A new strain of Covid-19 - which may be easier to catch - has been detected outside south-east England where it first came to light and has also been detected in Wales, Scotland, Denmark and Australia.
Dr Cillian de Gascun of the National Virus Reference Laboratory said it has not yet been found in Ireland.
The strain, which is under investigation by scientists, looks set to inevitably spread to this country.
There is also concern it could be a shape that is not well recognised by the immune system, making Covid-19 vaccines less effective.
But although an alert has been sounded, a lot of questions remain about how concerning it will turn out to be.
So far, the fear is this strain spreads more quickly but this has not been confirmed. There is no evidence that it does but it is increasing at a quicker rate than any other in the UK.
Figures announced on Monday in the UK suggest more than 1,000 people have been diagnosed with the strain in 60 local authorities.
The virus was found around the south-east of the country.
It was picked up by labs doing testing for the virus and scientists are examining it at the Public Health England lab at Porton Down.
Some people are cynical that it was mentioned by the UK government to justify tougher restrictions.
But the matter is being treated seriously, with the World Health Organisation having been told and waiting for more information.
It may turn out to be not much different than the common strain of the coronavirus but the authorities have learned they cannot be complacent about it.
The coronavirus has mutated on a number of occasions since the start of the pandemic so another strain was not unexpected.
A recent high-profile mutation was found in Denmark in mink, which in turn was passed on to some mink farm employees.
It led to a cull of the animals and travel restrictions into Denmark.
Cases were later found in other parts of Europe.
There is no evidence the new strain is more virulent and causes worse disease in those who catch it.
However, if it spreads more easily then vulnerable groups are at higher risk.
Its emergence is particularly worrying because it comes at a time of unprecedented levels of mixing since the start of the pandemic due to Christmas and New Year celebrations.
This reinforces the need to follow basic measures and to be careful with activities if people who are susceptible to the virus are making Christmas visits.
The initial reading of the new strain is that it is at low risk of having an impact on the Covid-19 vaccine's protection.
But experiments will be carried out in the coming weeks to confirm if this is the case.
If this virus is in the UK it will inevitably spread to Ireland, particularly with the increased travel over Christmas and the New Year.
It could also become the dominant virus over time but this is still speculative.
Anyone coming from the UK should follow guidance and not rely on a test alone because it might not pick up the virus due to its incubation period.
The advice remains to people not to travel to or from the country for non-essential journeys.
The virus has mutated many times since it was first discovered and scientists point out that none of these altered the way it behaves in any significant way.