Scientists measure the spread of infectious disease by its reproduction number, known as the ‘R’ number.

An infectious disease can be more easily contained when the R number is below 1.

For example, when the R number = 1, it means that 1 person will infect 1 other person. If the R number for the start of a virus outbreak is 3, and it takes a week for a person to become infectious, then that would mean 1 person infects 3 people after 7 days.

Using R = 3, calculate how many people will be infected in a 14-day period and in a 21-day period. You can check your answers and see the spread after 28 days by looking at the Transmission of the virus sheet. 

What do the results show?  

Scientist believe that when the R number is below 1, then the spread of a disease can be reduced. For example, if R = 0.5, every 10 people with the disease are likely to infect just five new people, and eventually the disease will stop reproducing.  

Look at the animations on the Washington Post website. You might wish to ask an adult to help.

The moving coloured dots indicate how quickly a disease can spread with and without different levels of social distancing. For those unable to social distance, PPE and testing and tracing have a vital role to play in reducing the spread of a disease like Covid-19. 

You can experiment with the effect of changing the R number by using the slider at the top of this Epidemic Calculator.

The calculator is set up with World Health Organisation data for Covid-19, including a basic reproduction number (R0) of 2.2. If you move the slider at the top of the web page, you can adjust the actual reproduction number (Rt) to make it lower or higher than 1.

As you do this, watch the effect on the number of cases (infectious people) and the number of deaths (fatalities). What do you notice?  

When you have got used to this, you can adjust the first slider at the bottom of the graph (Transmission Dynamics, Population Inputs) to the size of the population of your country and try it again.

If you want to investigate things further, you can visit the World Health Organisation Coronavirus Disease (COVID-19) Dashboard and look up your country to find out how many days after the first case (Day 0) a lockdown or other interventions took place.

You can then move the vertical dotted line on the epidemic calculator to show this and watch the effect on the number of cases and the shape of the curves. An important curve to watch is the number of hospitalisations. If this becomes higher than the number of hospital beds, then extra hospitals and staff and equipment may be needed. What would you recommend to keep the R number low?