With the outbreak of Coronavirus (COVID-19) pandemic, the 2011 movie ‘Contagion’ has suddenly gained popularity. An exclusive term of epidemiology has been referred in the movie called ‘R Naught (R_{})’. This article explains the concept of R Naught in simple words which sometimes also mistakenly referred as R Not. Have a look!
What is R Naught of COVID-19?
At present, the R Naught (R_{}) of SARS-CoV-2 is 2.2.
SARS-CoV-2 (commonly known as coronavirus) is the virus that causes the disease COVID-19. This means that one COVID-19 positive person can infect about 2.2 persons on average.
What is R Naught (R0)?
R Naught (R_{}) is a mathematical term used in epidemiology. It represents the average number of people who could be affected by one infected person of any particular contagious disease. It is also referred to as Basic Reproduction Number (BPR). R Naught reflects the extent of spread of disease to a new person.
R_{} is applicable only when the entire population is vulnerable to a particular disease. This precisely means that the disease is novel in itself and has never affected anyone before. As it has occurred for the first time, no one has received any vaccination and the spread of this disease cannot be controlled.
R_{} Value less than 1 | R_{} Value equal to 1 | R_{} Value more than 1 | |
Consequence | Each existing infection case causes less than one new infection. | Each existing infection case causes one new infection. | Each existing infection case causes more than one infection. |
Epidemiology Status | Disease will eventually die out. | Disease will be alive but within control. | Disease Outbreak or Epidemic |
Calculation of R Naught (R_{})
To calculate the R Naught of any contagious disease, some important factors are taken into consideration. These factors are – period of infectiousness, transmission mode of the disease and the contact rate. Period of infectiousness plays an important role in determining the spread of the disease. Longer the period of infectiousness, greater the spread of the disease. Transmission mode of the disease also plays a critical role in estimation of the R naught value. For e.g. an air-borne disease (COVID-19 & Measles) has more changes of spread that a disease transmitted through body fluid (HIV & Ebola). Lastly, the contact rate of the person also influences the spread of disease. A person living in isolation or self-quarantine will spread the disease much lesser than an otherwise social person not practicing social distancing. All the above three factors together determine the extent of the spread of a contagious disease.
Viral Disease | Transmission Mode | R Naught (R_{}) Value |
HIV | Body Fluids | 2-5 |
SARS | Airborne Droplets | 2-5 |
EBOLA | Body Fluids | 1.5-2.5 |
Polio | Fecal-Oral Route | 5-7 |
MERS | Airborne Droplets | 0.3-0.8 |