Introduction: Both the Roche and Abbott quantitative total/IgG spike antibody (S-Ab) assays are now traceable to the 1st WHO International Standard units (Binding antibody unit per mL [BAU/mL]). We performed this study to compare S-Ab to neutralizing antibody (N-Ab) responses up to 90 days post-vaccination in COVID-19 naïve subjects.
Methods: Between January to June 2021, seventy-three hospital staff from Changi General Hospital, Singapore, were tested for antibody levels pre-vaccination and 20-, 40-, 60- and 90-days post-vaccination. All subjects had no history of prior COVID-19 infection. Serum samples were assessed for total S-Ab (Roche Elecsys Anti-SARS-CoV-2 S), IgG S-Ab (Abbott quantitative IgG S-Ab), IgM S-Ab (Abbott qualitative IgM S-Ab), N-Ab (Snibe Maglumi) and total/IgG nucleocapsid antibodies (Roche/Abbott).
Results: Nucleocapsid antibodies were negative in all samples, corroborating their COVID-19 naïve status. All participants had high total/IgG S-Ab and N-Ab 20-days post-vaccination (Abbott IgG range 265-7764 BAU/mL, median 2412 BAU/mL; Roche total range 274-6127 BAU/mL, median 2146 BAU/mL; N-Ab range 0.51-15.7ug/mL, median 3.48ug/mL). All subjects experienced a significant decline in antibody levels by day 40 post-vaccination but antibodies appeared to plateau by day 60 and 90. Total/IgG S-Ab and N-Ab remained positive with high titers in all subjects throughout. In contrast, IgM displayed an increasing number of negative cases from day 20-90, with only 9.4% of subjects positive for IgM by day 90 (n=3/32). Younger patients had a significantly higher response than older patients. There was good agreement between N-Ab and S-Ab (total/IgG). (Total S-Ab r = 0.80, p<0.001, and IgG S-Ab r = 0.85, p<0.001).
Conclusion: Post-vaccination, Total/IgG S-Ab and N-Ab levels were high and correlated well. However, the IgM response was suboptimal and did not correlate with N-Ab. Total/IgG S-Ab expressed as BAU/mL were comparable and may be used as a surrogate for N-Ab.
Spike antibodies, Neutralizing antibodies, SARS-CoV-2, BNT162b2 mRNA vaccine, COVID-19, Immunoassays, WHO international standard units