COVID-19 convalescent plasma (CCP) has shown promise in halting disease progression in COVID-19 patients. However, CCP products are often underutilized and may be stored for extended periods before administration. The efficacy of these CCP products against emerging variants of SARS-CoV-2 that arise later in the pandemic is a topic of concern. Aims: In this study, we investigated the neutralizing capacity of plasma collected from non-vaccinated donors during the early stages of the pandemic against subsequent emergent variants of SARS-CoV-2.

Plasma donors, who had experienced COVID-19, were recruited. Levels of anti-SARS-CoV-2 antibodies (including Spike Trimer, Receptor Binding Domain (RBD), S1, S2, and nucleocapsid protein) were quantified using a multiplex serological assay (MULTICOV AB, NMI, Reutlingen, Germany). The neutralization capacity of RBD antibodies was assessed through an ACE2 binding inhibition assay. ACE2 binding inhibition was measured against wild type virus and variants of concern, including alpha, beta, gamma, delta, omicron BA.4, and omicron BA.5. Friedman test with Dunn's multiple comparisons was employed to compare ACE2 inhibition across different variants with the wild type.

A total of 47 non-vaccinated CCP donors (25 female; 53%) with a median age of 47 years (range 40-60) were included in the analysis. CCP donations were obtained between January 2021 and June 2021. Compared to the wild type (18.2%; 12.9-21.1%), ACE2 binding inhibition was similar against the alpha (17.7%; 13.7-23.4%) and delta (16.1%; 13.2- 18.5%) variants (p>0.05). However, ACE2 binding inhibition was significantly lower than that of the wild type against the beta (12.5%; 9.53-15.4%), gamma (10.1%; 6.41-13%), omicron BA.4 (5.22%; 1.99-6.92%), and omicron BA.5 (6.46%; 2.75-8.76%) variants (p<0.05).

Our findings indicate a potential decrease in the neutralizing efficacy of antibodies present in convalescent plasma from non-vaccinated donors against newly emerging SARS-CoV-2 variants. This emphasizes the critical need to evaluate the neutralization potential of therapeutic products against prevalent variants of SARS-CoV-2 before their clinical application, ensuring optimal clinical effectiveness.

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