Neutralizing activity to SARS-CoV-2 of convalescent and control plasma used in a randomized controlled trial

Transfusion. 2021 May;61(5):1363-1369. doi: 10.1111/trf.16283. Epub 2021 Feb 19.


Background: There are limited data on the neutralizing activity of convalescent plasma (CP) administered in randomized controlled trials (RCT) of COVID-19 infection.

Study design and methods: As part of an RCT, CP was collected per FDA guidelines from individuals recovered from COVID-19 infection. CP donors had to have ≥145 optical density (OD) units (ideal target ≥300) using a semiquantitative, immunochromatographic test for IgG antibody to the nucleocapsid protein (NP) of SARS-CoV-2 (typical range 0-500 OD units). A random subset of samples [14 control plasma, 12 CP "medium-anti-NP" (145-299 OD units), and 13 CP "high" anti-NP (≥300 OD units)] were tested for neutralizing antibodies using an established viral luciferase antibody inhibition assay to detect the infection of SARS-CoV-2 pseudovirus that encoded spike protein (SARS2-Strunc ) on a human immunodeficiency virus 1 vector (NL43dEnvNanoLuc), using ACE2-expressing 293 T cells. The titer needed to neutralize 50% of viral activity (NT50) was calculated.

Results: The uptake of SARS-CoV-2 pseudovirus by 293TACE2 cells was inhibited by pretreatment with CP compared to control CP (p < .001) with control plasma having a median (IQR) 50% neutralization titer (NT50) of 1:28 (1:16,1:36) compared to 1:334 (1:130,1:1295) and 1:324 (1:244,1:578), for medium anti-NP and high anti-NP CP units, respectively. The neutralizing activity of CP met minimum FDA criteria with neutralizing antibody titers >1:80 in 100% of randomly selected samples, using a conservative approach that excluded non-specific binding.

Discussion: Plasma from donors screened using an immunochromatographic test for IgG antibody to SARS-CoV-2 NP exhibited neutralizing activity meeting FDA's minimum standard in all randomly selected COVID-19 CP units.

Keywords: FFP transfusion; immunology (other than RBC serology).

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Antibodies, Neutralizing / blood*
  • Antibodies, Viral / blood*
  • Blood Donors*
  • COVID-19 / blood*
  • Convalescence*
  • Double-Blind Method
  • Female
  • Humans
  • Male
  • Middle Aged
  • SARS-CoV-2 / metabolism*


  • Antibodies, Neutralizing
  • Antibodies, Viral