Pulmonary lesions following inoculation with the SARS-CoV-2 Omicron BA.1 (B.1.1.529) variant in Syrian golden hamsters (preprint)

The Omicron BA.1 (B.1.1.529) SARS-CoV-2 variant is characterized by a high number of mutations in the viral genome, associated with immune-escape and increased viral spread. It remains unclear whether milder COVID-19 disease progression observed after infection with Omicron BA.1 in humans is due to reduced pathogenicity of the virus or due to pre-existing immunity from vaccination or previous infection. Here, we inoculated hamsters with Omicron BA.1 to evaluate pathogenicity and kinetics of viral shedding, compared to Delta (B.1.617.2) and to animals re-challenged with Omicron BA.1 after previous SARS-CoV-2 614G infection. Omicron BA.1 infected animals showed reduced clinical signs, pathological changes, and viral shedding, compared to Delta-infected animals, but still showed gross- and histopathological evidence of pneumonia. Pre-existing immunity reduced viral shedding and protected against pneumonia. Our data indicate that the observed decrease of disease severity is in part due to intrinsic properties of the Omicron BA.1 variant.

Omicron BA.1 and BA.2 are antigenically distinct SARS-CoV-2 variants (preprint)

The emergence and rapid spread of SARS-CoV-2 variants may impact vaccine efficacy significantly. The Omicron variant termed BA.2, which differs genetically substantially from BA.1, is currently replacing BA.1 in several countries, but its antigenic characteristics have not yet been assessed. Here, we used antigenic cartography to quantify and visualize antigenic differences between SARS-CoV-2 variants using hamster sera obtained after primary infection. Whereas early variants are antigenically similar, clustering relatively close to each other in antigenic space, Omicron BA.1 and BA.2 have evolved as two distinct antigenic outliers. Our data show that BA.1 and BA.2 both escape (vaccine-induced) antibody responses as a result of different antigenic characteristics. Close monitoring of the antigenic changes of SARS-CoV-2 using antigenic cartography can be helpful in the selection of future vaccine strains.

Extended Shedding and Enhanced Fitness of the SARS-CoV-2 Variant of Concern B.1.1.7 in Human Organoid Systems (preprint)

A new phase of the COVID-19 pandemic has started as SARS-CoV-2 variants are emerging globally, raising concerns for increased transmissibility. Early 2021 the B.1.1.7 (or Alpha) variant, became the dominant variant globally and epidemiological data suggests this variant spreads faster than its ancestors. However, this does not prove that a variant is intrinsically phenotypically different, let alone more transmissible or fit. Therefore, rapid phenotyping of SARS-CoV-2 variants of concern is urgently needed. We found that airway, intestinal and alveolar organoids infected with the B.1.1.7 variant produced higher levels of infectious virus late in infection compared to its 614G-containing ancestor. The B.1.1.7 variant also had a clear fitness advantage in human airway organoids. In alveolar organoids, the B.1.1.7 variant induced lower levels of innate immunity. These findings suggest that the B.1.1.7 variant is phenotypically different from its ancestor and may explain why this clade has spread rapidly across the globe.Funding Information: This work was supported by Netherlands Organization for Health Research and Development (10150062010008; B.L.H.), PPP allowance (LSHM19136; B.L.H.). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 874735. Declaration of Interests: H.C. is inventor on patents held by the Royal Netherlands Academy of Arts and Sciences that cover organoid technology. H.C.’s full disclosure is given at https://www.uu.nl/staff/JCClevers. All other authors have nothing to declare. Ethics Approval Statement: The Medical Ethical Committee of the Erasmus MC Rotterdam granted permission for this study (METC 2012-512). The study was approved by the UMC Utrecht (Utrecht, The Netherlands) ethical committee and was in accordance with the Declaration of Helsinki and according to Dutch law. This study is compliant with all relevant ethical regulations regarding research involving human participants.

Human airway cells prevent SARS-CoV-2 multibasic cleavage site cell culture adaptation (preprint)

Virus propagation methods generally use transformed cell lines to grow viruses from clinical specimens, which may force viruses to rapidly adapt to cell culture conditions, a process facilitated by high viral mutation rates. Upon propagation in VeroE6 cells, SARS-CoV-2 may mutate or delete the multibasic cleavage site (MBCS) in the spike protein that facilitates serine protease-mediated entry into human airway cells. We report that propagating SARS-CoV-2 on the human airway cell line Calu-3 - that expresses serine proteases - prevents MBCS mutations. Similar results were obtained using a human airway organoid-based culture system for SARS-CoV-2 propagation. Thus, in-depth knowledge on the biology of a virus can be used to establish methods to prevent cell culture adaptation.

Susceptibility of rabbits to SARS-CoV-2 (preprint)

Transmission of severe acute respiratory coronavirus-2 (SARS-CoV-2) between livestock and humans is a potential public health concern. We demonstrate the susceptibility of rabbits to SARS-CoV-2, which excrete infectious virus from the nose and throat upon experimental inoculation. Therefore, investigations on the presence of SARS-CoV-2 in farmed rabbits should be considered.