How contagious is the Delta variant of Covid-19? See how coronavirus can spread

This interactive feature was originally published in April 2020, it was updated on 27 August 2021 to compare the infectiousness of the Delta coronavirus variant.

One important characteristic of viruses and other pathogens is how contagious they are.

This is measured in a few ways. One key measure is the R0, or basic reproduction number, which indicates how many new cases one infected person generates.

For an R0 of three we would expect each new case of a disease to produce three other infections.

This is not just a measure of the inherent infectiousness of a disease. It also depends on other factors, including the rate of contact within a population and the duration of the infectious period. It’s a situation-dependent value, so in one city the R0 might be higher and in another lower. It also assumes that the entire population is susceptible to the disease.

Early studies of the behaviour of Covid-19 in Wuhan estimated the average R0 to be between 2.2 and 2.7, while smaller-scale outbreaks such as the one that occurred onboard the Diamond Princess had an R0 estimated at 2.2. Other studies suggest that values as low as 1.5 and as high as 3.8 were possible.

So what does that look like, and how does it compare with other diseases?

How does Covid-19 compare with other diseases?

Here, you can see the same spread patterns generated based on the R0 for a range of pathogens in a range of situations, ranging from Spanish flu to the highly contagious measles, for which R0 has been estimated to range between 12 and 18.

But the R0 is not the only important number. The effective reproduction number, R, is a value that takes into account the susceptibility of the population.