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A Short Introduction to the Basic Reproduction Number

And how it helps us understand the trend of the COVID-19 pandemic

Originated in China, the COVID-19 pandemic has caused more than 76.7 million infections as of December 2020, along with around 1.7 million deaths worldwide. R₀ values help governments decide appropriate measures such as vaccination and herd immunity to stop transmission of diseases.


In mathematical epidemiology, the basic reproduction number, denoted R₀, represents the expected number of secondary infections directly generated from a single primary infection into a susceptible population.

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R₀ and the COVID-19 Pandemic

The R₀ value of the current pandemic estimated by the WHO is around 3 and what that means is that if a person has the disease, then it is expected that they will infect 3 other people. And after some period of time, the 3 newly infected people will then infect another 3 people each. This process keeps on repeating and the number of total infections will grow dramatically.

Moghadas et al. CMAJ 2009

Different Values of R₀

The goal of public health interventions is to reduce the values of R₀ to a number below 1.

  • If R₀ is less than 1, the disease cannot spread and will die down because each current infection causes less than one new infection.
  • If R0 is more than 1, the disease can spread as each existing infection causes more than one infection in a susceptible population.

In general, the larger the value of R₀, the faster the disease will spread.

Factors affecting R₀ and it limitations

The basic reproduction number is affected by factors like how many people are susceptible to the disease at the start, the infectiousness of the organism, as well as contact rates and the mode of transmission. However, it’s only an estimate based on an idealized case where an infected person were put in a fully susceptible population. Each of the factors for calculating R₀ is estimated and adds to uncertainty in the overall value of R₀.

That said, the R₀ value of the COVID-19 pandemic has helped governments and scientists to monitor its transmission.

Goldilocks 17y/o traversing the tapestry of life

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