Infectious Disease · Medical Literature
Timing, Titre, and Host Determinants of Convalescent Plasma Efficacy in COVID-19: A Systematic Review With Design-Stratified Narrative Synthesis.
Abstract
The efficacy of convalescent plasma (CP) in COVID-19 has been contested, with large randomized trials producing conflicting results that led to recommendations against routine use. We performed a systematic review with design-stratified narrative synthesis (PRISMA 2020; SWiM; OSF: https://osf.io/tjcuq) to test whether these conflicting outcomes can be reconciled through three interacting determinants-timing, antibody titre, and host characteristics-using a standardized 91-field extraction matrix applied to each study. PubMed, Web of Science, and Scopus were searched through May 2026. Thirty-one studies (20,793 patients; 8 RCTs, 23 observational) were included. The synthesis indicates that CP is effective, but only under a specific convergence of conditions: when administered early in the disease course, with sufficient antibody titre, to seronegative or immunocompromised hosts who have not yet mounted an endogenous antibody response. Among RCTs analysing timing, 4 of 6 found benefit with earlier administration; for titre, 3 of 4 RCTs with explicit comparisons found benefit with higher-titre CP; and two large RCTs (combined n = 12,522) demonstrated preferential benefit in seronegative recipients. This interpretation is consistent with modelling estimates that CP, deployed largely to hospitalised patients, nonetheless saved tens of thousands of lives in the United States during the first pandemic year, an effect attributed to the subset treated early enough to modify disease progression. A second, previously unquantified finding concerns antibody class: characterisation of non-IgG immunoglobulins was nearly absent, with only 2 of 31 studies-both from our group-reporting IgA or IgM data, despite evidence that IgA and IgM are among the most potent neutralising classes against SARS-CoV-2. This hypothesis-generating observation identifies a systematic gap in CP product assessment. These findings reframe CP efficacy from a binary question to a conditional one and provide a template for the precision deployment of passive immunotherapy in future pandemics.
Clinical relevance
This systematic review addresses the persistent controversy surrounding convalescent plasma (CP) in COVID-19 by reconciling conflicting RCT outcomes through a conditional efficacy framework rather than a binary one. It directly affects immunocompromised and seronegative patient populations—those least able to mount endogenous humoral responses—for whom CP may represent a meaningful therapeutic option. Clinicians should engage with these findings now as they establish a precision-deployment model applicable to future pandemic preparedness and current management of immunocompromised hosts with viral infections.
Key findings
1. Among 31 included studies (20,793 patients; 8 RCTs, 23 observational), CP efficacy was conditional on the convergence of three determinants: early administration, high antibody titre, and a seronegative or immunocompromised host who has not yet mounted an endogenous antibody response. 2. Timing: 4 of 6 RCTs that stratified by timing of administration demonstrated a clinical benefit favoring earlier CP administration in the disease course. 3. Antibody titre: 3 of 4 RCTs with explicit high- versus low-titre comparisons found benefit associated with higher-titre CP products. 4. Serostatus: Two large RCTs (combined n = 12,522) demonstrated preferential clinical benefit in seronegative recipients compared to seropositive recipients at the time of CP administration. 5. Modelling estimates suggest CP—despite being deployed largely to hospitalized patients—saved tens of thousands of lives in the United States during the first pandemic year, an effect attributed to the subset treated early enough to modify disease progression. 6. Antibody class characterization was critically underreported: only 2 of 31 studies reported IgA or IgM data, representing a systematic gap in CP product assessment, despite evidence that IgA and IgM are among the most potent neutralizing immunoglobulin classes against SARS-CoV-2.
Practice implications
CP should not be regarded as uniformly ineffective; rather, it should be considered for early administration (prior to endogenous seroconversion) in seronegative or immunocompromised patients using verified high-titre products, as this phenotype-matched approach is where the consistent RCT signal of benefit resides. Clinicians and blood banks should prioritize titre verification and ideally characterize non-IgG immunoglobulin fractions (IgA, IgM) in CP units, given the emerging evidence of superior neutralizing potency of these classes and their near-total absence from current product assessment protocols. Future CP protocols—whether for COVID-19 in vulnerable hosts or for novel pandemic pathogens—should prospectively stratify by recipient serostatus and disease day at infusion, and collect IgA/IgM data, to enable precision passive immunotherapy rather than undifferentiated population-wide deployment.