In the last few months, we have seen many authorities imposing lockdown in coronavirus-hit cities or even nation-wide lockdown to contain the spread of the coronavirus. But have you ever wondered on what basis they decide to either impose the lockdown or lift it up? Many argue that it is based on common sense! If you think the same, you are terribly wrong.
Behind every coronavirus-related decision of the governments, the proper use of data and data models are involved. Based on the outcome of the complex calculations, they get to know the estimated number of cases and according to it, they decide their strategy.
In this blog, I will share the calculation methodology all governments are using to predict the number of coronavirus cases in the country or in the fixed area.
What is Ro?
Ro pronounces ‘R naught’ and refers to the Reproduction Number or value. In the simplest term, Ro is the mathematical term that denotes the contagious level of diseases like COVID-19. In other words, it defines the number of coronavirus positive patients that are estimated to increase in a homogeneous population as a result of infection spread by one COVID-19 positive patient. So, if a COVID-19 patient infects two people, the Ro value would be 2.
Based on the Ro value, the authorities get the idea of COVID-19 threat in an area.
- If Ro>1, each existing COVID-19 patient infects more than 1 individual. Which means, the threat remains high.
- If Ro=1, each existing COVID-19 patient infects 1 individual. Which means, the threat is limited.
- If Ro<1, each existing COVID-19 patient infects less than 1 individual. Which means, the threat is very minimal.
However, while applying Ro on a fixed population, there is a condition that should be considered: everyone is vulnerable to diseases. The meaning of this condition is very broad. It means,
- No one has taken the dose of vaccine
- No one has had this disease before
- There isn’t any way to cure the disease
The Ro value of Coronavirus
According to a study, the Ro value of coronavirus is a median of 5.7. This value is more than double than the Ro value of coronavirus found during its early stage. The 5.7 Ro value means a positive patient can infect 5 to 6 people.
5.7 Ro value of coronavirus is the major reason for concern as with 5.7 Ro value, almost 82% of the population should develop immunity against coronavirus to contain it.
Why Ro value matters – the examples
Imperial College London has been tracking the coronavirus cases and calculating Ro values regularly as it keeps changing.
Before the lockdown, the Ro value in the UK was around 4 which was really terrifying. However, as soon as people started following social distancing guidelines and lockdown measures imposed by the government, the Ro value in the UK fell significantly down below 1. This is the biggest achievement.
The same is happening in the country having the world’s second-largest population – India. During mid-May, the infection rate in India was at 1.22, after holding steady at 1.29 for about two weeks. Because of this rate, the government predicted that the number of active cases would not surpass 1,00,000 mark by 30 May and it turned out to be a true prediction.
However, In the month of April, the Ro value was 1.83 in India. This value would have translated to 1,00,000 active cases by 27 April, 33 days earlier!
How to calculate the Ro value of a disease?
To calculate the Ro value, we should consider three factors.
- Individual susceptibility
- Infectivity (the rate of the infection)
- The rate of removal of infection in the form of death or recovery
This is called the S-I-R model. Here, ‘S’ stands for Susceptibility, ‘I’ stands for Infectivity and ‘R’ stands for recovery rate. This model involves the linear equations and it was described by W.O Kermack and A.G McKendrick back in 1927. The following image shows their version of the model.
By entering the data in this model, the value of the reproduction number can be calculated. This is a complex calculation with lots of equations. You can know in detail in this PDF with all equations.
Bonus learning:
Our world has witnessed many viruses and diseases as a result of those viruses. However, not all the viruses are dangerous as not all of them have high Ro value. Even diseases having high Ro value are no longer dangerous as we have found vaccines and developed immunity against it.
The following is the Ro or reproduction value of the top communicable diseases.
