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/SummarySARS-CoV-2 variants bearing complex combinations of mutations that confer increased transmissibility, COVID-19 severity, and immune escape, were first detected after S:D614G had gone to fixation, and likely originated during persistent infection of immunocompromised hosts. To test the hypothesis that S:D614G facilitated emergence of such variants, S:D614G was reverted to the ancestral sequence in the context of sequential Spike sequences from an immunocompromised individual, and within each of the major SARS-CoV-2 variants of concern. In all cases, infectivity of the S:D614G revertants was severely compromised. The infectivity of atypical SARS-CoV-2 lineages that propagated in the absence of S:D614G was found to be dependent upon either S:Q613H or S:H655Y. Notably, Gamma and Omicron variants possess both S:D614G and S:H655Y, each of which contributed to infectivity of these variants. Among sarbecoviruses, S:Q613H, S:D614G, and S:H655Y are only detected in SARS-CoV-2, which is also distinguished by a polybasic S1/S2 cleavage site. Genetic and biochemical experiments here showed that S:Q613H, S:D614G, and S:H655Y each stabilize Spike on virions, and that they are dispensable in the absence of S1/S2 cleavage, consistent with selection of these mutations by the S1/S2 cleavage site. CryoEM revealed that either S:D614G or S:H655Y shift the Spike receptor binding domain (RBD) towards the open conformation required for ACE2-binding and therefore on pathway for infection. Consistent with this, an smFRET reporter for RBD conformation showed that both S:D614G and S:H655Y spontaneously adopt the conformation that ACE2 induces in the parental Spike. Data from these orthogonal experiments demonstrate that S:D614G and S:H655Y are convergent adaptations to the polybasic S1/S2 cleavage site which stabilize S1 on the virion in the open RBD conformation and act epistatically to promote the fitness of variants bearing complex combinations of clinically significant mutations.Abstract FigureHighlightsS:D614G is ubiquitous among SARS-CoV-2 B-lineage Spikes and is required for infectivity of the main Variants of ConcernIn an example of convergent evolution, SARS-CoV-2 A lineage viruses maintained transmission chains in the absence of S:D614G, but were instead dependent upon S:Q613H or S:H655YS:D614G and S:H655Y are both adaptations to the polybasic S1/S2 cleavage siteIncreased infectivity of S:D614G and S:H655Y is associated with a more open RBD conformation and increased steady-state levels of virion-associated S1

The recently identified, globally predominant SARS-CoV-2 Omicron variant (BA.1) is highly transmissible, even in fully vaccinated individuals, and causes attenuated disease compared with other major viral variants recognized to date1–7. The Omicron spike (S) protein, with an unusually large number of mutations, is considered the major driver of these phenotypes3,8. We generated chimeric recombinant SARS-CoV-2 encoding the S gene of Omicron in the backbone of an ancestral SARS-CoV-2 isolate and compared this virus with the naturally circulating Omicron variant. The Omicron S-bearing virus robustly escapes vaccine-induced humoral immunity, mainly due to mutations in the receptor-binding motif (RBM), yet unlike naturally occurring Omicron, efficiently replicates in cell lines and primary-like distal lung cells. In K18-hACE2 mice, while Omicron causes mild, non-fatal infection, the Omicron S-carrying virus inflicts severe disease with a mortality rate of 80%. This indicates that while the vaccine escape of Omicron is defined by mutations in S, major determinants of viral pathogenicity reside outside of S.

There is increasing evidence that the risk of SARS-CoV-2 infection among vaccinated individuals is variant-specific, suggesting that protective immunity against SARS-CoV-2 may differ by variant. We enrolled vaccinated (n = 39) and unvaccinated (n = 11) individuals with acute, symptomatic SARS-CoV-2 Delta or Omicron infection and performed SARS-CoV-2 viral load quantification, whole-genome sequencing, and variant-specific antibody characterization at the time of acute illness and convalescence. Viral load at the time of infection was inversely correlated with antibody binding and neutralizing antibody responses. Increases in antibody titers and neutralizing activity occurred at convalescence in a variant-specific manner. Across all variants tested, convalescent neutralization titers in unvaccinated individuals were markedly lower than in vaccinated individuals. For individuals infected with the Delta variant, neutralizing antibody responses were weakest against BA.2, whereas infection with Omicron BA.1 variant generated a broader response against all tested variants, including BA.2.

Clinical features of SARS-CoV-2 Omicron variant infection, including incubation period and transmission rates, distinguish this variant from preceding variants. However, whether the duration of shedding of viable virus differs between omicron and previous variants is not well understood. To characterize how variant and vaccination status impact shedding of viable virus, we serially sampled symptomatic outpatients newly diagnosed with COVID-19. Anterior nasal swabs were tested for viral load, sequencing, and viral culture. Time to PCR conversion was similar between individuals infected with the Delta and the Omicron variant. Time to culture conversion was also similar, with a median time to culture conversion of 6 days (interquartile range 4-8 days) in both groups. There were also no differences in time to PCR or culture conversion by vaccination status.

The US Centers for Disease Control and Prevention recommends rapid testing for SARS-CoV-2 infection as a key element of epidemic control. The Abbott BinaxNow is in widespread use in the United States for self-testing and as part of public health screening campaigns, but has not been evaluated for use with the omicron variant of SARS-CoV-2. We recruited individuals testing positive for COVID-19 PCR at an academic medical center. Anterior nasal swabs were stored in viral transport media and evaluated by viral load quantification and whole genome sequencing. We created serial dilutions from 2.5×103-2.5×105 viral copies/specimen for two delta and omicron specimens, respectively, and tested each with the BinaxNow assay per manufacturer instructions. Results were interpreted by three readers, blinded to the specimen variant and concentration. All omicron and delta specimens with concentrations of 100,000 copies/swab or greater were positive by the BinaxNow Assay, a concentration similar to previously reported limits of detection for this assay. Assay sensitivity diminished below that. This study demonstrates that Omicron variant SARS-CoV-2 infections are detected by the BinaxNow rapid antigen assay. Additional laboratory and clinical validation assessments are needed to better determine their limits of detection and performance in real-world settings.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) monoclonal antibodies (mAbs) are among the treatments recommended for high-risk ambulatory persons with coronavirus 2019 (COVID-19). Here, we study viral culture dynamics post-treatment in a subset of participants receiving the mAb bamlanivimab in the ACTIV-2 trial (ClinicalTrials.gov: NCT04518410). Viral load by qPCR and viral culture are performed from anterior nasal swabs collected on study days 0 (day of treatment), 1, 2, 3, and 7. Treatment with mAbs results in rapid clearance of culturable virus. One day after treatment, 0 of 28 (0%) participants receiving mAbs and 16 of 39 (41%) receiving placebo still have culturable virus (p 0.0001). Recrudescence of culturable virus is detected in three participants with emerging mAb resistance and viral RNA rebound. While further studies are necessary to fully define the relationship between shed culturable virus and transmission, these results raise the possibility that mAbs may offer immediate (household) and public-health benefits by reducing onward transmission.

Quantitative Real-time PCR (qPCR) based Hepatitis B virus (HBV) DNA load estimation is crucial for the initiation of treatment and serves as a strong predictor of liver disease progression in HBV infected individuals. HBV DNA quantification has been ever evolving with the addition of new qPCR based kits on a regular basis. The study was carried with an objective to evaluate the performance characteristics of a commercially available qPCR kit (TRUPCR(®), 3B Black Bio Biotech, India Ltd.) and compare with CE approved qPCR kit (Artus HBV Real-time PCR, Qiagen, Germany). 121 HBV infected patients were prospectively enrolled from July to December 2016. Aliquots of serum samples were tested in parallel by TRUPCR(®) and Artus for HBV DNA levels. Genotype D was most predominant genotype in 36.9% (38/121) of patients followed by genotype A in 14.6% (15/121) patients. Median viral load as seen by Artus was log(10)IU/ml 3.37 (interquartile range log(10)IU/ml 2.10–10.89) as compared to TRUPCR(®) where it was log(10)IU/ml 3.54 (interquartile range log(10)IU/ml 2.67–11.52). A very good correlation was seen between the two assays (R(2) = 0.964) with a concordance rate of 92.6% (112/121). The TRUPCR(®) qPCR HBV kit is capable of providing reliable and rapid HBV DNA quantitation and together with its much lower costs, presents itself as a good alternative.