What can Italy's epidemic tell us about how the outbreak will develop elsewhere?
Deaths and confirmed cases of coronavirus in the UK double every three days and, on Friday, the country has experienced its biggest increase in deaths so far.
Models of the epidemic provide very different estimates of their potential final death toll, from tens of thousands to one published on Friday that projected a figure below 7,000.
So how do you understand the projections and what the patterns of coronavirus deaths in other countries tell us about what could happen next in the UK?
How are things in the UK?
Confirmed cases in the UK are doubling every three or four days. Deaths are growing faster, doubling every two to three days.
These data do not show all cases, only confirmed ones. This is because the test is performed mainly only on patients who are sick enough to be hospitalized, not on patients with mild symptoms and, therefore, the actual number of cases is greater.
Field experts would expect these broader cases to also follow a similar pattern: doubling every few days. This is because viruses multiply and the number of people infected by them also. They continue to multiply at a constant rate until they run out of people to infect or measures to slow the spread take effect.
It is difficult to see this multiplication constant in the chart above, but it is easier to see if you plot the same figures on a different scale.
On the scale shown below, a straight line means "to bend at a constant speed". We added dotted guidelines to the chart to show what would be expected if cases or deaths doubled every two or three days.
In reality, doubling speeds generally fluctuate until an epidemic reaches a large enough number, say 100 cases. Since that point, confirmed cases in the UK have doubled every 3.3 days.
Fortunately, there have not yet been enough deaths in the UK for us to chart an established trend of 100, so our trend line starts at 10.
At the moment, deaths are growing faster than confirmed cases, doubling every 2.5 days.
On March 27, the UK recorded 759 deaths. If the speed of doubling continues, we would expect to see another 750 deaths in the next three days and 1,500 in the next 2.5 days. But is that speed faster or slower in the UK than in other countries?
Is the United Kingdom on the same path as Italy?
Italy has the most advanced epidemic in Europe, with the most confirmed cases and the most deaths.
On average, the number of deaths in Italy has doubled every three days, but this masks the rapid growth of the first 1,000 deaths, followed by a slower growth rate, suggesting that the course of the epidemic is changing.
In other European countries, the initial death figures also follow this pattern of doubling every two to three days.
A notable exception is Spain, where the number of deaths appears to be increasing more rapidly than anywhere else (every two days and fifteen) after the 1,000 deaths, although it is not clear why.
Does this mean that the UK is just a few weeks behind a peak like Italy's? Not necessarily. Each country has a different health system and is taking different measures to control the spread of the virus. The number of deaths depends on both the spread of the virus and the treatment that people can access when they have it.
The future in each country will depend on the actions that governments and citizens take.
Analysis by Rachel Schraer, health reporter
AN work released on Friday projected that less than 7,000 people would die from coronavirus in the UK in total. This number is much lower than the model used by the government.
So, where did they get their numbers from? To arrive at these projections, Professor Tom Pike used the trajectory of death figures in China to predict the progress of outbreaks in the United Kingdom and other countries.
But virus and epidemic experts warned against the assumption that countries will follow the same trajectory, even though there are similarities in each country's initial figures.
There are some things that will be the same all over the world, like how long it takes the virus to become infectious in someone's body. But how an outbreak develops after that depends on what measures countries take and when they act, and China imposed restrictions earlier than many other countries.
Small changes in the rate of infection increase over time, with large reductions in the number of new infections.
Scientists expect each infected person to infect about 2.5 other people on average. As each of them infects another 2.5 and so on, a month multiplying at this rate leads to more than 400 new infections.
Halving this infection rate means that, after a month, we would expect to see only 15 new infections – a 95% reduction. This is because a small difference in the infection rate increases and increases to make a big difference in the number of people infected.
The path of the epidemic in China and South Korea shows how it is possible to slow the spread.
China implemented severe blockades in Wuhan and Hubei province in late January, before 30 deaths occurred. At that point, the epidemic was growing rapidly. About 10 days later, the number of deaths began to slow, decreasing to double every three days and now becoming much slower than that.
The total number of deaths continued to increase, but the number of new deaths each day decreased and ended up decreasing.
South Korea and Japan have never seen the same growth in deaths as other countries. They grew consistently at a slower pace, taking more than a week to double.
South Korea quickly began testing and tracking at scale, using nearly 30 hospitals where suspected or confirmed cases could be isolated.
When will we see a change?
Professor Neil Ferguson of Imperial College, who developed the modeling used by the government, says it takes time for any measures, such as social distance, to take effect.
People are infected, incubate the virus, develop symptoms, get worse and need hospital treatment before being confirmed to be carrying the virus. After that, it takes a while for a case to reach the stage where intensive care is needed and then it succeeds or someone dies.
The number of confirmed cases may give a hint earlier, as it takes less time to get to the test than the result of death.
It is just a tip, as changes in policy or testing capacity can change the number of confirmed cases.
But Professor Colin Baigent of the University of Oxford says the evidence is that the roadblocks are working.