Thus, it is now possible to compare and harmonize different commercially available test systems as well as clinical and serological studies. In this standard preparation, the quantity of antibodies is defined as 1000 neutralization antibody units (IU) corresponding to 1000 binding antibody units (BAU) per mL. To solve this problem and to facilitate the comparison among different serological tests, the World Health Organization (WHO) has recently established and distributed reference material based on a standard serum pool collected from human donors having suffered from COVID-19. Until now, it has been difficult to compare the results obtained by different assay systems since the quantification of the viral ABs is not standardized. Most often, such assays measure antibodies of the classes IgG, IgA, and IgM. Usually, these ABs are directed against the SARS-CoV-2 surface S1-protein, the so-called spike protein, and the inner nucleocapsid protein N of the virus. So far, the detection of the humoral immune response after a SARS-CoV-2 infection has mainly been based on immunoassay systems measuring antibodies (ABs). Most likely, the wide use of the WHO reference preparation will be very useful in determining the individual immune status of patients after an infection with SARS-CoV-2 or after vaccination. Vaccination resulted in a rapid boost of antibodies to S1-protein but, as expected, not to the N-protein. Sera from all patients retained the ability to neutralize SARS-CoV-2 for more than a year. After natural infection, the antibodies (IgA, IgG) against the S1-protein remained elevated above the established cut-off to positivity (S-IgA 60 BAU/mL and S-IgG 50 BAU/mL, respectively) for over a year in all patients, while this was not the case for ABs against the N-protein (cut-off N-IgG 40 BAU/mL, N-IgA 256 BAU/mL). Furthermore, in two individuals, the effects of an additional vaccination with a mRNA vaccine containing the S1-RBD sequence on these antibodies were examined. Serum samples were collected frequently during a period of over one year. In this study, the concentration of antibodies (ABs) against both the S- and the N-protein of SARS-CoV-2 as well as serum neutralization activity were evaluated in three patients after a mild course of COVID-19. Omicron-specific quantitative IgG neutralization levels must be established to inform preventative care.The new WHO reference standard allows for the definition of serum antibodies against various SARS-CoV-2 antigens in terms of binding antibody units (BAU/mL) and thus to compare the results of different ELISA systems. Variability in vaccine responses and Ab declines show the utility of measuring spike Ig Ab levels rather than using empiric time frames for booster guidance. Ab levels decreased at an average rate of 91 IU/mL per month after primary immunization. In the pre-omicron era, primary COVID immunization with a mRNA vaccine generally yielded adequate Ab responses, although inadequate responses were observed in 19% of Pfizer, 6% of Moderna, and 49% of J&J vaccine recipients. Only 1/7 breakthrough COVID-19 infections occurred post booster (6 days later Conclusion. While some variation was observed, rates of Ab decay were generally consistent across vaccine, HIV status and CD4 count strata (Figure 2). In those with >=2 Ab tests within six months between vaccination and boosting, Ab levels declined at a rate of 91 IU/mL per month (95% CI -138, -44). Overall, 83% had an adequate immune response to vaccination (Pfizer 82%, Moderna 94%, J&J 51%), with similar findings regardless of HIV status and CD4 count (Figure 1). Out of 1979 patients, 869 completed their primary vaccinations, of whom 825 (95%) had >=1 eligible Ab test ( HIV+ 512 HIV- 313 Table). Vaccine response was assessed at the first Ab test and considered adequate (>250 IU/mL) or inadequate (low >=51 to 200 cells/muL cutoff). Adults who were fully vaccinated against SARS-CoV-2 virus (i.e., 2 Pfizer, 2 Moderna or 1 J&J vaccine) were included if >1 Roche SARS-CoV-2 Semi-Quant Spike Ig Ab test was performed >21 days after vaccination and before any booster (through 03DEC2021). In a New York City healthcare clinic where those guidelines were adopted, we aimed to quantify vaccination responses in HIV + and HIV- individuals to assess the utility of quantifying antibodies to guide booster timing. In-vitro neutralizing antibody (Ab) titers correlated with ~250 IU/ mL Spike Ig Ab level for the Delta COVID-19 variant, establishing the 2021 French and Swiss cutoff for booster guidance.
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