The UK Health Security Agency (UKHSA) has reported a stark increase in certain Covid cases in the month of June. Some 8% of Covid tests analysed at the start of June were positive for the new Nimbus strain, the agency said, a two-fold increase from 4% in May. People who are experiencing symptoms have been urged to stay home as countries that have been hit with the strain have seen an increase in hospitalisations due to Covid. The NB.1.8.1 variant is now being monitored by the World Health Organisation (WHO).
Data gathered by UKHSA reveals an increase in overall positive Covid tests, regardless of strain. Nimbus is thought to be the most contagious variant of Covid to date, and it has been flagged in the UK, US and Australia, a spike which has been linked to an increase in cases in countries such as Thailand, India, Hong Kong and Singapore. Experts have warned that Nimbus could drive a new wave of Covid cases this summer as he public has been warned to be wary of symptoms.
Earlier this month, Dr Gayatri Amirthalingam, deputy director at UKHSA, warned: “NB.1.8.1 has been detected in small numbers in the UK to date, but international data suggests that it is growing as a proportion of all Covid-19 cases.”
People have also been warned that the immunity they have built up from previous vaccinations and COVID-19 infections has likely faded. This means that more people could be at higher risk of infection.
Other data collected by UKHSA shows an increase of 5.6% to 6.8% in COVID cases between May and June. This represents the highest figure recorded so far in 2025.
Dr Naveed Asif, GP at The London General Practice, explained: “The NB.1.8.1 variant, also referred to as Nimbus, is a new strain of the Covid-19 virus that has emerged due to mutations in its genetic material.”
Studies have suggested that Nimbus can infect human cells more efficiently, meaning it may be better at dodging your body’s immune system and causing you to become ill. However, there is no evidence to suggest that this strain of the virus is more deadly or likely to cause severe illness than others.
Dr Chun Tang, GP at Pall Mall Medical, explained: “Variants like this pop up when the virus mutates, which is normal for viruses, especially ones that spread widely. NB.1.8.1 was first picked up in early 2025 and has since been detected in several countries, including the UK, China and [the] US. It’s got some new mutations that scientists are keeping a close eye on.”