April 28, 2020

What is R0 And Why Should We Care About It?

Basic Reproduction number (R0)

R0 assumes that EVERYONE is susceptible to the disease.

Definition: Average number of secondary cases produced by a typical case of an infection in a population, i.e., the average number of people who will contract the disease from someone who has the disease.

Example: If the R0 is 2, for each new case of COVID-19, there would be 2 new secondary cases, and this replication will continue unless there is a vaccine or another intervention. Thus, if R0 is less than 1, the infection will eventually peter out. But if R0 is greater than 1, it will keep spreading and can cause an epidemic.

What R0 is: Epidemiologic metric used to describe the contagiousness of infectious agents, which is a function of (a) human behavior and (b) biological characteristics of the pathogen.

What R0 is NOT: Biological constant for a specific pathogen, a rate over time, measure of disease severity.

Effective Reproductive number (Rt)

Definition: Average number of secondary cases per infectious case in a population made up of both susceptible and non-susceptible hosts.

Formula: R = R0 * x
[x = Fraction of the host population that is susceptible]

Example: If the R0 is 4 and half the population is immune, the effective reproductive rate is 2. Thus, a single case would produce an average of 2 new secondary cases.

Conclusion:
Rt is different from R0 in that it does not assume the total population is susceptible and thus takes immunity into consideration. When R is defined at a specified time interval, it can be presented as Rt. Therefore, it is best to use Rt when assessing the implications of vaccination campaigns and other public health interventions. However, since we are unclear about immunity with COVID-19, we generally use R0. Essentially, we should aim to bring R0 down to below 0 if we want to move towards eradicating the virus. Social distancing can help catalyze this but until we have a vaccine, immunity build-up or a cure, we will not be able to permanently eliminate the contagion from the system.

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