# Controlling the R Value

The R Value is the Reproduction Number, which is the rate at which people are passing the virus on - it is used to track how many people, on average, will be infected for every one person who has the disease. If R is greater than 1 the number of cases of infection will increase exponentially, but if it is below 1 then the disease will peter out.

The natural reproduction rate is called R0, pronounced “R naught,” and is a mathematical term that indicates how contagious an infectious disease is without any control measures, and enables different diseases to be compared. SARS COV-2 has a natural reproduction number of about 3. Measles has one of the highest infection rates at 15.

However, although the disease has a natural R value, R isnt fixed as a disease can mutate changing its infectiousness over time. Also it can be affected by a range of external factors such as population density (the more densely populated a city is the higher the R),and he level of immunity of a population from past infection or vaccination.

By using methods such as social distancing, face coverings and lockdowns etc governments are able to move R to a level lower than the natural reproduction rate and thus to some extent control the disease as when it is low enough they can start to implement more precise measures such as contract tracing and targeted local lockdowns if an area's R gets too high.

However with COVID-19 as there is a lag between when people are infectious and when they start to show symptoms, so by using hospitalisation or death rates would mean the R value is weeks behind the actual spread of the disease and so is always out of date. A better way would be randomised testing in the community which will give a more timely value.