Overview

• NB.1.8.1 was first identified in January 2025 and has since been detected in over 20 countries, including confirmed cases in the UK, US, China and India.
• The UN health agency labelled Nimbus a variant under monitoring on May 23 after its global sequence share jumped from 2.5% in early April to 10.7% by late April.
• Mutations on the spike protein, notably at position 445, suggest higher transmissibility but health officials report no increase in disease severity compared with earlier Omicron lineages.
• UK Health Security Agency data show a 97% rise in positive tests since March, reaching a 6.1% positivity rate by June 1, prompting renewed guidance on testing, masking and distancing.
• Existing COVID-19 vaccines are expected to remain protective against NB.1.8.1, and authorities are urging eligible people to receive their next booster to limit severe illness.