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COVID-19 Pandemic Explained: What Is Reproduction (R) Number And Why Does It Matter?

According to experts, R number helps in understanding how fast and wide is the infection spreading; but what exactly is the R number and why does it matter during the ongoing COVID-19 pandemic?

COVID-19 Pandemic Explained: What Is Reproduction (R) Number And Why Does It Matter?
Highlights
  • The latest R effective estimate for the country is 0.90
  • R value tells how many people are infected by one infected person
  • An R value of less than 1 suggests that the rate of infection is slow

New Delhi: In the past 11 months of the COVID-19 pandemic, we have gained a decent grasp on once-unpopular terms and concepts like personal protective equipment (PPE), social distancing, doubling rate and “flattening of the curve”. We have all seen ourselves grow from being completely clueless about the infectious disease to becoming budding armchair virologists. However, there is one phrase used widely but may still confuse many: the Reproduction or R number. According to experts, R number is an important parameter to know the spread of an infectious disease. The aim while fighting a pandemic should be to shrink the R number below 1 and keep it there, say experts. NDTV spoke with Dr Chandrakant Lahariya, epidemiologist, vaccinologist, and public health and policy expert based in Delhi and Dr Preeti Kumar, Vice President- Public Health System Support at the Public Health Foundation of India to know what is R number and why does it matter.

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According to Dr Lahariya, R refers to an infectious disease’s capacity to spread. It signifies the average number of people that one infected person will pass the virus to. In other words, it measures the number of new cases from one infected person. Explaining further he said,

R number changes over the course of time due to various factors like how infectious the disease is, how it develops, how a population behaves, immunity among people from the infection or vaccine, and other such factors.

He further added that a place that is densely populated is likely to have a higher R than a sparsely populated area.

R0 And Rt

R number is represented by R0 or ‘R nought’ and Rt or ‘Effective Reproduction Number’ or simply ‘R Effective’.

According to Dr Kumar, R0 is the average number of secondary infections by a typical case, where everyone is susceptible or is not immune to the disease which happens at the beginning of an epidemic or a pandemic. For example, if R0 is 5, it indicates that each infected person will infect at least five other people and each of those five persons will infect five more persons and so on. She said that R0 is affected by the rate of contacts in the population, the probability of infection being transmitted, and the duration of infectiousness. She said,

An R0 value matters because above 1 means that the epidemic will grow exponentially, while below 1 means it is not growing and the disease will eventually stop spreading. We all want R0 below 1 which shows that not enough new people are being infected to sustain the outbreak.

R0 can be brought below 1 once the infected persons transmit the disease to less than 1 person, on an average. This can be achieved when people gain immunity against the infection either via vaccine or herd immunity which is a stage where enough people become immune by getting infected naturally and fighting it that more people are not catching the infection. As per experts, Measles is one of the diseases that have a high R0 of 15 in populations with immunity. This implies that on average, one person will spread measles to 15 others.

Also Read: COVID-19 Explained: How Can Winter Season Impact India’s Current Declining COVID Trend?

Rt, on the other hand, is a measure of 1 person infecting the number of other persons in a population among whom the precautionary measures to avoid the infection are being followed strictly. In the case of COVID-19, the precautionary measures include wearing masks in public, maintaining social distancing, washing hands regularly, wiping high-touch surfaces frequently among others. If the Rt is 3, this means that one infected person can pass on the infection to three more persons and those three will further infect three each on an average. But by following recommended safety measures Rt can be brought down to less than 1 which means that the transmission is slowing down.

According to Dr Lahariya, R is calculated using data such as the number of people dying, admitted to hospital or testing positive for the virus over time.

During the initial days of the pandemic, in March and April, the Rt value in the country was more than 2 which decreased to less than 1 in September. The latest R effective estimate for the country is 0.90. Regionally if we see, on Thursday, November 5, Himachal Pradesh recorded the highest Rt value in India at 1.20. Haryana recorded the second-highest Rt value at 1.19. This was followed by Delhi and Mizoram at 1.18, Ladakh at 1.08 and Chandigarh at 1.05. On the other hand, Karnataka recorded the lowest Rt value in the country at 0.54. This was followed by Puducherry at 0.62, Nagaland at 0.66, Assam at 0.73, Bihar at 0.77, and Arunachal Pradesh and Tripura at 0.83.

The experts warn against complacency that may set in among the public and the authorities as the R number goes below 1. This is because until the immunity is built against the virus, any negligence can result in an increase in R number.

Also Read: One Of The Puzzles Of COVID-19 Is The Large Proportion Of People Who Are Asymptomatic: Dr Vincent Rajkumar Of Blood Cancer Journal

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