A-Z Index × Submit A-Z Index × Submit A-Z Index Search Dropdown × Submit Facebook Twitter LinkedIn Syndicate Emerging Infectious Disease journal ISSN: 1080-6059 Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.
Norovirus incidence increased in England during 2022–2025, when GII.17 replaced GII.4 as the dominant genotype. By using nationally linked norovirus testing and fatality data, we found age and care setting, but not genotype, were associated with case-fatality risk. Increased incidence might reflect changes in transmissibility or population immunity.
Norovirus is a highly contagious virus that causes diarrhea and vomiting. Although symptoms from norovirus are not inherently severe, norovirus places substantial pressure on healthcare systems, particularly in closed settings where transmission rates are high. Older adults and hospitalized patients are disproportionately affected and are at greater risk of severe outcomes and complications ( 1 , 2 ).
Noroviruses comprise 10 genogroups (GI–GX) and 48 genotypes ( 3 ), and cross-immunity is limited ( 4 ). During the past decade, the GII.4 genotype predominated globally ( 5 ), with semiregular strain replacement ( 6 ). During the 2023–24 winter season, GII.17 reemerged and replaced GII.4 as the dominant genotype in multiple countries ( 7 ). In the 2024–25 season, record-high laboratory reports of norovirus were recorded in England, driven by the co-circulation of GII.17 and GII.4 genotypes ( 8 ). The increase in reported norovirus cases might reflect greater disease severity, increased transmissibility, or reduced population immunity to GII.17, although distinguishing among those explanations is challenging ( 9 ), particularly with aggregate data ( 10 ).
Whereas previous studies have investigated norovirus severity across genotypes ( Appendix ), we directly compared the severity of disease caused by GII.4 and GII.17 genotypes during the recent shift in genotype dominance. We estimated genotype-specific case-fatality risk to compare the relative severity of norovirus strains by linking individual-level laboratory, genotyping, hospitalization, and death data from England during the 2022–2025 seasons.
In this retrospective cohort study, we included persons in England with a positive norovirus test, linking routine individual-level laboratory test results to death and hospitalization records. We included positive test results from the UK Health Security Agency’s modular open laboratory information system (MOLIS) ( 11 ) for genotyped specimens and the second generation surveillance service (SGSS) ( 11 ), which lacks genotyping information but captures a larger, more representative set of tests. We obtained death registrations from the Office of National Statistics through July 23, 2025 ( 12 ).
Figure 1 . Number and proportion of positive norovirus test results and linked severe outcomes from a study of norovirus case-fatality risk in England during the 2022–23 and 2024–25 seasons. A) Number of...
We linked records by using National Health Service number (unique patient identifier) and relevant dates ( Figure 1 ; Appendix ). By using MOLIS and SGSS data, we attributed deaths to norovirus infection episodes by linking each death to any positive norovirus test within the preceding 14 days ( Appendix ). From those data, we generated 3 linked datasets: MOLIS-deaths, SG…