Fig. 1.

Evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike antibodies (U/mL) in individuals with previous SARS-CoV-2 infection (red points) and in seronegative persons without declared history of infection (blue points). Blood samples before the first vaccine dose were obtained a maximum of 2 days before. Geometric means with 95% CI are shown, if applicable. The grey dotted line corresponds to the positivity cut-off (0.8 U/mL) of the Elecsys anti-SARS-CoV-2 spike quantitative electrochemiluminescent immunoassay. An automatic dilution of 1/100 at >250 U/mL was performed by the analyser to extend the measurement domain up to 25 000 U/mL. Forty-two samples were rounded to 25 000 U/mL out of 1038 (4%). Results <0.4 U/mL (limit of quantification) were rounded up to 0.4. Up to day 4, blood samples taken 1 day earlier or later compared with the expected blood collection times were allowed. From day 7, 2 days were allowed. Individuals with incomplete samplings were not excluded from the analysis.