Fig 1.

Increasing trend of spike IgG antibodies in a tertiary hospital patient population. The 3-year period between April 2020 and February 2023 was divided into five time periods to represent the alpha variant (up to January 2021), the alpha variants with partial vaccinations (February to June 2021), the delta variants (July to December 2021), the omicron BA.1 and 2 (January to July 2022), and the omicron BA.4/5 with BQ.1 and XBB variants (August to February 2023). A total of 15,820 sera are shown by the individual points, contributed by 11,022 patients. Most patients (8,451, 77%) were tested only once during the 3 years, while the remaining 2,571 underwent sequential measurements. The box plot in each of the five time periods represents the middle 50% of the observations, bordered at the 25th and 75th percentiles, and contain a line indicating the median antibody value. The 3.5 ELISA optical density value suggested by the FDA as cutoff for COVID-19 convalescent plasma donations is shown by the red dashed line. Presently, high antibody level patient samples require a 1–5,000 serum dilution in contrast to 1:101 suggested in the original Euroimmun. The conversion of Euroimmun AU 1 = 14 RU/mL = 45 BAU/mL such that on the low range 3.5 AU ~ 49 RU/mL ~ 157 BAU/mL and on the high range 150 AU ~ 2,100 RU/mL ~ 6,720 BAU/mL.