2.2 Measuring infectiousness Te ine whakapokenga
2.2. Measuring infectiousness | Te ine whakapokenga
Scientists use a measure to describe the infectiousness of a disease called the basic reproduction number – also known as R0 or 'R naught.' This number tells how many people, on average, each sick person will in turn infect, when all individuals are susceptible to infection. Figure 2 shows what this looks like for nine diseases. Measles is one of the most infectious diseases in humans, with a R0 range of 12–18. This means a single infected person will infect, on average, 12 to 18 people.
The actual or 'effective' reproduction number (denoted Re or Rt) is typically smaller than R0. It is affected by other factors, such as the number of people an infected person comes into contact with, how susceptible other people are to the disease (for example, whether they have been vaccinated or have previously caught the disease and built up an immunity), their overall health, and any actions they have taken to protect themselves (such as wearing a mask). Public health measures such as lockdowns typically aim to lower the effective reproduction number.
While measles may be the most virulent disease listed in Figure 2, widespread vaccination efforts and herd immunity typically curb its spread.
Figure 2: The reproduction number (R0) of communicable diseases
A measure of how many people each sick person will infect on average
Source: Graphic by Harrison Schell, from Nicholas LePan 'Visualizing the History of Pandemics'(14 March 2020), https://www.visualcapitalist.com/history-of-pandemics-deadliest/
When the effective reproduction number of a disease drops below one, numbers of new infections will shrink. Should it climb above one, numbers of affected people can increase exponentially, as newly infected people become contagious and pass the infection on. If everyone has the same chance of becoming infected, diseases with an R0 of two (such as Ebola or the influenza strain that caused the 1918–1919 pandemic) can create 15 cases after three generations of infection (Figure 3). It only takes an increase in R0 from two to three (for example, the original strain of COVID-19) and this increases to 40 cases. In two more generations, cases will be around 60 (for R0 = 2) and 360 (for R0 = 3).
Figure 3: Impact of increases in the reproduction number on growth in cases
Source: Based on Joseph Eisenberg, 'R0: How scientists quantify the intensity of an outbreak like coronavirus and predict the pandemic's spread', The Conversation, updated 28 March 2020, https://theconversation.com/r0-how-scientists-quantify-the-intensity-of-an-outbreak-like-coronavirus-and-predict-the-pandemics-spread-130777
The combination of the effective reproduction number and the average time between infections determines the speed of disease spread.