Health officials are investigating a new Covid variant which has caused 49 cases of infection, mostly in Yorkshire and the Humber region.
Public Health England (PHE) has been monitoring the VUI-21MAY-01 or AV.1 variant since April.
PHE said there was "currently no evidence that this variant causes more severe disease or renders the vaccines currently deployed any less effective".
The agency said additional testing and tracing was being carried out.
Dr Kev Smith, from PHE, said scientists had been watching and sequencing the variant since the "strange combination of mutations" were spotted a few weeks ago.
"So far the people that we have identified are not particularly infectious, they're not really getting more sick than other cases of coronavirus and we're not seeing anything particularly worrying about it," he added.
Dr Smith said that PHE had classified it as a "variant under investigation" and would continue monitoring its spread through extra testing.
"At the moment, we don't think it's more infectious than the other coronaviruses that we are seeing around."
'Don't be alarmed'
Greg Fell, director of public health in Sheffield, said his team had been monitoring the new strain "as we do with all outbreaks across the city".
"Please don't be alarmed, we want you to continue doing what you have been for the past year. Follow the guidance, continue to wash your hands regularly and wear a mask indoors.
"Where cases have been identified, additional follow-up of cases, testing of contacts and targeted case-finding will be used to limit the spread of variants."
Viruses mutate and change all the time.
A "variant under investigation" is one that acquires mutations that give it the potential to spread more easily, cause more severe disease or evade immunity from vaccines or previous infection.
This new variant - one of eight under investigation - will now be monitored to see how it spreads and analysed in the laboratory.
It would only be designated a "variant of concern" - like forms of the virus first detected in Kent, India and South Africa - if scientists conclude it does pose a heightened threat.
The origin of this variant - also known as AV.1 - is unclear. There have also been cases detected in Greece and Chad.
New variants are a story that we will all become very familiar with in the years to come. Most will have little effect in the UK if the vaccination campaign continues to be a success.
But it will be important to keep monitoring them in case one emerges that means the Covid vaccines need to be updated.
Dr Matthew Parker from the University of Sheffield is one of the scientists who first identified the variant.
He said it was a mutation of the variant B11482 which was first spotted in other countries last year and it was not possible to say how it had arrived in Yorkshire.
"It's just that we happened to notice some sequences in South Yorkshire and the rest of the UK recently. It might not have arisen in the UK, we don't know that yet."