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How Fast Has COVID-19 Been Spreading?


Friday, March 27, 2020

By Guillaume Vandenbroucke, Research Officer and Economist

One question surrounding COVID-19 is: How fast has it spread? The table below may help answer this question.

The first column of data indicates the number of cases as of the evening of March 26. Each subsequent column indicates the number of days it took for confirmed cases to double from 1,000 to 2,000, then from 2,000 to 4,000, etc.

Number of Days for COVID-19 Cases to Double
Cases as of March 23 From 1,000 to 2,000 From 2,000 to 4,000 From 4,000 to 8,000 From 8,000 to 16,000 From 16,000 to 32,000 From 32,000 to 64,000
China 81,782 2 1 3 3 4 8
France 29,551 3 3 4 5    
Germany 43,938 4 1 3 3 5
Iran 29,406 2 2 5 7
Italy 80,589 3 3 3 5 4 6
South Korea 9,241 2 3 12      
Spain 57,786 2 2 2 4 4  
United Kingdom 11,809 3 3 4      
United States 83,507 3 2 3 1 3 3
SOURCE: GitHub data repository for the 2019 Novel Coronavirus Visual Dashboard operated by the Johns Hopkins University Center for Systems Science and Engineering (JHU CSSE).

Take France, for example. It took three days for the number of cases to double from 1,000 to 2,000, then another three days for it to double again, four days to double a third time and, finally, five days to double a fourth time to reach 16,000 cases. (France has not reached 32,000 cases, so there are no data after that.)

These numbers are similar across countries with one obvious exception: South Korea. The pattern in South Korea reveals a remarkable slowdown in the progression of the disease: It doubled from 1,000 to 2,000 in two days, then doubled again in three days, but took 12 days to double a third time.

A similar slowdown can be seen in China, where the number of days for doubling cases is increasing. This indicates a plateau in the progression of the diseases in these countries and, perhaps, that COVID-19 already peaked there.

The U.S. is seeing a doubling of cases every few days, which could be a combination of two factors:

  • The spread of the disease itself, which might be negatively affected by social distancing
  • The magnitude of testing, which is going to positively affect the number of cases if testing becomes more frequent

When a quantity is multiplied by two every three days, the original quantity ends up being multiplied by 10 in about 10 days and by 100 in about 20 days. This is a very fast rate of growth. As a point of comparison, when the U.S. gross domestic product grows at a rate of 2% per year (its historical rate of growth), it takes more than 116 years for it to be multiplied by 10—not 10 days.

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