This blog post was reviewed and updated in June 2025.
With the emergence of the NB.1.8.1 COVID-19 variant - sometimes referred to as the Nimbus variant - some people are asking how this new lineage - or group of genetically similar viruses - compares to other variants. NB.1.8.1 is a SARS-CoV-2 variant derived from the recombinant variant XDV.1.5.1. The earliest sample of NB.1.8.1 was collected in January 2025 and it was declared a Variant Under Monitoring (VUM) by the World Health Organization (WHO) in May.
What have we seen so far?
NB.1.8.1 has been detected in small numbers in the UK to date, and international data suggests that it is growing as a proportion of all COVID-19 cases.Although cases and hospitalisations are increasing in some countries where NB.1.8.1 is widespread, current data does not indicate that this variant leads to more severe illness than other variants in circulation.
The most important thing to do is to get your vaccination when it is due if you’re eligible. It’s normal for viruses to mutate and change, and as more data becomes available on this variant we’ll have a better understanding of how it interacts with our immune systems and how to optimise our protection, as well as actions we can take to keep the most vulnerable safe and live our lives as normally as possible. .
If people become unwell, and are unsure if they have COVID-19, what should they do?
If you have symptoms of a respiratory infection, such as COVID-19, and you have a high temperature or do not feel well enough to go to work or carry out normal activities, you should avoid contact with vulnerable people and stay at home if possible.
For those of us who absolutely can’t stay at home, our Living with COVID guidance is unchanged, and outlines how to prevent transmission to others.
What vaccination programme is in place for COVID-19 in the UK?
Vaccines remain our best defence against severe disease and hospitalisation from flu and COVID-19. Our spring 2025 COVID-19 vaccination programme remains open and eligible people can book via the NHS App, website or by phoning 119 until 17 June 2025.
What surveillance systems are in place?
We publish the latest surveillance data for COVID-19 and other respiratory illnesses weekly, to the UKHSA data dashboard, and we publish our COVID-19 and flu surveillance report fortnightly at the moment. We’re also getting vital data from those who are admitted to hospital with symptoms, and we are utilising genome sequencing to understand which variants people are most vulnerable to.
There are also specific surveillance programmes in place, where small sample groups are tested regularly. These allow us to monitor trends in the wider community.
Hospital is where we will see the more severe cases, and we will be monitoring the numbers of people attending with COVID-19 symptoms very carefully. This will help us understand the growth rate and transmission potential of the new variant.
We continue to collaborate globally with health organisations in other countries, WHO and initiatives such as the Global Influenza Surveillance & Response System (GISAID) to ensure that we have the most current data.
There are also specific surveillance programmes in place, where small sample groups are tested regularly. These allow us to monitor trends in the wider community.
What is the UKHSA doing to tackle the new variant?
When a new variant appears on our radar, at the initial stages it is often quite difficult to know whether the mutations provide any advantages to the virus. Genetic mutations happen all the time, and in some cases have been known to make a virus less transmissible or cause a milder reaction in people.