INTRODUCTION
The use of convalescent plasma to treat patients infected with emerging respiratory viruses, including severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) and avian influenza, has been a topic of study for decades (
1–3). Since the start of the SARS-CoV-2 pandemic, convalescent plasma was identified as a potential therapeutic candidate for clinical trials (
4,
5). Those clinical trials identified mixed efficacy of convalescent plasma and the potential for early use of high-titer convalescent plasma in immunocompromised patients infected with SARS-CoV-2 (
6–10). A revision of the U.S. Food and Drug Administration (FDA) emergency use authorization (EUA) for the use of COVID-19 convalescent plasma identified immunocompromised individuals as clinical trial candidates for high-titer COVID-19 convalescent plasma (
11). Work on convalescent plasma has also led to further studies on protective immunity to SARS-CoV-2 (
12–15) and has informed our incomplete understanding of the correlation of protection against SARS-CoV-2 (
16,
17). Earlier convalescent plasma qualification approaches relied on low-throughput culture-based 50% plaque reduction neutralization (PRNT
50) assays (
18). Other, more rapid and easier-to-utilize approaches, including virus-like particle (VLP), competition assays, and enzyme-linked immunosorbent assays, were also used to identify high-titer plasma and study immune responses in individuals previously infected with SARS-CoV-2 (
8,
12,
15,
19).
The Abbott SARS-CoV-2 IgG II Quant assay (Abbott anti-spike [S]; Abbott, Chicago, IL, USA; here referred to as the Abbott Quant assay) is a high-throughput assay that is simpler to operationalize than PRNT
50. This assay generates semiquantitative results which can be converted into binding antibody units [BAU] per milliliter (
20). A prior study noted that a cutoff of 7.1 × 10
3 BAU/mL might be used to screen for neutralizing high-titer plasma against wild-type, Alpha, Beta, Gamma, and Delta SARS-CoV-2 (
12). High-throughput semiquantitative technologies enable researchers to screen large numbers of plasma donations for unique specimens that might contain high-titer anti-SARS-CoV-2 neutralizing plasma against wild-type and variant of concern (VOC) SARS-CoV-2 (
21).
SARS-CoV-2 VOC Omicron has shown an ability to partially evade both infection and vaccine-generated pre-Omicron neutralizing antibody capacity (
22–25). In individuals with a prior BA.1 or BA.2 infection, there is also a marked decrease in neutralizing capacity against BA.2.12.1, BA.4, and BA.5 (
26). Compared to BA.5, Omicron BQ.1.1 and XBB.1 subvariants were more likely to escape neutralizing antibodies after both monovalent and bivalent mRNA vaccine boosting (
27). There is growing evidence that screening plasma using high-throughput immunosorbent assays at a threshold of ≥2 × 10
4 BAU/mL may identify high-titer neutralizing plasma against Omicron BA.1 that could then be used in convalescent plasma clinical trials (
28–30).
Assessments of neutralizing capacity of plasma or serum may be impacted by local and temporal factors. Prior to the emergence of Omicron, less than 10% of Canadians were estimated to have been naturally infected with SARS-CoV-2 (
31,
32). Until January to March 2021, most infections in Canada were likely due to wild-type or Alpha SARS-CoV-2 (
33). Vaccination campaigns were initiated in December 2020, with 96% of all Canadian blood donors showing evidence of measurable antibodies to anti-spike (S) by August 2021 (
34). Canadian Blood Services was able to determine donor vaccination status most effectively for the time from January to March 2021 (
12,
13).
This study used PRNT50 to determine the neutralizing capacity of vaccinated and unvaccinated donor plasma collected from January to March 2021against Omicron BA.1. This study also used the Abbott Quant assay to screen a larger number of donor plasma specimens collected from this time period for individual specimens potentially containing high-titer neutralizing capacity against Omicron BA.1.
DISCUSSION
For the period from January to March 2021, plasma collected from vaccinated Canadian blood donors was more likely to have measurable neutralizing antibodies (measured by PRNT
50 against wild type, Alpha, Beta, Gamma, Delta, and Omicron BA.1) than plasma from unvaccinated blood donors. In the unvaccinated group, none of the plasma specimens had measurable PRNT
50 titers versus Omicron BA.1. As previously noted, specimens were collected when seroprevalence to SARS-CoV-2 was <10% and when most Canadians with a history of SARS-CoV-2 infection would have been infected with wild-type or Alpha SARS-CoV-2 (
32,
33). Only a minority (8%) of vaccinated donors in this study claimed to be fully vaccinated (
12,
32), and only 2% of Canadians had received two doses of a SARS-CoV-2 vaccine (
35). Wastewater studies and clinical specimens suggest that Omicron emergence occurred much later in Canada, during the period from November 2021 to January 2022 (
36–40).
As previously described, Omicron BA.1 can escape neutralization from patients infected with non-Omicron strains. These trends are independent of specific geographic regions. In the United States, convalescent-phase serum collected from a small number of patients infected with Delta (
n = 19) had lower levels of pseudovirus neutralization against BA.1 than convalescent-phase serum from BA.1-infected patients (
n = 31) (
41). In another U.S. study, postinfection serum panels (1 month postinfection [
n = 64] and 6 months postinfection [
n = 36]) collected prior to the emergence of BA.1 exhibited decreased neutralization against BA.1 than wild-type SARS-CoV-2 when measured with a 50% fluorescent focus reduction neutralization titer (FFRNT
50) assay (
42). Convalescent serum from Chinese patients hospitalized from January to April 2020 with no vaccination history (
n = 24) or 1 dose of vaccine (
n = 20) also exhibited reduced neutralization against BA.1 compared to wild type using a pseudovirus assay (
43). A small number of specimens collected from Austrian patients with ancestral infection (March and April 2020 [
n = 10]) had reduced neutralization of BA.1, using a focus-forming neutralization assay (
44).
None of the specimens screened with the Abbott Quant assay had a value of ≥2 × 10
4 BAU/mL, which has been previously associated with high-titer plasma against Omicron BA.1 (
28). This is not unexpected, as convalescent plasma collected during earlier waves of the pandemic may have reduced efficacy against Omicron subvariants as they arise (
45). However, this finding does not imply that the donors tested lacked protection against SARS-CoV-2 disease and death. Immunity to SARS-CoV-2 is complex and involves neutralizing antibodies, binding antibodies, antibody-dependent cellular cytotoxicity (
46), complex mechanisms of cell-mediated immunity (
47), and elements of innate immunity (
48). Due to this complexity, there is still no wide consensus on correlations of protection to SARS-CoV-2 (
16,
17). Apart from a potential role as a cutoff for high-titer convalescent plasma by convalescent plasma trials (
28,
29), there is also no international consensus on the protective utility of the binding antibody value of ≥2 × 10
4 BAU/mL (
30,
49).
A full year of the pandemic would need to pass before the Canadian population developed high BAU per milliliter values. A larger Canadian seroprevalence study (10,000 to 40,000 specimens/month) first identified median BAU/mL levels of ≥2 × 10
4 BAU/mL in February of 2022 after the emergence of Omicron. However, the low frequency of anti-N and high frequency of anti-S in the population suggests that high BAU per milliliter values were being driven by COVID-19 vaccination programs rather than natural infection (
50). This study does not discriminate between the impacts of boosters or new bivalent vaccines. However, it is important to note the benefit of SARS-CoV-2 vaccines in reducing disease burden and death in the Canadian population, even in an environment dominated by Omicron (
51–53). The rollout of SARS-CoV-2 vaccines in Canada can be seen as a success story, with 85% of Canadians receiving at least one dose and 82% receiving a primary series by 11 September 2022. However, some Canadians expressed antivaccine sentiments, lacking understanding of vaccines and herd immunity (
54), and vaccine-hesitant individuals often expressed a preference for natural immunity (
55).
This study has several additional caveats. Different cell culture conditions were used for wild type, Alpha, Beta, Gamma, and Delta than for Omicron. To account for this, the study focused on identifying the presence or absence of any neutralizing antibody capacity against SARS-CoV-2 VOCs. This study included a small number of specimens for the time from January 2021 to March 2021 used for PRNT
50 (
13). Due to the time taken to develop Omicron BA.1 PRNT
50 assays, this study did not assess donor plasma for neutralization against later sublineages of BA.1, BA.2, BA.3, BA.4, BA.5, or recombinants that have circulated in Canada (
56). It is also important to acknowledge that donor-declared histories of vaccination may be confounded by recall bias and may be incomplete (
57). The collection of vaccination histories, as approved in the study ethics proposal, was also limited to the specimens used for PRNT
50 and not linked to data broadly tested with the Abbott Quant assay.
Although this work relies on specimens collected early in the pandemic, it does have applicability to understanding humoral immunity in individuals who are partially vaccine hesitant (receiving less than a full series of wild-type SARS-CoV-2 vaccine) or completely vaccine hesitant (relying on immunity from an earlier infection with wild-type or Alpha SARS-CoV-2). Those individuals may have impaired humoral protection against Omicron BA.1 SARS-CoV-2 infection. Therefore, even in populations with high rates of SARS-CoV-2 infection, vaccination (including boosting with monovalent or bivalent vaccines) is an important strategy in reducing the burden of severe disease and death (
58,
59). This protection is broad and ensures the safety of adults and children in the population from outcomes including intensive care admission and death, even when Omicron is dominant (
51).
ACKNOWLEDGMENTS
Canadian Blood Services staff and leadership were instrumental in supporting this project. The operations of the University of Alberta biosafety level 3 (BSL3) laboratory were supported by M. Desaulniers.
The following authors have no conflicts of interest: Sheila F. O’Brien, Qi-Long Yi, Ashleigh Tuite, Karen Colwill, Bhavisha Rathod, Kento T. Abe, and Yi-Chan J. Lin. Stated conflicts of interest are as follows. David H. Evans consults for, and holds research contracts from, Tonix Pharmaceuticals, New York, related to the construction of COVID-19 vaccines. Support for the operations of the University of Alberta BSL3 facility was also received from the Li Ka Shing Institute of Virology, Canada Foundation for Innovation, and Alberta Innovates. Steven J. Drews has functioned as a content expert for respiratory viruses for Johnson & Johnson (Janssen) and has received funding in kind from Abbott. Guillermo Orjuela and Ninette F. Robbins are current employees and shareholders of Abbott Laboratories. Anne-Claude Gingras has received research funds from a research contract with Providence Therapeutics Holdings, Inc., for other projects.
S.J.D., D.H.E., and S.F.O. received funding through the Canadian Institutes of Health Research (CIHR; VR2-172723) and Alberta Innovates (G2020000360 Drews). Commercial Abbott Architect SARS-Cov-2 IgG assay kit costs were supported by Abbott Laboratories, Abbott Park, Illinois. Abbott analyzers used by Canadian Blood Services were supplied by the COVID-19 Immunity Task Force (CITF). Publication charges for this manuscript were funded by the CITF. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Methodology, Y.-C.J.L., K.T.A., A.-C.G., B.R., K.C., A.T., D.H.E., and S.J.D.; Investigation, D.H.E., A.-C.G., K.C., A.T., S.F.O., and S.J.D.; Funding acquisition, S.F.O., D.H.E., G.O., N.F.R., A.T., A.-C.G., and S.J.D.; Supervision, D.H.E., A.-C.G., S.F.O., and S.J.D.; Manuscript drafting, S.J.D., Y.-C.J.L., S.F.O., A.T., A.-C.G., G.O., N.F.R., and D.H.E.; Data collation and analysis, A.-C.G., S.F.O., Q.-L.Y., D.H.E, Y.-C.J.L., and S.J.D.; and Administration, S.J.D.