NB.1.8.1 'Nimbus' Variant Becomes Dominant US Strain, ER Visits Rise in 27 States

CDC genomic surveillance confirmed the NB.1.8.1 ('Nimbus') Omicron JN.1-lineage subvariant, first detected in China in early 2025, became the dominant circulating strain in the United States, accounting for roughly 43% of sequenced cases by late June and continuing to climb through July. Wastewater surveillance detected rising viral levels in at least 10 states, and emergency-department visits for COVID-like illness increased across 27 states as the summer wave built. WHO classifies Nimbus as a 'Variant Under Monitoring' with low assessed public-health risk; it shows enhanced ACE2 binding and infectivity but no evidence of increased severity or significant vaccine escape, and current vaccines are expected to remain protective against severe disease.