Characterizing SARS-CoV-2 convalescent plasma

As the current pandemic of COVID-19 continues to spread over the world, the lack of vaccines or therapeutic drugs has led to the use of convalescent plasma on an experimental basis. However, it is necessary to be able to measure how effective this therapy is without the need for biocontainment facilities. A new study published on the preprint server bioRxiv* in June 2020 uses serologic correlates to assess the efficacy of convalescent plasma. This will also help to classify donor plasma as more or less potent at neutralizing the virus.

The first recorded use of convalescent plasma (CP) was over a hundred years ago. In fact, this won Emil Behring the first-ever Nobel Prize in Medicine in 1901. The use of plasma for this pandemic was spurred by the evidence of some therapeutic efficacy in the earlier coronavirus outbreaks, namely, SARS and MERS. This led to the relatively early consideration of its use in the current pandemic.

How are Convalescent Plasma Donors Identified?

Different methods are used to identify potential CP donors, such as diagnosis of acute infection by RT-PCR testing during the acute phase of infection, antibody testing by ELISA or other serologic assays in the convalescent period, or detecting neutralizing antibodies using virus microneutralization testing (MNT).

The last-mentioned is the only assay for the functional efficacy of an antibody, in actually neutralizing the virus. This is, therefore, the gold standard of antibody testing. The disadvantage is that the use of live and, therefore, infectious viral particles mandates level biosafety 3 containment restrictions (BSL3). Moreover, this procedure takes a few days and is relatively involved.

Neutralizing Titer Assessment

The Austrian Red Cross Blood Service began the collection of CP by plasmapheresis, having inactivated the virus first, at the Vienna Blood Center. The first hundred units of CP were assayed using virus microneutralization testing (MNT), to look for functional or neutralizing antibodies. Their results show a high neutralizing antibody titer, with the mean MNT titer being 50%. In other words, the NT50 was about 1:230.

These figures need to be assessed by international reference standards, which are unavailable as yet. However, for some recovered patients, with proof of COVID-19, one plasma unit lacked any neutralizing antibodies at all, while with 6 units, which comes to about 6% of the total, the NT50s were less than 1:23.

The investigators also did a binding assay for the antibodies, on the same units of CP, to find an association between the MNT and ELISA. While the first is perhaps more reliable with respect to its function, the ELISA test is more readily available, and the lower threshold means that we can detect CP with lower potency, and define the threshold for the usability of CP for transfusion.

Comparing MNT and ELISA

The current study is focused on finding a more straightforward, scalable, and reliable test for antibodies in CP, on identifying potential donors, which compares favorably with the level of functional antibody activity detected by MNT.

Secondly, the researchers aimed to evaluate donor characteristics associated with more potent CP, such as disease severity, duration from the earliest symptom to plasma harvesting, and donor age or sex.

There were 83 samples for which both MNT and ELISA results were available. The researchers found a highly significant correlation, but with low magnitude. In order to achieve the aim of using ELISA to establish the lower limit for CP transfusion in COVID-19, the analysis was performed again, this time without units that had very high NT59 titers, such as >1:500. This came to 12 units or about 15% of the total. This improved the size of the correlation.

The next step was to consider the Euroimmun ELISA 1.1 threshold as the qualification for CP transfusion. The researchers found that 6 units, or about 7%, would be left out, at a mean NT50 of 1:29. This would mean that the NT50 would go up from 1:233 for all the 83 CP collected to  1:249, for the remaining 77 units.

The researchers comment, “A final verdict about whether this cut-off is suitable for the use of CP in the treatment of COVID-19 will have to await an evaluation of clinical efficacy in correlation to these antibody measurements.”

Evaluating Donor Characteristics

For the second aim, the researchers classified 90/100 CP donors as having WHO severity scores 1 and 2, with a mean NT50 of 1:208. There were 6 donors who had severe disease scores 3-6, with a mean NT50 of 1:696 – more than 3 times as high as the first batch. The small number of high-titer donors makes it difficult to estimate the significance of this association with severe disease, unfortunately.

A significant correlation was found between the donor age, but the predictive value was low. Thus, it may not be possible to pick CP donors based on their age. Thirdly, female donors had a significantly lower mean NT50 than male donors, with a difference of 1:220. However, the meaning of this difference in real life is not yet clear.

The study concludes, “We have established an ELISA-based correlate to the MNT, with a threshold proposal that could be used to eliminate lower titer units from the clinical supply for COVID-19 treatment.” However, donor characteristics associated with a significantly higher antibody titer are not yet known.

*Important Notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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