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The Role of SARS-Cov-2 Antibody Testing; Current and Future Applications

Emma Heffernan, Margaret M Hannan, Maria Fitzgibbon

SARS-CoV-2 which causes coronavirus disease (COVID-19) is still challenging health care systems and governments all over the world. Although molecular testing remains the most reliable laboratory method available for establishing active infection, serological tests can identify past infection and measure immune response in vaccinated individuals. Serological tests have a number of useful applications in the management and control of the COVID-19 pandemic including as indicators of past infection, an adjunct to molecular testing in certain clinical situations, the diagnosis of late presentation COVID-19, seroprevalence studies and in assessing the efficacy of vaccines in development and the follow-up and monitoring of vaccinated individuals. Initial SARS-CoV-2 antibody assays which were qualitative and had different antigens as their target proteins have been revised to include the Spike (S) protein or the Receptor Binding Domain (RBD) protein as their target antigens and have the potential to quantify the antibody response. These assay revisions will facilitate the quantification of specific antibody titre which will in turn enable the monitoring of antibody response in individuals over time and the response to different available vaccines. Serological assays designed to assess antibody response to vaccination should include a good correlation of antibody results with neutralizing activity and evidence suggests that S protein based immunoassays correlate better with neutralizing activity than Nucleocapsid (N) protein based assays. Studies comparing both the N and S protein based assays have shown an additional utility of serological assays in differentiating between a SARS-CoV-2 infected antibody response and a vaccine induced immune response. Additionally, quantitative antibody assays could play a role in a more targeted distribution of vaccines by assessing antibody levels after one dose of vaccine which will allow for wider vaccine coverage. Standardisation of SARS-CoV-2 antibody assays is essential and the recent availability of an International Standard (IS) and International Reference Panel for anti-SARS-CoV-2 immunoglobulins will facilitate the future development and evaluation of serological assays. This will in turn help define protective levels of antibody and aid in the assessment of the efficacy and durability of antibody responses to the different vaccines available and in development.