Peering through the window of my study because of an unusual noise in the street last night, I saw three loud men walk by, one of whom saw me at the window, and gave a sardonic half-acknowledgement. Then, shortly afterward three women walked after them. It may not be relevant, but they were all young, well-dressed and slim. No self-isolation for them.

Even those neighbours who claim to be self-isolating talk to each other within a 1 metre distance: breathing range. They have also been shopping, and altruistically collecting prescriptions for older folk. Bluntly, the virus is finding stepping stones.

Today, the UK government finally closed pubs, restaurants, cafes and gyms, but kept open take-away food providers.

People are in search of facts, and there are facts out there to be debated and understood. In the absence of the summary I’m looking for, I’d like to given an explanation from the point of view of the virus. Yes, I know that a moiety of RNA does not have a point of view, but bear with me.

Viruses propagate by finding hosts. Once they enter a person, they must jump to the next one. Hosts are stepping stones. Increase the distance between the stepping stones and R0 falls below 1.

So, calculations depend on transmissibility and lethality: viruses can cause a lot of damage, and also be easy to transmit. The difficulties in these calculations are well known.

Not everyone gets tested, and not everyone with the virus shows symptoms, so it is often hard to get a reliable base rate. Testing random members of the population, or a variety of sub-groups who differ in potential exposure would help refine calculations. Absent those figures, the time between infection and symptoms, and then the most severely affected people eventually requiring hospitalisation is hard to calculate. When people die, countries vary in what is written on the death certificate. In Italy it is assumed that if an elderly person dies in a hospital which is dealing with coronavirus, then coronavirus is implicated in the death. Perhaps not.

Data are now more plentiful, and approximations can be made. The disease is transmissible enough and damaging enough to swamp most health-care systems. The public health message is very simple: Don’t be a stepping-stone.

Recent modelling suggests that as many as 23 million UK citizens may already have the virus, the London School of Hygiene and Tropical Medicine suggests.

For a scenario with a death rate of 1 per cent, where each infected person infects three more – which is closest to what is currently thought to be happening – the team at LSHTM found that one death points to a minimum of 37 cases, a maximum of 138,624 and a median average of 1,733.

With the current number of deaths at 167, it means that between 6,179 and 23 million people could already be infected, with an average of nearly 290,000.

More from The Daily Telegraph

Scientists at the Universities of Oxford, Warwick and Lancaster found that contact tracing could reduce the transmission rate from 3.11 people to 0.21 – enabling the outbreak to be contained.

They carried out a postal and online survey, asking 5,802 people how many social interactions they had on a given day and found the average number of contacts over a 14-day period was 217.
Of these total encounters, an average of 59 contacts (27 per cent) met the definition of a close contact (within two metres for 15 minutes) and 36 (61 per cent) could be traced.
However, the team said that even with contact tracing, they would still expect 15 per cent of all infected people to generate at least one case that could not be identified.
Separate modelling by the LSHTM also found that if an infected person infected another 1.5 people, tracing fewer than 50 per cent of contacts was enough to control the outbreak.

The six young revellers will probably be alright themselves. I think of them dancing as they face the wave of infection. Certainly, they have helped that wave hit us, by providing the stepping stones the virus needs to thrive.