Disease Spread Background

Additional factors can be used to measure how a pathogen spreads and predict whether an epidemic may occur. In this section, you’ll learn about two such factors: the basic reproduction number (R0) and the effective reproduction number (Re).

Measuring Early Disease Spread

One way to measure the potential spread of a pathogen at the very beginning of an epidemic is by calculating the basic reproduction number (R0). R0, pronounced R-naught, is the average number of individuals a single infectious individual will infect throughout the duration of their infection, if everyone else in the population is susceptible.

R0 is related to the transmission rate (t) and the recovery rate (r) as follows:

R 0 = t r
Icons for Susceptible, Susceptible and Close Contact, and Infectious individuals.
When R0 = 3, each infectious individual will infect three susceptible individuals throughout the duration of their infection. As each new infectious individual infects three additional individuals, the pathogen spreads through the population.

Table 1 explains how to interpret different values of R0. Table 2 lists values of R0 for some example diseases.

Table 1. R0 can indicate how the number of disease cases may change and whether an epidemic is likely to result.
Disease Cases Likely Outcome
R0 > 1 Increase Disease will spread to an increasing number of individuals (exponentially) and can become an epidemic.
R0 < 1 Decrease Disease will eventually die out. No epidemic.
R0 = 1 Stay the same Disease will be maintained at baseline level. No epidemic.
Table 2. Estimated values of R0 for various infectious disease epidemics.
Disease Pathogen Estimated R0
Flu Influenza virus (seasonal) 1.1 – 3.4
Common cold Rhinovirus 2 – 3
COVID-19 SARS-CoV-2 (ancestral strain) 2.4 – 3.4
HIV/AIDS Human immunodeficiency virus (HIV) 2 – 5
Measles Measles virus 12 – 18

Measuring Later Disease Spread

As a disease spreads, more individuals typically gain immunity to the pathogen and enter the removed group. Since they are no longer susceptible to becoming infected, there are fewer individuals that the pathogen can spread to.

The body’s ability to effectively recognize and destroy certain pathogens. When an individual has immunity against a pathogen (also called being immune), they cannot be infected by that pathogen.

To measure the spread of a pathogen at this point, we can use the effective reproduction number (Re) instead of R0. Re, which is also known as Rt, is the average number of individuals a single infectious individual will infect throughout the duration of their infection, if some individuals in the population are immune.

Re is related to R0, the number of susceptible individuals (S), and the total number of individuals in the population (N) as follows:

R e = R 0 × ( S N )

Re can also measure disease spread in a population that has implemented control measures: policies that aim to reduce transmission by reducing the number of susceptible individuals or their interactions with infectious individuals. Examples of control measures include vaccines, lockdowns and social distancing.

A substance, typically injected into the body, that helps an individual develop immunity against a pathogen. Vaccines work by triggering the body’s response against the pathogen without causing disease. The body is then better prepared to recognize and destroy the same type of pathogen in the future.
A control measure in which individuals stay where they are to reduce contact with others. This can reduce the transmission of some pathogens.
A control measure in which individuals maintain a larger-than-usual physical distance from one another (e.g., staying six feet apart). This can reduce the transmission of some pathogens.

Values of Re can be interpreted similarly to the values of R0 in Table 1. For example, reducing Re below 1 using control measures can make the disease eventually die out and prevent an epidemic from occurring.

Icons for Susceptible and Close Contact, Infectious, and Removed or Immune individuals.
When Re = 2, each infectious individual will infect two susceptible individuals during the duration of their infection. Removed individuals cannot be infected, which reduces the spread of the pathogen. Dashed lines from infectious individuals to removed individuals indicate no transmission.
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