INSUBCONTINENT EXCLUSIVE:
the contagiousness of a disease.It refers to the average number of people a person infected with coronavirus will pass the virus to,
assuming no pre-existing immunity.The R number varies in different populations, depending on factors such as age and how frequently people
come into contact with each other.R0 for COVID-19 in Europe was estimated to be around 3
But social distancing measures can reduce the number of new infections
By isolating some infected people the R number will decrease.This new R number is called the effective reproduction number (Re) or the
real-time estimation of the reproduction number (Rt).It is a measure of the disease's transmission rate at any given moment, and it changes
according to people's behaviour and the level of immunity to the disease in the population.If the number is above 1, the number of cases
increases exponentially.If the number is well below 1 then the disease will die out.Researchers from the London School of Hygiene and
Tropical Medicine are estimating the real-time transmission rates of COVID-19 across the world.Analysis of their data shows that
transmission rates have fallen in most European countries since the beginning of March, and that by mid-April the R number was approaching
1.Data from the study also suggests transmissions rates have dropped by an average of one point in all UK regions since the beginning of
March.Transmission rates can be reduced by changes in the way we behave, but not all changes have the same impact.A study from Imperial
College London estimates lockdown can reduce the Rt number by up to 70% and avoiding public events can reduce the number by 50%.Meanwhile
other types of measures, such as school closures, have a more modest impact.Voluntary measures can also be effective.A recent publication
from the German Robert Koch Institut suggests the infection rates in Germany might have decreased even before the lockdown, which could be
because people were already choosing to change their behaviour.Crucially transmission rates can go up as well as down, and changes in
behaviour can also result in R increasing.The study from the London School of Hygiene and Tropical Medicine also projects how the R number
could be performing in more recent days.Their projections show there is still a possibility that transmission rates could jump again to over
1.This is why R will be monitored closed as countries try to work out how to end lockdown restrictions.Calculating accurate R numbers is a
complicated process, as it relies on the daily number of reported cases of the disease being correct and there are some limitations in the
accuracy of this figure.The first problem is that there is a lag in the number of daily cases reported, as it takes around 10 to 14 days for
a person to catch the virus, develop the symptoms and be tested for COVID-19.To correct for this gap between infection and when a patient
tests positive, the researchers use a statistical model called nowcast.This shows that in the UK the R number is decreasing, but that does
not mean there won't be new cases.In fact, the study suggests the UK is among the six countries with highest number of expected new cases,
according to data up to 20 April.The UK is the only European country in this group
The other five countries are Brazil, Ecuador, Turkey, Russia and the US.The second problem with the official daily number of cases is that
the figure is underreported."Underreporting as such doesn't affect the Rt estimates, as long as the proportion of underreported cases stays
constant and affects the whole population equally," says Dr Sebastian Funk, the head of the research group at the London School of Hygiene
and Tropical Medicine.But changes in the testing criteria and practices can affect estimates of the reproduction number."If cases are more
likely to be reported in some parts of the population (if, for example, there is a focus on testing healthcare workers), then the
reproduction number estimate will be biased towards this population and no longer reflective of the general population," Dr Funk
says.Furthermore "if there is a ramp-up in testing, more cases might be reported even if the overall number of infections is in decline", he
adds.Professor Mike Tildesley from the University of Warwick says the effective reproduction number (Rt) is "an extremely important quantity
when it comes to determining whether lockdown measures can be lifted"."If Rt is above 1, this means that any epidemic will continue to grow
and any relaxation of control will result in Rt growing further."If Rt is significantly below one, this is an indication that a number of
measures could be relaxed and Rt may remain below 1 meaning that cases will continue to decline."However, if Rt is very close to 1, this
does not give us much leeway in terms of lifting control measures
So it is important for us not just to establish whether Rt is below 1, but how far below 1 the value is."
