High incidence of imperforate vagina in ADGRA3-deficient mice.

BACKGROUND: Ten percent of the female population suffers from congenital abnormalities of the vagina, uterus, or oviducts, with severe consequences for reproductive and psychological health. Yet, the underlying causes of most of these malformations remain largely unknown. ADGRA3 (GPR125) is involved in WNT signaling and planar cell polarity, mechanisms vital to female reproductive tract development. Although ADGRA3 is a well-established spermatogonial stem cell marker, its role within the female urogenital system remains unclear. RESULTS: In this study, we found Adgra3 to be expressed throughout the murine female urogenital system, with higher expression pre-puberty than after sexual maturation. We generated a global Adgra3-/- mouse line and observed imperforate vagina in 44% of Adgra3-/- females, resulting in distension of the reproductive tract and infertility. Ovarian morphology, plasma estradiol, ovarian Cyp19a1, and vaginal estrogen receptor α (Esr1) expression were unaffected. However, compared to controls, a significantly lower bone mineral density was found in Adgra3-/- mice. Whereas vaginal opening in mice is an estrogen-dependent process, 17ß-estradiol treatment failed to induce vaginal canalization in Adgra3-/- mice. Furthermore, a marked reduction in vaginal and ovarian progesterone receptor expression was observed concomitant with an upregulation of apoptotic regulators Bcl2, Bid, and Bmf in adult Adgra3-/- females with a closed vagina. CONCLUSIONS: Our collective results shed new insights into the complex mechanisms by which the adhesion receptor ADGRA3 regulates distal vaginal tissue remodeling during vaginal canalization via altered sex hormone responsiveness and balance in apoptotic regulators. This highlights the potential of ADGRA3 as a target in diagnostic screening and/or therapy for obstructive vaginal malformations in humans.

Loss of Adgra3 causes obstructive azoospermia with high penetrance in male mice.

The adhesion receptor ADGRA3 (GPR125) is a known spermatogonial stem cell marker, but its impact on male reproduction and fertility has not been examined. Using a mouse model lacking Adgra3 (Adgra3-/- ), we show that 55% of the male mice are infertile from puberty despite having normal spermatogenesis and epididymal sperm count. Instead, male mice lacking Adgra3 exhibited decreased estrogen receptor alpha expression and transient dilation of the epididymis. Combined with an increased estradiol production, this indicates a post-pubertal hormonal imbalance and fluid retention. Dye injection revealed a blockage between the ejaculatory duct and the urethra, which is rare in mice suffering from infertility, thereby mimicking the etiologies of obstructive azoospermia found in human male infertility. To summarize, male reproductive tract development is dependent on ADGRA3 function that in concert with estrogen signaling may influence fluid handling during sperm maturation and storage.

Patterns of human and porcine gammaherpesvirus-encoded BILF1 receptor endocytosis.

BACKGROUND: The viral G-protein-coupled receptor (vGPCR) BILF1 encoded by the Epstein-Barr virus (EBV) is an oncogene and immunoevasin and can downregulate MHC-I molecules at the surface of infected cells. MHC-I downregulation, which presumably occurs through co-internalization with EBV-BILF1, is preserved among BILF1 receptors, including the three BILF1 orthologs encoded by porcine lymphotropic herpesviruses (PLHV BILFs). This study aimed to understand the detailed mechanisms of BILF1 receptor constitutive internalization, to explore the translational potential of PLHV BILFs compared with EBV-BILF1. METHODS: A novel real-time fluorescence resonance energy transfer (FRET)-based internalization assay combined with dominant-negative variants of dynamin-1 (Dyn K44A) and the chemical clathrin inhibitor Pitstop2 in HEK-293A cells was used to study the effect of specific endocytic proteins on BILF1 internalization. Bioluminescence resonance energy transfer (BRET)-saturation analysis was used to study BILF1 receptor interaction with ß-arrestin2 and Rab7. In addition, a bioinformatics approach informational spectrum method (ISM) was used to investigate the interaction affinity of BILF1 receptors with ß-arrestin2, AP-2, and caveolin-1. RESULTS: We identified dynamin-dependent, clathrin-mediated constitutive endocytosis for all BILF1 receptors. The observed interaction affinity between BILF1 receptors and caveolin-1 and the decreased internalization in the presence of a dominant-negative variant of caveolin-1 (Cav S80E) indicated the involvement of caveolin-1 in BILF1 trafficking. Furthermore, after BILF1 internalization from the plasma membrane, both the recycling and degradation pathways are proposed for BILF1 receptors. CONCLUSIONS: The similarity in the internalization mechanisms observed for EBV-BILF1 and PLHV1-2 BILF1 provide a foundation for further studies exploring a possible translational potential for PLHVs, as proposed previously, and provides new information about receptor trafficking.

First cases of SARS-CoV-2 BA.2.86 in Denmark, 2023.

We describe 10 cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant BA.2.86 detected in Denmark, including molecular characteristics and results from wastewater surveillance that indicate that the variant is circulating in the country at a low level. This new variant with many spike gene mutations was classified as a variant under monitoring by the World Health Organization on 17 August 2023. Further global monitoring of COVID-19, BA.2.86 and other SARS-CoV-2 variants is highly warranted.

Selective Boosting of CCR7-Acting Chemokines; Short Peptides Boost Chemokines with Short Basic Tails, Longer Peptides Boost Chemokines with Long Basic Tails.

The chemokine receptor CCR7 and its ligands CCL19 and CCL21 regulate the lymph node homing of dendritic cells and naïve T-cells and the following induction of a motile DC-T cell priming state. Although CCL19 and CCL21 bind CCR7 with similar affinities, CCL21 is a weak agonist compared to CCL19. Using a chimeric chemokine, CCL19CCL21N-term|C-term, harboring the N-terminus and the C-terminus of CCL21 attached to the core domain of CCL19, we show that these parts of CCL21 act in a synergistic manner to lower ligand potency and determine the way CCL21 engages with CCR7. We have published that a naturally occurring basic C-terminal fragment of CCL21 (C21TP) boosts the signaling of both CCL19 and CCL21. Boosting occurs as a direct consequence of C21TP binding to the CCR7 N-terminus, which seems to free chemokines with basic C-termini from an unfavorable interaction with negatively charged posttranslational modifications in CCR7. Here, we confirm this using a CCL19-variant lacking the basic C-terminus. This variant displays a 22-fold higher potency at CCR7 compared to WT CCL19 and is highly unaffected by the presence of C21TP. WT CCL19 has a short basic C-terminus, CCL21 a longer one. Here, we propose a way to differentially boost CCL19 and CCL21 activity as short and long versions of C21TP boost CCL19 activity, whereas only a long C21TP version can boost chemokines with a full-length CCL21 C-terminus.

Epidemiological characterisation of the first 785 SARS-CoV-2 Omicron variant cases in Denmark, December 2021.

By 9 December 2021, 785 SARS-CoV-2 Omicron variant cases have been identified in Denmark. Most cases were fully (76%) or booster-vaccinated (7.1%); 34 (4.3%) had a previous SARS-CoV-2 infection. The majority of cases with available information reported symptoms (509/666; 76%) and most were infected in Denmark (588/644; 91%). One in five cases cannot be linked to previous cases, indicating widespread community transmission. Nine cases have been hospitalised, one required intensive care and no deaths have been registered.

Methods for Studying Endocytotic Pathways of Herpesvirus Encoded G Protein-Coupled Receptors.

Endocytosis is a fundamental process involved in trafficking of various extracellular and transmembrane molecules from the cell surface to its interior. This enables cells to communicate and respond to external environments, maintain cellular homeostasis, and transduce signals. G protein-coupled receptors (GPCRs) constitute a family of receptors with seven transmembrane alpha-helical domains (7TM receptors) expressed at the cell surface, where they regulate physiological and pathological cellular processes. Several herpesviruses encode receptors (vGPCRs) which benefits the virus by avoiding host immune surveillance, supporting viral dissemination, and thereby establishing widespread and lifelong infection, processes where receptor signaling and/or endocytosis seem central. vGPCRs are rising as potential drug targets as exemplified by the cytomegalovirus-encoded receptor US28, where its constitutive internalization has been exploited for selective drug delivery in virus infected cells. Therefore, studying GPCR trafficking is of great importance. This review provides an overview of the current knowledge of endocytic and cell localization properties of vGPCRs and methodological approaches used for studying receptor internalization. Using such novel approaches, we show constitutive internalization of the BILF1 receptor from human and porcine γ-1 herpesviruses and present motifs from the eukaryotic linear motif (ELM) resources with importance for vGPCR endocytosis.

EBI2 overexpression in mice leads to B1 B-cell expansion and chronic lymphocytic leukemia-like B-cell malignancies.

Human and mouse chronic lymphocytic leukemia (CLL) develops from CD5+ B cells that in mice and macaques are known to define the distinct B1a B-cell lineage. B1a cells are characterized by lack of germinal center (GC) development, and the B1a cell population is increased in mice with reduced GC formation. As a major mediator of follicular B-cell migration, the G protein-coupled receptor Epstein-Barr virus-induced gene 2 (EBI2 or GPR183) directs B-cell migration in the lymphoid follicles in response to its endogenous ligands, oxysterols. Thus, upregulation of EBI2 drives the B cells toward the extrafollicular area, whereas downregulation is essential for GC formation. We therefore speculated whether increased expression of EBI2 would lead to an expanded B1 cell subset and, ultimately, progression to CLL. Here, we demonstrate that B-cell-targeted expression of human EBI2 (hEBI2) in mice reduces GC-dependent immune responses, reduces total immunoglobulin M (IgM) and IgG levels, and leads to increased proliferation and upregulation of cellular oncogenes. Furthermore, hEBI2 overexpression leads to an abnormally expanded CD5+ B1a B-cell subset (present as early as 4 days after birth), late-onset lymphoid cancer development, and premature death. These findings are highly similar to those observed in CLL patients and identify EBI2 as a promoter of B-cell malignancies.

Rationally designed chemokine-based toxin targeting the viral G protein-coupled receptor US28 potently inhibits cytomegalovirus infection in vivo.

The use of receptor-ligand interactions to direct toxins to kill diseased cells selectively has shown considerable promise for treatment of a number of cancers and, more recently, autoimmune disease. Here we move the fusion toxin protein (FTP) technology beyond cancer/autoimmune therapeutics to target the human viral pathogen, human cytomegalovirus (HCMV), on the basis of its expression of the 7TM G protein-coupled chemokine receptor US28. The virus origin of US28 provides an exceptional chemokine-binding profile with high selectivity and improved binding for the CX3C chemokine, CX3CL1. Moreover, US28 is constitutively internalizing by nature, providing highly effective FTP delivery. We designed a synthetic CX3CL1 variant engineered to have ultra-high affinity for US28 and greater specificity for US28 than the natural sole receptor for CX3CL1, CX3CR1, and we fused the synthetic variant with the cytotoxic domain of Pseudomonas Exotoxin A. This novel strategy of a rationally designed FTP provided unparalleled anti-HCMV efficacy and potency in vitro and in vivo.

Identification and functional comparison of seven-transmembrane G-protein-coupled BILF1 receptors in recently discovered nonhuman primate lymphocryptoviruses.

UNLABELLED: Coevolution of herpesviruses with their respective host has resulted in a delicate balance between virus-encoded immune evasion mechanisms and host antiviral immunity. BILF1 encoded by human Epstein-Barr virus (EBV) is a 7-transmembrane (7TM) G-protein-coupled receptor (GPCR) with multiple immunomodulatory functions, including attenuation of PKR phosphorylation, activation of G-protein signaling, and downregulation of major histocompatibility complex (MHC) class I surface expression. In this study, we explored the evolutionary and functional relationships between BILF1 receptor family members from EBV and 12 previously uncharacterized nonhuman primate (NHP) lymphocryptoviruses (LCVs). Phylogenetic analysis defined 3 BILF1 clades, corresponding to LCVs of New World monkeys (clade A) or Old World monkeys and great apes (clades B and C). Common functional properties were suggested by a high degree of sequence conservation in functionally important regions of the BILF1 molecules. A subset of BILF1 receptors from EBV and LCVs from NHPs (chimpanzee, orangutan, marmoset, and siamang) were selected for multifunctional analysis. All receptors exhibited constitutive signaling activity via G protein Gαi and induced activation of the NF-κB transcription factor. In contrast, only 3 of 5 were able to activate NFAT (nuclear factor of activated T cells); chimpanzee and orangutan BILF1 molecules were unable to activate NFAT. Similarly, although all receptors were internalized, BILF1 from the chimpanzee and orangutan displayed an altered cellular localization pattern with predominant cell surface expression. This study shows how biochemical characterization of functionally important orthologous viral proteins can be used to complement phylogenetic analysis to provide further insight into diverse microbial evolutionary relationships and immune evasion function. IMPORTANCE: Epstein-Barr virus (EBV), known as an oncovirus, is the only human herpesvirus in the genus Lymphocryptovirus (LCV). EBV uses multiple strategies to hijack infected host cells, establish persistent infection in B cells, and evade antiviral immune responses. As part of EBV's immune evasion strategy, the virus encodes a multifunctional 7-transmembrane (7TM) G-protein-coupled receptor (GPCR), EBV BILF1. In addition to multiple immune evasion-associated functions, EBV BILF1 has transforming properties, which are linked to its high constitutive activity. We identified BILF1 receptor orthologues in 12 previously uncharacterized LCVs from nonhuman primates (NHPs) of Old and New World origin. As 7TM receptors are excellent drug targets, our unique insight into the molecular mechanism of action of the BILF1 family and into the evolution of primate LCVs may enable validation of EBV BILF1 as a drug target for EBV-mediated diseases, as well as facilitating the design of drugs targeting EBV BILF1.

Unbiased Virus Detection in a Danish Zoo Using a Portable Metagenomic Sequencing System.

Metagenomic next-generation sequencing (mNGS) is receiving increased attention for the detection of new viruses and infections occurring at the human-animal interface. The ability to actively transport and relocate this technology enables in situ virus identification, which could reduce response time and enhance disease management. In a previous study, we developed a straightforward mNGS procedure that greatly enhances the detection of RNA and DNA viruses in human clinical samples. In this study, we improved the mNGS protocol with transportable battery-driven equipment for the portable, non-targeted detection of RNA and DNA viruses in animals from a large zoological facility, to simulate a field setting for point-of-incidence virus detection. From the resulting metagenomic data, we detected 13 vertebrate viruses from four major virus groups: (+)ssRNA, (+)ssRNA-RT, dsDNA and (+)ssDNA, including avian leukosis virus in domestic chickens (Gallus gallus), enzootic nasal tumour virus in goats (Capra hircus) and several small, circular, Rep-encoding, ssDNA (CRESS DNA) viruses in several mammal species. More significantly, we demonstrate that the mNGS method is able to detect potentially lethal animal viruses, such as elephant endotheliotropic herpesvirus in Asian elephants (Elephas maximus) and the newly described human-associated gemykibivirus 2, a human-to-animal cross-species virus, in a Linnaeus two-toed sloth (Choloepus didactylus) and its enclosure, for the first time.

Rapid and Flexible RT-qPCR Surveillance Platforms To Detect SARS-CoV-2 Mutations.

Multiple mutations in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs) increase transmission, disease severity, and immune evasion and facilitate zoonotic or anthropozoonotic infections. Four such mutations, ΔH69/V70, L452R, E484K, and N501Y, occurred in the SARS-CoV-2 spike glycoprotein in combinations that allow the simultaneous detection of VOCs. Here, we present two flexible reverse transcription-quantitative PCR (RT-qPCR) platforms for small- and large-scale screening (also known as variant PCR) to detect these mutations and schemes for adapting the platforms to future mutations. The large-scale RT-qPCR platform was validated by pairwise matching of RT-qPCR results with whole-genome sequencing (WGS) consensus genomes, showing high specificity and sensitivity. Both platforms are valuable examples of complementing WGS to support the rapid detection of VOCs. Our mutational signature approach served as an important intervention measure for the Danish public health system to detect and delay the emergence of new VOCs. IMPORTANCE Denmark weathered the SARS-CoV-2 crisis with relatively low rates of infection and death. Intensive testing strategies with the aim of detecting SARS-CoV-2 in symptomatic and nonsymptomatic individuals were available by establishing a national test system called TestCenter Denmark. This testing regime included the detection of SARS-CoV-2 signature mutations, with referral to the national health system, thereby delaying outbreaks of variants of concern. Our study describes the design of the large-scale RT-qPCR platform established at TestCenter Denmark in conjunction with whole-genome sequencing to report mutations of concern to the national health system. Validation of the large-scale RT-qPCR platform using paired WGS consensus genomes showed high sensitivity and specificity. For smaller laboratories with limited infrastructure, we developed a flexible small-scale RT-qPCR platform to detect three signature mutations in a single run. The RT-qPCR platforms are important tools to support the control of the SARS-CoV-2 endemic in Denmark.

Partial functional complementation between human and mouse cytomegalovirus chemokine receptor homologues.

The human cytomegalovirus (CMV) proteins US28 and UL33 are homologous to chemokine receptors (CKRs). Knockout of the mouse CMV M33 protein (UL33 homologue) results in substantial attenuation of salivary gland infection/replication and reduced efficiency of reactivation from tissue explants. M33-mediated G protein-coupled signaling is critical for the salivary gland phenotype. In this report, we demonstrate that US28 and (to a lesser degree) UL33 restore reactivation from tissue explants and partially restore replication in salivary glands (compared to a signaling-deficient M33 mutant). These studies provide a novel small animal model for evaluation of therapies targeting the human CMV CKRs.

Improvements in metagenomic virus detection by simple pretreatment methods.

Early detection of pathogens at the point of care helps reduce the threats to human and animal health from emerging pathogens. Initially, the disease-causing agent will be unknown and needs to be identified; this often requires specific laboratory facilities. Here we describe the development of an unbiased detection assay for RNA and DNA viruses using metagenomic Nanopore sequencing and simple methods that can be transferred into a field setting. Human clinical samples containing the RNA virus SARS-CoV-2 or the DNA viruses human papillomavirus (HPV) and molluscum contagiosum virus (MCV) were used as a test of concept. Firstly, the virus detection potential was optimized by investigating different pretreatments for reducing non-viral nucleic acid components. DNase I pretreatment followed by filtration increased the proportion of SARS-CoV-2 sequenced reads > 500-fold compared with no pretreatments. This was sufficient to achieve virus detection with high confidence and allowed variant identification. Next, we tested individual SARS-CoV-2 samples with various viral loads (measured as CT-values determined by RT-qPCR). Lastly, we tested the assay on clinical samples containing the DNA virus HPV and co-infection with MCV to show the assay's detection potential for DNA viruses. This protocol is fast (same day results). We hope to apply this method in other settings for point of care detection of virus pathogens, thus eliminating the need for transport of infectious samples, cold storage and a specialized laboratory.

A qualitative RT-PCR assay for the specific identification of the SARS-CoV-2 B.1.1.529 (Omicron) Variant of Concern.

OBJECTIVES: The aim of this study was to develop a RT-PCR assay for the specific detection of the SARS-CoV-2 Omicron Variant of Concern (VOC) as a rapid alternative to sequencing. METHODS: A RT-PCR was designed in silico and then validated using characterised clinical samples containing Omicron (both BA.1 and BA.2 lineages) and the Omicron synthetic RNA genome. As negative controls, SARS-CoV-2 positive clinical samples collected in May 2020, and synthetic RNA genomes of the isolate Wuhan Hu-1 and of the Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Kappa (B.1.617.1), Iota (B.1.526), Epsilon (B.1.429) and Delta (B.1.617.2) SARS-CoV-2 VOC were used. RESULTS: Experiments performed using as templates the synthetic RNA genomes demonstrate the high specificity of the PCR-method for the SARS-CoV-2 Omicron. Despite the synthetic RNAs were used at high copy numbers, specific signal was mainly detected with the Omicron synthetic genome. Only a non-specific late signal was detected using the Alpha variant genome, but these results were considered negligible as Alpha VOC has been replaced by the Delta and it is not circulating anymore in the world. Using our method, we confirmed the presence of Omicron on clinical samples containing this variant but not of other SARS-CoV-2 lineages. The method is highly sensitive and can detect up to 1 cp of the Omicron virus per µl. CONCLUSIONS: The method presented here, in combination with other methods in use for detection of SARS-CoV-2, can be used for an early identification of Omicron.

A RT-qPCR system using a degenerate probe for specific identification and differentiation of SARS-CoV-2 Omicron (B.1.1.529) variants of concern.

Fast surveillance strategies are needed to control the spread of new emerging SARS-CoV-2 variants and gain time for evaluation of their pathogenic potential. This was essential for the Omicron variant (B.1.1.529) that replaced the Delta variant (B.1.617.2) and is currently the dominant SARS-CoV-2 variant circulating worldwide. RT-qPCR strategies complement whole genome sequencing, especially in resource lean countries, but mutations in the targeting primer and probe sequences of new emerging variants can lead to a failure of the existing RT-qPCRs. Here, we introduced an RT-qPCR platform for detecting the Delta- and the Omicron variant simultaneously using a degenerate probe targeting the key ΔH69/V70 mutation in the spike protein. By inclusion of the L452R mutation into the RT-qPCR platform, we could detect not only the Delta and the Omicron variants, but also the Omicron sub-lineages BA.1, BA.2 and BA.4/BA.5. The RT-qPCR platform was validated in small- and large-scale. It can easily be incorporated for continued monitoring of Omicron sub-lineages, and offers a fast adaption strategy of existing RT-qPCRs to detect new emerging SARS-CoV-2 variants using degenerate probes.

Epstein-Barr Virus-Encoded BILF1 Orthologues From Porcine Lymphotropic Herpesviruses Display Common Molecular Functionality.

Infection of immunosuppressed transplant patients with the human γ-herpesvirus Epstein-Barr virus (EBV) is associated with post-transplant lymphoproliferative disease (PTLD), an often fatal complication. Immunosuppressed miniature pigs infected with γ-herpesvirus porcine lymphotropic herpesvirus 1 (PLHV1) develop a similar disease, identifying pigs as a potential preclinical model for PTLD in humans. BILF1 is a G protein-coupled receptor (GPCR) encoded by EBV with constitutive activity linked to tumorigenesis and immunoevasive function downregulating MHC-I. In the present study, we compared BILF1-orthologues encoded by the three known PLHVs (PLHV1-3) with EBV-BILF1 to determine pharmacological suitability of BILF1 orthologues as model system to study EBV-BILF1 druggability. Cell surface localization, constitutive internalization, and MHC-I downregulation as well as membrane proximal constitutive Gαi signaling patterns were conserved across all BILFs. Only subtle differences between the individual BILFs were observed in downstream transcription factor activation. Using Illumina sequencing, PLHV1 was observed in lymphatic tissue from PTLD-diseased, but not non-diseased pigs. Importantly, these tissues showed enhanced expression of PLHV1-BILF1 supporting its involvement in PTLD infection.

A guide to adhesion GPCR research.

Adhesion G protein-coupled receptors (aGPCRs) are a class of structurally and functionally highly intriguing cell surface receptors with essential functions in health and disease. Thus, they display a vastly unexploited pharmacological potential. Our current understanding of the physiological functions and signaling mechanisms of aGPCRs form the basis for elucidating further molecular aspects. Combining these with novel tools and methodologies from different fields tailored for studying these unusual receptors yields a powerful potential for pushing aGPCR research from singular approaches toward building up an in-depth knowledge that will facilitate its translation to applied science. In this review, we summarize the state-of-the-art knowledge on aGPCRs in respect to structure-function relations, physiology, and clinical aspects, as well as the latest advances in the field. We highlight the upcoming most pressing topics in aGPCR research and identify strategies to tackle them. Furthermore, we discuss approaches how to promote, stimulate, and translate research on aGPCRs 'from bench to bedside' in the future.

Risk of hospitalisation associated with infection with SARS-CoV-2 omicron variant versus delta variant in Denmark: an observational cohort study.

BACKGROUND: Estimates of the severity of the SARS-CoV-2 omicron variant (B.1.1.529) are crucial to assess the public health impact associated with its rapid global dissemination. We estimated the risk of SARS-CoV-2-related hospitalisations after infection with omicron compared with the delta variant (B.1.617.2) in Denmark, a country with high mRNA vaccination coverage and extensive free-of-charge PCR testing capacity. METHODS: In this observational cohort study, we included all RT-PCR-confirmed cases of SARS-CoV-2 infection in Denmark, with samples taken between Nov 21 (date of first omicron-positive sample) and Dec 19, 2021. Individuals were identified in the national COVID-19 surveillance system database, which included results of a variant-specific RT-PCR that detected omicron cases, and data on SARS-CoV-2-related hospitalisations (primary outcome of the study). We calculated the risk ratio (RR) of hospitalisation after infection with omicron compared with delta, overall and stratified by vaccination status, in a Poisson regression model with robust SEs, adjusted a priori for reinfection status, sex, age, region, comorbidities, and time period. FINDINGS: Between Nov 21 and Dec 19, 2021, among the 188 980 individuals with SARS-CoV-2 infection, 38 669 (20·5%) had the omicron variant. SARS-CoV-2-related hospitalisations and omicron cases increased during the study period. Overall, 124 313 (65·8%) of 188 980 individuals were vaccinated, and vaccination was associated with a lower risk of hospitalisation (adjusted RR 0·24, 95% CI 0·22-0·26) compared with cases with no doses or only one dose of vaccine. Compared with delta infection, omicron infection was associated with an adjusted RR of hospitalisation of 0·64 (95% CI 0·56-0·75; 222 [0·6%] of 38 669 omicron cases admitted to hospital vs 2213 [1·5%] of 150 311 delta cases). For a similar comparison by vaccination status, the RR of hospitalisation was 0·57 (0·44-0·75) among cases with no or only one dose of vaccine, 0·71 (0·60-0·86) among those who received two doses, and 0·50 (0·32-0·76) among those who received three doses. INTERPRETATION: We found a significantly lower risk of hospitalisation with omicron infection compared with delta infection among both vaccinated and unvaccinated individuals, suggesting an inherent reduced severity of omicron. Our results could guide modelling of the effect of the ongoing global omicron wave and thus health-care system preparedness. FUNDING: None.