The SARS-CoV-2 Omicron BA.1 variant emerged in 20211 and has multiple mutations in its spike protein2. Here we show that the spike protein of Omicron has a higher affinity for ACE2 compared with Delta, and a marked change in its antigenicity increases Omicron's evasion of therapeutic monoclonal and vaccine-elicited polyclonal neutralizing antibodies after two doses. mRNA vaccination as a third vaccine dose rescues and broadens neutralization. Importantly, the antiviral drugs remdesivir and molnupiravir retain efficacy against Omicron BA.1. Replication was similar for Omicron and Delta virus isolates in human nasal epithelial cultures. However, in lung cells and gut cells, Omicron demonstrated lower replication. Omicron spike protein was less efficiently cleaved compared with Delta. The differences in replication were mapped to the entry efficiency of the virus on the basis of spike-pseudotyped virus assays. The defect in entry of Omicron pseudotyped virus to specific cell types effectively correlated with higher cellular RNA expression of TMPRSS2, and deletion of TMPRSS2 affected Delta entry to a greater extent than Omicron. Furthermore, drug inhibitors targeting specific entry pathways3 demonstrated that the Omicron spike inefficiently uses the cellular protease TMPRSS2, which promotes cell entry through plasma membrane fusion, with greater dependency on cell entry through the endocytic pathway. Consistent with suboptimal S1/S2 cleavage and inability to use TMPRSS2, syncytium formation by the Omicron spike was substantially impaired compared with the Delta spike. The less efficient spike cleavage of Omicron at S1/S2 is associated with a shift in cellular tropism away from TMPRSS2-expressing cells, with implications for altered pathogenesis. 

/pdf/altered-tmprss2-usage-by-sars-cov-2-omicron-impacts-2idrm9tt.pdf

Altered TMPRSS2 usage by SARS-CoV-2 Omicron impacts infectivity and fusogenicity

/pdf/altered-tmprss2-usage-by-sars-cov-2-omicron-impacts-2s9ev14t.pdf

http://www.cell.com/article/S0092867422007103/pdf

Antibody escape of SARS-CoV-2 Omicron BA.4 and BA.5 from vaccine and BA.1 serum

At the end of 2021 a new SARS-CoV-2 variant, Omicron, emerged and quickly spread across the world. It has been demonstrated that Omicron’s high number of Spike mutations lead to partial immune evasion from even polyclonal antibody responses, allowing frequent re-infection and vaccine breakthroughs. However, it seems unlikely these antigenic differences alone explain its rapid growth; here we show Omicron replicates rapidly in human primary airway cultures, more so even than the previously dominant variant of concern, Delta. Omicron Spike continues to use human ACE2 as its primary receptor, to which it binds more strongly than other variants. Omicron Spike mediates enhanced entry into cells expressing several different animal ACE2s, including various domestic avian species, horseshoe bats and mice suggesting it has an increased propensity for reverse zoonosis and is more likely than previous variants to establish an animal reservoir of SARS-CoV-2. Unlike other SARS-CoV-2 variants, however, Omicron Spike has a diminished ability to induce syncytia formation. Furthermore, Omicron is capable of efficiently entering cells in a TMPRSS2-independent manner, via the endosomal route. We posit this enables Omicron to infect a greater number of cells in the respiratory epithelium, allowing it to be more infectious at lower exposure doses, and resulting in enhanced intrinsic transmissibility.

/pdf/the-sars-cov-2-variant-omicron-shows-rapid-replication-in-1r8sgs62.pdf

The SARS-CoV-2 variant, Omicron, shows rapid replication in human primary nasal epithelial cultures and efficiently uses the endosomal route of entry

Epidemiologic surveillance has revealed decoupling of Coronavirus Disease 2019 (COVID-19) hospitalizations and deaths from case counts after emergence of the Omicron (B.1.1.529) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant globally. However, assessment of the relative severity of Omicron variant infections presents challenges because of differential acquired immune protection against Omicron and prior variants and because longer-term changes have occurred in testing and healthcare practices. Here we show that Omicron variant infections were associated with substantially reduced risk of progression to severe clinical outcomes relative to time-matched Delta (B.1.617.2) variant infections within a large, integrated healthcare system in Southern California. Adjusted hazard ratios (aHRs) for any hospital admission, symptomatic hospital admission, intensive care unit admission, mechanical ventilation and death comparing individuals with Omicron versus Delta variant infection were 0.59 (95% confidence interval: 0.51–0.69), 0.59 (0.51–0.68), 0.50 (0.29–0.87), 0.36 (0.18–0.72) and 0.21 (0.10–0.44), respectively. This reduced severity could not be explained by differential history of prior infection among individuals with Omicron or Delta variant infection and was starkest among individuals not previously vaccinated against COVID-19 (aHR = 0.40 (0.33–0.49) for any hospital admission and 0.14 (0.07–0.28) for death). Infections with the Omicron BA.2 subvariant were not associated with differential risk of severe outcomes in comparison to BA.1/BA.1.1 subvariant infections. Lower risk of severe clinical outcomes among individuals with Omicron variant infection should inform public health response amid establishment of the Omicron variant as the dominant SARS-CoV-2 lineage globally. Comparison of outcomes of SARS-CoV-2 Delta and Omicron infections shows reduced severity of Omicron infections, most notably in unvaccinated individuals, and no differential risk of severe outcomes between subvariants BA.1 and BA.2. The findings highlight the importance of continual assessment of clinical outcomes associated with SARS-CoV-2 variants of concern to inform both medical interventions and healthcare resource management. 

/pdf/clinical-outcomes-associated-with-sars-cov-2-omicron-b-1-1-ikejxhpy.pdf

Clinical outcomes associated with SARS-CoV-2 Omicron (B.1.1.529) variant and BA.1/BA.1.1 or BA.2 subvariant infection in Southern California

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant of concern evades antibody-mediated immunity that comes from vaccination or infection with earlier variants due to accumulation of numerous spike mutations. To understand the Omicron antigenic shift, we determined cryo-electron microscopy and x-ray crystal structures of the spike protein and the receptor-binding domain bound to the broadly neutralizing sarbecovirus monoclonal antibody (mAb) S309 (the parent mAb of sotrovimab) and to the human ACE2 receptor. We provide a blueprint for understanding the marked reduction of binding of other therapeutic mAbs that leads to dampened neutralizing activity. Remodeling of interactions between the Omicron receptor-binding domain and human ACE2 likely explains the enhanced affinity for the host receptor relative to the ancestral virus. 

/pdf/structural-basis-of-sars-cov-2-omicron-immune-evasion-and-1cnsjyr0.pdf

Eakapol Wankahart

Papers

Structural basis of SARS-CoV-2 Omicron immune evasion and receptor engagement