American Red Cross Hanover, Pennsylvania, United States
Background/Case Studies: SARS-CoV-2 nucleocapsid (N) antibody (Ab) boosts attributed to reinfections are difficult to identify due to variability of N Ab waning/boosting between donors in addition to variable inter-donation intervals resulting in testing gaps. Increases in N Ab signal-to-cutoff (S/CO) may be used to identify reinfections. Although no standard threshold of boosting exists that defines reinfection, we sought to investigate whether a 1.5x ratio increase in N Ab S/COs from a pre-infection donation to a post-infection donation, based on a previous study by Héma-Québec, coincides with donor reported reinfection.
Study
Design/Methods: A cohort of 3437 blood donors were tested at each donation for Ab against the spike (S) protein (Ortho VITROS S Total Ig), and S-reactive samples were tested for N Ab (Roche Elecsys Total Ig) between June 2020 and June 2021. Ortho VITROS N Total Ig replaced Roche in July 2021 and testing continued through October 2022. Quarterly surveys in 2022 were used to identify donors with self-reported infections. Reinfections were defined as a second reported infection. We evaluated whether reinfections were corroborated by a 1.5x increase in N Ab S/CO from consecutive donations after the first observed N Ab decline from a previously tested peak. The proportion of donors with confirmed reinfection was calculated. Inter-assay variability by correlation and mean log10 paired difference for samples tested on both assays were assessed.
Results/Findings: Among 88 donors with reported reinfections, 77 had donated specimens tested before and after the infection; 69/77 (90%) were identified using the 1.5x ratio criteria. For all reinfections (n=69), median boost inter-donation interval was 90 days (IQR: 63-126 days); median boost ratio, 10.9x (IQR: 5.3-29.1x, Range: 1.6-644.2x); and median post-infection N Ab S/CO was 277.0 (IQR: 229.0-322.0). Comparison of 1112 dual N Ab tested samples showed strong correlation (r=0.91); VITROS N S/COs were slightly higher (mean log10 diff: 0.2, SD: 0.56). Conclusions: The ratio-based boosting method had good sensitivity, with 90% of reinfections identified. However, the approach may be limited if N Abs are measured infrequently due to long inter-donation intervals where unobserved boosts may be negated by waning. Almost all confirmed reinfections occurred after July 2021 (n=68), a likely consequence of increasing primary infections along with immunity escaping virus variants. As reinfections increase, so too does the importance of their identification to monitor the burden of SARS-CoV-2.
Importance of research: Understanding SARS-CoV-2 nucleocapsid antibody (N Ab) dynamics, including waning following first infection and boosting from reinfection (RI), is crucial to serological identification of incident infections. N Ab levels wane and RI lead to N Ab boosting facilitating identification of RI although complicating interpretation of seroprevalence studies and disease surveillance. Among blood donors reporting RI, 90% demonstrated a 1.5x boost in N Ab signal-to-cutoff from pre-RI to post-RI.
