A Covid strain that is a spin-off of Omicron has swept the US and already arrived in the UK, health chiefs have confirmed. Researchers discovered that the mutant strain ‘Circada’ surged globally in September 2025. Virologists worry about the catalogue of mutations it carries on its spike protein – the part of virus that allows to bind to human cells.

US scientists have recently raised concerns that the current Covid vaccines may be less effective against this new variant. However, others say that it is still too early to draw firm conclusions or cause alarm. Others, however, have warned it’s far too early to panic and that there is no evidence Circada is any different from other strains in terms of the risk it poses.

The UK Health Security Agency (UKHSA) has not revealed exactly how many cases of the strain, known scientifically as BA3.2, have been spotted so far in the UK. Its consultant said the agency is constantly monitoring all available data and the severity of the virus.

According to the US Centers for Disease Control and Prevention (CDC), it is currently surging across the US and has been spotted in 25 states.

Below is a brief summary of everything we know so far about the virus.

The Circada variant has now been identified in at least 23 countries. It has also been picked up in voluntary nasal swab samples taken from international travellers arriving at US airports.

Recent estimates suggest the variant accounts for around 30% of cases in Denmark, Germany, and the Netherlands. Despite this spread, it has not led to a notable rise in overall Covid infection rates compared with previous years.

Weekly figures from the UK Health Security Agency show that, among positive cases analysed in England between February and March, only 2.13% were identified as the BA.3 strain.

There are no signs yet that the variant poses any more of a danger than other variants.

Symptoms of BA.3.2 appear similar to other recent variants, including sore throat, cough, congestion, fatigue, headache and fever.

Some people also experience gastrointestinal issues like nausea or diarrhoea, according to the CDC.

Professor Brandon Dionne, an associate clinical professor of pharmacy and health systems sciences at Northeastern University, said: “There definitely are quite a few mutations with this one, so there’s concern that the current vaccine is not going to be a great match.”

Professor Kyle Enfield, an associate professor of medicine at the University of Virginia, added: “BA.3.2 is the new kid in the block.

“It is different enough from the JN.1 strains that the vaccine may not do as good a job of priming the immune system against it, allowing it to evade detection.”

There are concerns that the Circada variant could be better at bypassing vaccine protection than other Omicron subvariants.

However, even if vaccines are somewhat less effective against it, overall immunity is still expected to provide a level of protection.