Studying temporal titre evolution of commercial SARS-CoV-2 assays reveals significant shortcomings of using BAU standardization for comparison
Authors: Kroidl Inge , Winter Simon , Rubio-Acero Raquel , Bakuli Abhishek , Geldmacher Christof , Eser Tabea M. , Déak Flora , Horn Sacha , Zielke Anna , Ahmed Mohamed I. M. , Diepers Paulina , Guggenbühl Jessica , Frese Jonathan , Bruger Jan , Puchinger Kerstin , Reich Jakob , Falk Philine , Markgraf Alisa , Fensterseifer Heike , Paunovic Ivana , Thomschke Angelika , Pritsch Michael , Riess Friedrich , Saathoff Elmar , Hoelscher Michael , Olbrich Laura , Castelletti Noemi , Wieser Andreas ,   KoCo19/ORCHESTRA Study Group, Alamoudi Emad , Anderson Jared , Baldassarre Valeria , Baumann Maximilian , Becker Marc , Bednarski Franziska , Behlen Marieke , Bemirayev Olimbek , Beyerl Jessica , Bitzer Patrick , Böhnlein Rebecca , Brand Isabel , Brauer Anna , Britz Vera , Bünz Franziska , Caroli Friedrich , Coleman Josephine , Contento Lorenzo , Czwienzek Alina , Deák Flora , Diefenbach Maximilian N , Diekmannshemke Jana , Do Anna , Dobler Gerhard , Durner Jürgen , Eser Tabea , Eberle Ute , Eckstein Judith , Feyereisen Manuela , Fingerle Volker , Fischer Stefanie , Forster Felix , Fröschl Günter , Fuchs Christiane , Geisenberger Otto , Garí Mercè , Gasser Marius , Gauder Sonja , Geier Raffaela , Gillig Kristina , Gezgin Keisha , Gilberg Leonard , Gillig Kristina , Girl Philipp , Golschan Elias , Grauvogl Vitus , Noller Jessica Michelle Guggenbuehl , Guglielmini Elena Maria , Gutierrez Pablo , Haderer Anslem , Halfmann Celina , Hannes Marlene , Hartinger Lena , Haselwarter Timm , Hasenauer Jan , Hernandez Alejandra , Heller Luca , Heiber Arlett , Herrmann Matthias , Hillari Leah , Hillmann Stefan , Hinske Christian , Hoefflin Janna , Hofberger Tim , Höfinger Michael , Hofmann Larissa , Huber Kristina , Janke Christian , Karger Lilian , Kappl Ursula , Keßler Antonia , Khan Zohaib , Kiani Charlotte , Klugherz Isabel , Kreider Norah , Kresin Johanna , Kroidl Arne , Kunder Pratik , Lang Magdalena , Lang Clemens , Lange Silvan , Lapteva Ekaterina , Laxy Michael , Le Gleut Ronan , Leidl Reiner , Liedl Leopold , Lindner Felix , Lucaj Xhovana , Lucke Elisabeth , Luppa Fabian , Nafziger Alexandra Sophie , Maczka Alexander , Mang Petra , Matcau Paula , Mayrhofer Rebecca , Mekota Anna-Maria , Metaxa Dafni , Mohr Emily , Müller Hannah , Müller Katharina , Nascimento Nathalia , Niermeyer Kasimir , Nikolaides Sophia , Noreña Ivan , Pattard Leonie , Plank Michael , Pleimelding Claire , Pletschette Michel , Poll Viona , Prückner Stephan , Pusl Konstantin , Pütz Peter , Radon Katja , Raimúndez Elba , Raschka Julius , Reinkemeyer Christina , Rothe Camilla , Ruci Viktoria , Schäfer Nicole , Schälte Yannik , Schandelmaier Paul , Schluse Benedikt , Schneider Annika , Schneider Lara , Schultz Sophie , Schunk Mirjam , Schwettmann Lars , Sedlmeier Josefin , Sintu-Sempta Linda , Soler Alba , Sothmann Peter , Strobl Katharina , Strüber Aida , Strüber Laura , Tang Jeni , Theis Fabian , Thiel Verena , Thumser Eva , Thur Niklas , Thiesbrummel Sophie , Ullrich Julian , Vollmayr Vincent , Von Lovenberg Emilia , Von Lovenberg Jonathan , Vos Carsten , Waibel Julia , Wallrauch Claudia , Weigl Nikolas , Wölfl Roman , Wolff Julia , Wullinger Pia , Würfel Tobias , Wustrow Patrick , Zange Sabine , Zeggini Eleftheria , Zimmer Thorbjörn , Zimmermann Thomas , Zuche Lea 
Published: September 2023

Abstract
Background
Measuring specific anti-SARS-CoV-2 antibodies has become one of the main epidemiological tools to survey the ongoing SARS-CoV-2 pandemic, but also vaccination response. The WHO made available a set of well-characterized samples derived from recovered individuals to allow normalization between different quantitative anti-Spike assays to defined Binding Antibody Units (BAU).

Methods
To assess sero-responses longitudinally, a cohort of ninety-nine SARS-CoV-2 RT-PCR positive subjects was followed up together with forty-five vaccinees without previous infection but with two vaccinations. Sero-responses were evaluated using a total of six different assays: four measuring anti-Spike proteins (converted to BAU), one measuring anti-Nucleocapsid proteins and one SARS-CoV-2 surrogate virus neutralization. Both cohorts were evaluated using the Euroimmun Anti-SARS-CoV-2-ELISA anti-S1 IgG and the Roche Elecsys Anti-SARS-CoV-2 anti-S1 assay.

Results
In SARS-CoV-2-convalesce subjects, the BAU-sero-responses of Euroimmun Anti-SARS-CoV-2-ELISA anti-S1 IgG and Roche Elecsys Anti-SARS-CoV-2 anti-S1 peaked both at 47 (43–51) days, the first assay followed by a slow decay thereafter (> 208 days), while the second assay not presenting any decay within one year. Both assay values in BAUs are only equivalent a few months after infection, elsewhere correction factors up to 10 are necessary. In contrast, in infection-naive vaccinees the assays perform similarly.

Conclusion
The results of our study suggest that the establishment of a protective correlate or vaccination booster recommendation based on different assays, although BAU-standardised, is still challenging. At the moment the characteristics of the available assays used are not related, and the BAU-standardisation is unable to correct for that.