So we’ve reached 10,000 cases in the US about a day earlier than our projection estimated.  What does that mean?

There are several things it could mean.  I’m going to go through a list of possibilities in roughly the order of increasing probability.

1) The virus has gotten stronger.
This is possible, but highly unlikely.  To my mind, it’s sufficiently unlikely that I wouldn’t even mention it except that I want to allay any fears about this.  Even if the virus had mutated to a more virulent strain, it would start in one location and would then have to spread outward.  We haven’t seen any real indication of this.

2) It’s just noise.

This is possible and there is undoubtedly some noise in the data, there always is.  “Noise” is just randomness in the data. Even though we all know that the weather gets warmer from February to June, if you record the temperature every day, sometimes there will be a few days in a row that get colder.  In fact, you’re almost bound to have some periods like that in any given year. It doesn’t mean anything about the warming of the seasons, it’s just noise. But it would take a particularly large amount of noise to cause the difference we’ve seen in the time to reach 10,000 cases.

3) Sloppy parameters.

This almost certainly plays a role.  When I projected the numbers back on March 14th, I used a starting point of 2500 cases and a doubling period of 3 days.  Those numbers were almost certainly already wrong when I hit the button to post. I chose those numbers because they were round numbers that were pretty close.  We did cross 2500 that morning, but the real doubling period was unlikely to be exactly 3 days. For example, if the doubling period was really 2.5 days, then we would have projected 10,000 cases for today instead of tomorrow.  I chose 3 because it made the projection easy to communicate and my goal was to make some general points about what to expect rather than to make precise predictions. So my choice to use round parameters probably plays a significant role in 10,000 coming early, but I don’t think it’s the whole story.

4) Increased testing.

For the last couple of months Coronavirus tests have been hard to come by.  I personally know people who had symptoms but could not get a test. As testing increases, we’ll see a rapid increase in cases just because we’re looking more.  We may also see an increase in deaths as we look beyond deaths involving flu-like symptoms to what actually causes those symptoms. There are some indications that we may have missed early cases by thinking they were influenza.  To see how increased testing can affect apparent growth rates, let’s look at a chart.

This chart is from the Johns Hopkins tracker. ( )  The orange and yellow lines show the number of cases inside and outside of China respectively starting on January 20.  Notice the weird wiggle in the China line (orange). It starts to level and then suddenly jumps way up before levelling again.  That jump is when China relaxed its criteria and started testing more people. They saw a lot more cases because they tested a lot more people.

New York has been aggressively expanding its virus testing and it shows.  Monday saw the number of cases in NY increase by 30%. On Tuesday, they increased by 74%!  On Wednesday They increased by 83%! Right now, New York has about one third of the cases in the US.  When their growth rate goes up this dramatically, it makes a big impact on the growth rate in the nation as a whole.  If they continue at those high testing rates, and other locations join them, the numbers could get very big very fast.  Just remember, this kind of increase in the numbers is a GOOD thing. I’ve mentioned before that there are a lot of unreported cases, likely far more than the reported cases.  Getting a clearer picture of the actual extent of the virus will help us respond to it more effectively. So, while the numbers may start growing faster now, that may actually be a good thing.

I have some other things I want to talk about, but I’m going to save them for another post.  As always, I’m happy to take questions and comments and anyone is welcome to share this if they like.

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