From February 26, 2020
I will try to explain why my views of the impact of COVID-19 dramatically changed due to the cluster of cases in Italy. Of course, I’ll use a graph to try to make the concepts clearer.
One of the terms we use in epidemiology is called the incubation period. That is the time from which a person is first exposed to a communicable disease until they start showing symptoms of the disease. They also can start spreading the disease later during this time period. I’ve represented that with three horizontal bars with gradients somewhat indicating that they get more likely to spread infection as they get closer to the time they are identified as a patient. The incubation period for this disease is thought to be 14 days, which is the lengths of the bars I used.
The first two cases (purple) were a husband and wife who were tourists from China. It is thought that he caused her illness as well as well as 12 others. There is only one other case (orange) that shows up on 2/7. No more are identified until 2/21, when over the course of five days we have seen an additional 319 cases so far. That is one of the things that is so alarming to me. There are three possibilities that I can think of:
1. There were other cases that haven’t been identified that spread the disease. This could be good or bad, depending on how much of a proportion of the population are asymptomatic spreaders.
2. These three individuals combined are somehow responsible for the next 319 cases (so far), or worse, maybe just 1 or 2 of them are. If this is the case, that means at a minimum that one person was responsible for over 100 cases, and if it’s only 1 or 2 of them causing most of the spread, a number much higher.
3. In some individuals, the incubation period may be longer than 14 days.
People like this are called super spreaders. For comparison, during SARS it is thought that the majority of the disease in Singapore was spread by five of these types of individuals, the highest causing 76 cases.
This is a concept I covered earlier called the reproductive number, or R0 of a disease. It’s the average people infected by an individual that is infected. The WHO estimate had been 1.4-2.5, but a recent analysis (link below) of various literature indicates that the value is 1.4 to 6.49 and with an average of 3.28. For comparison, the Spanish Flu of 1918 was 1.2-3.0
The reproductive number of COVID-19 is higher compared to SARS coronavirus
Combining this with what I had described earlier of the differences between how I expected this to travel in the West versus the East is very alarming. We are looking at a disease that could easily rival 1918 in scope. The mortality numbers I have calculated assuming a 30% attack rate (the percentage of the world population that get infected), which is the estimated rate from 1918, are simply shocking. However, we do not know exactly how many people will be infected, so this could be far worse or far better.
I’m sorry this one was probably a little more technical and harder to grasp but some concepts but it’s late and I don’t have the energy to wordsmith. I will try to answer any questions though as I have time.
Disclaimer: This commentary is my own interpretation and does not represent the analysis by the government or my employer. The data is from the Johns Hopkins University’s Center for Systems Science and Engineering.