ABSTRACT
Widespread and frequent testing is critical to prevent the spread of COVID-19, and rapid antigen tests are the diagnostic tool of choice in many settings. With new viral variants continuously emerging and spreading rapidly, the effect of mutations on antigen test performance is a major concern. In response to the spread of variants the National Institutes of Healths Rapid Acceleration of Diagnostics (RADx(R)) initiative created a Variant Task Force to assess the impact of emerging SARS-CoV-2 variants on in vitro diagnostic testing. To evaluate the impact of mutations on rapid antigen tests we developed a lentivirus-mediated mammalian surface-display platform for the SARS-CoV-2 Nucleocapsid protein, the target of the majority of rapid antigen tests. We employed deep mutational scanning (DMS) to directly measure the effect of all possible Nucleocapsid point mutations on antibody binding by 17 diagnostic antibodies used in 11 commercially available antigen tests with FDA emergency use authorization (EUA). The results provide a complete map of the antibodies epitopes and their susceptibility to mutational escape. This approach identifies linear epitopes, conformational epitopes, as well as allosteric escape mutations in any region of the Nucleocapsid protein. All 17 antibodies tested exhibit distinct escape mutation profiles, even among antibodies recognizing the same folded domain. Our data predict no vulnerabilities of rapid antigen tests for detection of mutations found in currently and previously dominant variants of concern and interest. We confirm this using the commercial tests and sequence-confirmed COVID-19 patient samples. The antibody escape mutation profiles generated here serve as a valuable resource for predicting the performance of rapid antigen tests against past, current, as well as any possible future variants of SARS-CoV-2, establishing the direct clinical and public health utility of our system. Further, our mammalian surface-display platform combined with DMS is a generalizable platform for complete mapping of protein-protein interactions.
ABSTRACT
BackgroundUpper respiratory samples for SARS-CoV-2 detection include the gold standard nasopharyngeal (NP) swab, and mid-turbinate (MT) nasal swabs, oropharyngeal (OP) swabs, and saliva. Following the emergence of the omicron (B.1.1.529) variant, limited preliminary data suggest that OP swabs or saliva samples may be more sensitive than nasal swabs, highlighting the need to understand differences in viral load across different sites. MethodsMT, OP, and saliva samples were collected from symptomatic individuals presenting for evaluation in Atlanta, GA, in January 2022. Longitudinal samples were collected from a family cohort following COVID-19 exposure to describe detection of viral targets over the course of infection. ResultsSARS-CoV-2 RNA and nucleocapsid antigen measurements demonstrated a nares-predominant phenotype in a familial cohort. A consistent dominant location for SARS-CoV-2 was not found among 54 individuals. Positive percent agreement for virus detection in MT, OP and saliva specimens were 66.7 [54.1-79.2], 82.2 [71.1-93.4], and 72.5 [60.3-84.8] by RT-PCR, respectively, and 46.2 [32.6-59.7], 51.2 [36.2-66.1], and 72.0 [59.6-84.4] by ultrasensitive antigen assay. The composite of positive MT or OP assay was not significantly different than either alone for both RT-PCR and antigen assay (PPA 86.7 [76.7-96.6] and 59.5 [44.7-74.4], respectively). ConclusionsOur data suggest that SARS-CoV-2 nucleocapsid and RNA exhibited similar kinetics and diagnostic yield in three upper respiratory sample types across the duration of symptomatic disease. Collection of OP or combined nasal and OP samples does not appear to increase sensitivity versus validated nasal sampling for rapid detection of viral antigen.
