Prime Minister Boris Johnson says the UK is “now seeing a second wave” of Covid-19.

Expanding “local” restrictions mean more than 13 million people (one-fifth of the UK population) have extra curbs on their lives.

And the surge in cases is not contained to just the hotspots, but is widespread across the UK. Local restrictions do not suppress a virus that is spreading outside of those areas.

It is against this backdrop the government is deciding what to do next. One idea is a “circuit-break” – a short, sharp period of tightened restrictions to curb the spread of coronavirus.

So why might a circuit break be needed and what could it achieve?

Let’s do some rough maths.

Take 6,000 cases a day, double them every week – as the Scientific Advisory Group for Emergencies (Sage) suggests is happening – and by mid-October you have more than 100,000 infections a day as we did at the peak.

That is not sophisticated disease modelling, it is not written in stone and measures such as the “rule of six” should slow the spread.

But that simple sum gives a sense of how quickly a small problem can be become a huge one.

A circuit break is all about trying to change that trajectory.

“The evidence is hospitalisations are increasing, it is a worry and the concern is what happens if we don’t do something,” Dr Mike Tildesley, from the University of Warwick, told me.

He is part of the government’s disease modelling group of scientists, called SPI-M, which has been discussing circuit-breakers this week.

Dr Tildesley added: “To be perfectly frank, none of us want this, but we’re stuck between a rock and a hard place.

“However, with a managed short-term lockdown you buy yourself some time.”

A bout of tighter restrictions should result in cases falling instead of rising, but how far they drop is uncertain and will depend on how severe the restrictions are.

It is suggested schools and workplaces would remain open, but the hospitality sector (think bars and restaurants) would be hit. This is not Lockdown 2.0.

“The over-arching aim is you don’t want intensive care units filling up again, but you also have increased options at lower levels of the virus,” said Dr Adam Kucharski, another SPI-M member and researcher at the London School of Hygiene and Tropical Medicine.

When levels of the virus are low, it is easier to spot outbreaks and use highly targeted measures, that are less disruptive than national ones, to curb the spread of Covid-19.

Dr Kucharski told the 360aproko.com”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.

“That’s the difference, options decline substantially as cases rise.”

Circuit breaks have been used in other countries. The temporary lockdown in New Zealand can be seen as a circuit break that gave contact tracers the time to get on top of their outbreak.

In the UK, a break could buy time to improve the government’s beleaguered Test and Trace programme, which is already struggling with current levels of coronavirus.

But the problem is once the circuit break is over, cases would begin to rise again.

“You may find yourself in a cycle of short-term lockdowns until you have an exit strategy like a vaccine or herd immunity,” Dr Tildesley says.

Remember it is only September.

Spring, when coronavirus should be easier to contain and we may have a vaccine, is still a very long way away.