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How has Covid-19 spread so quickly?

Two measures are relevant to this question: how many people someone with Covid-19 infects and how long it takes to do so.

Experts have calculated the basic reproduction number (R0) of Covid-19 to estimate how contagious the disease is. R0 is the number of people an infected person is likely to infect at the start of an outbreak. Think of it as the “potential” of the disease.

The National Institute of Communicable Diseases (NICD) estimated that in March, shortly after the first cases were recorded in South Africa, R0 was between 1.7 and 2.5. That means the average infected person would infect about two others. In May – after the introduction of the lockdown – the NICD estimated the actual reproduction number (R) to be 1.5. (Note: This number is not constant.)

When R is greater than 1, the epidemic grows because each person infects more than one person.

In addition, Covid-19 has a “short” serial interval of four to eight days. This is the time between when someone starts to show symptoms and when the person they infect does so.

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