RESULTS
The validity, specificity, and reliability of the SARS-CoV-2 neutralization assay used in the current study were demonstrated by the analysis of 100 convalescent plasma donations from polymerase chain reaction (PCR)-confirmed SARS-CoV-2 cases [9] and was the basis for the correlation of function against diverse binding assays [10]. As controls, 2 plasma samples collected before the emergence of SARS-CoV-2 did not contain SARS-CoV-2 nAbs, as expected, but neutralized HCoV-229E with NT50 values of 1:43 and 1:26, respectively.

Testing of IVIG
SARS-CoV-2 nAb titers were below the limit of detection for all 54 IVIG lots tested, irrespective of geographic origin of the plasma (Europe vs United States) and plasma collection modality (recovered vs source) (Figure 1A).
Figure 1.  Coronavirus neutralizing antibody titers in IVIG lots (n = 54) against (A) SARS-CoV-2 and (B) HCoV-229E. The IVIG lots were manufactured from plasma either donated by plasmapheresis (S), or recovered from whole blood donations (R), in the United States or central Europe. Each dot represents the mean of 2 independent experiments, except in (B) from 1 EU-R IVIG lot the titer of a single determination is shown. The lines represent the median in each group. Paired t tests were used for determination of significance. Abbreviation: EU, Europe; HCoV, human coronavirus; IVIG, intravenous immunoglobulin; LOD, limit of detection; µNT50, 50% neutralization titer; R, recovered; S, source; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2. In contrast, HCoV-229E nAb titers between 1:43 and 1:196 (mean = 1:98) were measured for the 54 IVIG lots tested (Figure 1B). IVIG lots produced from recovered plasma contained significantly higher levels of nAb to HCoV-229E as compared to IVIG lots produced from source plasma, independent of the geographic origin. A significant, although quantitatively minor, difference was also found between IVIG lots manufactured from source plasma collected in either the United States or Europe.

Testing of Human Plasma
To evaluate the potential development of SARS-CoV-2 antibodies in the plasma donor community, samples of plasma pools of 6 donations each were tested. The use of pools enabled testing of a high amount of plasma donations for SARS-CoV-2 nAbs with the biosafety level-3 functional assay. As the mean SARS-CoV-2 µNT50 of plasma donations is rather high (approximately 1:230 [9]) even the nAbs of only 1 positive sample within a pool are detectable in this assay.
Testing a total of 560 plasma pools of 6 donations each, in total reflective of 3360 plasma donations, from week 13 (ie, last week of March) until week 28 (ie, first week of July) 2020 revealed that most of these pools had SARS-CoV-2 µNT titers below the limit of detection (Table 1). The first pool with detectable nAbs to SARS-CoV-2 was collected in week 14. Further positive pools were found in weeks 15, 16, 24, and 28. Up to 7% of the tested pools showed nAbs to SARS-CoV-2, which indicates that up to 1.17% of the plasma donors were positive for SARS-CoV-2 nAbs at a cumulative incidence of COVID-19 in Austria of 0.21% (Table 1).
Table 1.  SARS-CoV-2 Neutralizing Antibodies in Tested Plasma Pools
Week in 2020
Parameter  13  14  15  16  20  24  28
Plasma pools ≥LOD, %  0.0  1.3  1.3  6.3  0.0  5.0  7.0
µNT50 of plasma pool titers ≥LOD, 1:X  n.a.  37  5  6, 18, 26, 34, 87  n.a.  7, 10, 20, 22  4, 7, 8, 9, 12, 18, 24
Number of plasma pools tested  40  80  80  80  100  80  100
SARS-CoV-2 nAb positive plasma donors in Austria, %  0.00  0.21  0.21  1.04  0.00  0.83  1.17
Cumulative COVID-19 incidence in Austria, %a  0.10  0.14  0.16  0.17  0.18  0.19  0.21
Abbreviations: µNT50, neutralization titer for 50% of the wells; COVID-19, coronavirus disease 2019; LOD, limit of detection; n.a., not applicable; nAb, neutralizing antibody; PCR, polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.
aSARS-CoV-2 PCR-positive individuals per 100 000 population in Austria are described as cumulative COVID-19 incidence in percent.