubation or death by day 30. Secondary outcomes were: time to intubation or death; ventilator-free days by day 30; in-hospital death by day 90; time to in-hospital death; death by day 30; length of stay in critical care and hospital; need for extracorporeal membrane oxygenation; need for renal replacement therapy; convalescent plasma-associated adverse events; and occurrence of ≥3 grade adverse events by day 30 (classification of adverse events was performed using MedDRA (https://www.meddra.org/) and was graded by Common Terminology Criteria for Adverse Events, v4.03). All transfusion-related adverse events were classified and graded by International Society for Blood Transfusion (www.isbtweb.org) definitions. All patients were followed to day 30, including a 30-d telephone visit for patients who were discharged from hospital. Patients who were in hospital beyond day 30 were followed until discharge for the purpose of determining in-hospital mortality up to day 90.

Statistical analysis

The primary analysis was based on the intention-to-treat population, which included all individuals who were randomized and for whom primary outcome data were available. The per-protocol population was comprised of eligible patients who were treated according to the randomized allocation of the intervention and received two units (or equivalent) of convalescent plasma within 24 h of randomization.

The effect of convalescent plasma on the composite primary outcome of intubation or death by day 30 was assessed by testing the null hypothesis that the composite event rate was the same under convalescent plasma and standard of care. The RR for the primary outcome (convalescent plasma versus standard of care) was computed with a 95% CI. Secondary outcomes were analyzed as described in the statistical analysis plan (Appendix B in the (Supplementary Information). No multiplicity adjustments were implemented for the secondary analyses. Procedures planned for addressing missing data and subgroup analyses are described in the statistical analysis plan. Forest plots were used to display point estimates, and CIs across subgroups with interaction tests were used to assess effect modification.

The effect-modifying role of antibody content on the primary outcome was assessed via logistic regression controlling for the blood supplier, treatment and the antibody marker. Antibody markers were log-transformed, centered and then divided by the corresponding standard deviation before being entered into logistic regression models (see statistical analysis plan, Appendix B in the Supplementary Information). A multivariate logistic regression model was then fitted adjusting for all four markers. Generalized additive models were used to examine the joint effect of each pair of serologic markers on the primary outcome⁴⁴.

The results from CONCOR-1 were subsequently included in a meta-analysis based on the 20 May 2021 update of the Cochrane systematic review⁸ and known randomized trials published since comparing convalescent plasma to placebo or standard care in patients with COVID-19. These were divided based on whether they used plasma with high antibody titer or not. For each trial, we compared the observed number of deaths at 30 d (or closest available time point before a crossover, if applicable) of patients allocated to convalescent plasma or the control group. Summary estimates for RR with 95% CI were calculated using random effects meta-analysis to account for variation in effect size among studies. Heterogeneity was quantified using inconsistency index (I²) and P values from the chi-square test for homogeneity.

With a 2:1 randomization ratio, 1,200 patients (800 in the convalescent plasma group and 400 in the standard of care group) were needed to provide 80% power to detect an RR reduction of 25% with convalescent plasma for the primary outcome with a 30% event rate under standard of care, based on a two-sided test at the 5% significance level. An interim analysis by a biostatistician unblinded to the allocation of the intervention was planned for when the primary outcome was available for 50% of the target sample. An O’Brien–Fleming stopping rule was employed⁴⁵ to control the overall type I error rate at 5%. Conditional power was used to guide futility decisions with the nominal threshold of 20% to justify early stopping.

Reporting Summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Extended data

is available for this paper at 10.1038/s41591-021-01488-2.

Supplementary information

The online version contains supplementary material available at 10.1038/s41591-021-01488-2.