There have been lots of news reports about the R rate over the last few days, particularly whether the North West has gone above 1.
Antony asked the Health Secretary about this explicitly. He confirmed that, on the basis of all the evidence, the rate was still below 1 in the North West, including in Burnley.
What is R?
The reproduction number (R) is the average number of secondary infections produced by 1 infected person.
An R number of 1 means that on average every person who is infected will infect 1 other person, meaning the total number of new infections is stable. If R is 2, on average, each infected person infects 2 more people. If R is 0.5 then on average for each 2 infected people, there will be only 1 new infection. If R is greater than 1 the epidemic is generally seen to be growing, if R is less than 1 the epidemic is shrinking.
R can change over time. For example, it falls when there is a reduction in the number of contacts between people, which reduces transmission.
R is not the only important measure of the epidemic. R indicates whether the epidemic is getting bigger or smaller but not how large it is. The number of people currently infected with coronavirus (COVID-19) – and so able to pass it on – is very important.
R should always be considered alongside the number of people currently infected. If R equals 1 with 100,000 people currently infected, it is a very different situation to R equals 1 with 1,000 people currently infected.
Limitations of R
R is an average value that can vary in different parts of the country, communities, and subsections of the population. It cannot be measured directly so there is always some uncertainty around its exact value.
How R is estimated
Individual modelling groups use a range of data to estimate R including:
- epidemiological data such as hospital admissions, ICU admissions and deaths – it generally takes 2 to 3 weeks for changes in R to be reflected in these data sources, due to the time between infection and needing hospital care
- contact pattern surveys that gather information on behaviour – these can be quicker (with a lag of around a week) but can be open to bias as they often rely on self-reported behaviour
- household infection surveys where blood samples and swabs are performed on individuals which can provide estimates of how many people are infected – longitudinal surveys (which sample the same people repeatedly) allow a direct estimate of the infection rates
Different modelling groups use different data sources to estimate R using complex mathematical models that simulate the spread of infections. Some may even use all these sources of information to adjust their models to better reflect the real-world situation. There is uncertainty in all these data sources, which is why R estimates can vary between different models, and why we do not rely on one model; evidence is considered, discussed and R is presented as a range.
Who estimates R?
R is estimated by a range of independent modelling groups based in universities and Public Health England (PHE). The modelling groups present their individual R estimates to the Science Pandemic Influenza Modelling group (SPI-M) - a subgroup of SAGE - for discussion. Attendees compare the different estimates of R and SPI-M collectively agrees a range which R is very likely to be within.
𝗪𝗵𝗮𝘁 𝗶𝘀 𝘁𝗵𝗲 current 𝗥 𝗿𝗮𝘁𝗲?
The Scientific Advisory Group for Emergencies confirmed on Friday that its estimates, taking into account 10 different models, are that R remains between 0.7 to 0.9, and that it is below 1 in every region of the country.
That means the number of new infections is expected to continue to fall as every person who catches Covid-19 will infect less than 1 other person.
𝗜𝘀 𝘁𝗵𝗶𝘀 𝘁𝗵𝗲 𝗼𝗻𝗹𝘆 𝗺𝗲𝗮𝘀𝘂𝗿𝗲 𝗯𝗲𝗶𝗻𝗴 𝗺𝗼𝗻𝗶𝘁𝗼𝗿𝗲𝗱?
There are many factors that could influence the infection rate in an area, including population density, age and the position of an area on the pandemic curve.
There are 10 models that go into SAGE and different scientific models will come out with different results, and it is right that the scientists should publish and discuss those—that is how science advances. But having looked at all those models together, SAGE has determined every UK region remains below 1.
By looking at all the available data, decisions can be taken on the most intelligence-led basis; always supported by the scientific evidence.
𝗪𝗵𝘆 𝗱𝗶𝗱 𝘀𝗼𝗺𝗲 𝗻𝗲𝘄𝘀 𝗿𝗲𝗽𝗼𝗿𝘁𝘀 𝘀𝗮𝘆 𝗶𝘁 𝗶𝘀 𝗮𝗯𝗼𝘃𝗲 𝟭?
One study put the R rate in the North West at 1.01 which understandably generated a lot of media attention. But this should have been reported in the round, and alongside the opinion of SAGE which is that looking at all models, R is still below 1.
