Wales will go into lockdown for a two-week circuit-breaker from Friday, to try to stem rising coronavirus cases.
Everyone will be told to stay at home, while pubs, restaurants and non-essential shops will shut.
The government's science advisers have recommended a circuit-breaker in England, Northern Ireland is pressing ahead and Scotland is doing something similar.
What is a circuit-breaker?
A circuit-breaker is a tight set of restrictions - it could feel a lot like the original lockdown - but crucially it would be for a fixed period of time.
It is designed to reverse the tide of the epidemic and bring the number of cases back down.
How far they drop would be uncertain and would depend on how severe the restrictions were, and how much we stick to them.
The hope is a circuit-breaker would be less damaging - to the economy and people's mental health - than a lockdown, because people could plan ahead as there would be a clear end in sight.
And it has been suggested that a mini-lockdown would coincide with school holidays, so children's education would be less disrupted.
What would a circuit-breaker achieve?
Dr Adam Kucharski, a researcher at the London School of Hygiene and Tropical Medicine, said: "The overarching aim is you don't want intensive care units filling up again, but you also have increased options at lower levels of the virus."
The government's test-and-trace programme is currently overwhelmed by the number of cases it is trying to handle.
Dr Kucharski said: "As cases and hospitalisations increase, there is less information on what the outbreak is doing, as test and trace can't pick it all up, you don't know where the outbreak is."
But when levels of the virus are low, it is easier to spot outbreaks and use highly targeted measures, which are less disruptive than national ones, to curb the spread of Covid-19.
How long would a circuit-breaker last?
The minimum duration is seen as two weeks.
Anything shorter and the risk is people who are infected just before the start of the circuit-breaker may still be able to spread the virus when it ends.
The longer the circuit-breaker, the more it would reduce cases - but the greater the other societal impacts.
When would we know if it was working?
Not until after the circuit-breaker was lifted.
It can take a week between being infected and developing symptoms, the same again between getting sick and needing hospital treatment, and up to a month before people die.
All of these lags mean the statistics on deaths and hospital admissions are expected to look worse throughout the whole of a circuit-breaker.
It is only later that the benefits begin to trickle through.
What happens after a circuit breaker?
Cases will go up again unless something fundamentally changes.
"With a managed short-term lockdown, you buy yourself some time," says Dr Mike Tildesley, from the University of Warwick.
"But you may find yourself in a cycle of short-term lockdowns until you have an exit strategy like a vaccine or herd immunity."
Has it been used anywhere else?
Israel is currently using a circuit-breaker after its "traffic-light" system did not get infections under control. The government said there were some "preliminary signs of success".
The temporary lockdown in New Zealand, which successfully allowed contact tracers to get back on top of the outbreak there, can be seen as a circuit-breaker even though it was not called that.
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