The British government has adopted a stricter lockdown in an effort to suppress the transmission of the coronavirus. New stringent measures – significantly limiting freedom of movement across the UK – have been introduced. They were brought in after an alarming body of evidence suggested that the UK is now just two weeks behind Italy in the course of the UK’s COVID-19 epidemic.

But what is the logic behind comparisons of the UK and Italy?

To start with, both countries have a similar size population. But what really has caused concern among experts are possible similarities in the number of deaths and the rates of exponential growth in the number of fatalities in particular.

In week 10 of 2020, ending on 6 March, Italy had announced 176 new deaths from COVID-19, and at the end of last week (week 12), the UK had reported 166.

And on Sunday, research from both the Sanger Institute in Cambridge and Professor David Spiegelhalter at the Centre for Risk and Evidence Communication at the University Cambridge, also suggested that the UK’s daily increase in the underlying death rate in week 12 could be following a similar path to that which Italy’s had in week 10.

This was when logarithms of the number of deaths each day in both Italy and the UK were calculated. This method is useful because it accounts for skewed data caused by outliers and other anomalies and gives a clearer picture of the overall trends in both countries.

This is more accurate than comparing case numbers for positive tests for the virus. The testing capacity in Italy and the UK is not yet the same, and the countries’ testing strategies have varied, making it hard to compare like-with-like. At the moment, looking at deaths provides a better way of making a crude comparison.

The reason why this evidence was so potentially concerning is because it suggests that, if fatality rates were to continue to increase in this way, the NHS would experience the type of overwhelming exponential growth in critical cases experienced in Italy over the last two weeks.

This does not mean that the UK will necessarily be in the same situation as Italy in two weeks’ time. Experts point out that there is a danger in over-interpreting the data so early on in the UK’s epidemic. There are not yet enough data points to accurately predict such an outcome.

Professor Sheila Bird, former Programme Leader at the MRC Biostatistics Unit at the University of Cambridge, said that “all this means is that we (Italy and the UK) have comparable population sizes and had a comparable number of deaths. We may be on a less steep trajectory than Italy, but it’s too early to tell.” What we have is a snapshot of a particular moment in the course of the UK’s trajectory, not a pre-determined path.

And because there is a period between when someone becomes infected and death, there is also effectively a time lag in the data. It is too early to tell what impact the UK’s stringent lockdown will have on critical cases. It will take days and weeks for this to feed into the data.

Professor Spiegelhalter has said that the outlook remains “very uncertain”. His comparison of the daily increase in underlying death rate in the UK and Italy over the 12 days following each country reaching 5 deaths, for instance, showed the UK to have a marginally higher rate (37%) than Italy (35%).

But he has since compared the data for the 14 days following each country reaching 5 deaths. This suggests that the UK’s deaths have risen on average by about 30% per day, a lower rate when compared to 37% in the equivalent period in Italy’s epidemic. It’s too early to tell whether this will be a permanent reduction or only a fluctuation.

Pulling below the 33% increase marker may also be important, Dr James Gill at Warwick Medical School, has said. China dropped below this marker at around 21 days after their 100th case. The UK did so after 5 days after the 100th British death, while Italy only pulled below the 33% increase line 16 days after experiencing their 100th fatality.

There are also many other factors at play when comparing Italy and the UK – from the epidemiological and demographic to the cultural and behavioural.

The aggregate impact of these will become clearer over the next few weeks. For the moment, experts agree, observing the UK’s strict lockdown will be crucial if the NHS is to have a chance of pulling down the upward trend in critical cases. We need to stay at home to protect public services and save lives.