Does more testing increase cases? Does lack of testing decrease cases?

After tracking Covid-19 cases early on and trying to use them to predict deaths, I quickly realized that, since the number of actual cases is so much more than the ones that have been identified through testing, the only thing constraining case numbers was the number of tests – which, then as today, was far too low. So I have been saying for a while that more reported cases just reflect more testing.

Of course, President Trump picked up on that (I didn’t realize he was a regular reader), when he started saying that he wanted to keep tests down since that would keep cases down – but he of course seemed to think that actual cases were the same as reported cases. It would be nice if decreasing testing would cause actual cases to fall, just as if not getting on the scale would cause one’s weight to fall; unfortunately, that’s not how things work.

Two days ago, I posted that there had been a sharp drop in daily cases in one day – too much to be explained by an increase in mask-wearing or something like that. I admitted that this might just be a fluke, but I also speculated that it may be because people are just not getting tested because the long wait time for results makes it not worth the trouble.

If that were true (and there have been lots of news stories about exactly that phenomenon), then the drop in new cases would be bad news, since actual cases would probably grow faster as a result of that (since people who are asymptomatic but untested would continue to walk around infecting others). Daily cases have rebounded some, but not to what they were just four days ago. I’m hoping this means we’re really in a downward trend, but it’s too early to say that.

However, Steve Parker pointed out that, if I wasn’t ready to credit a big one-day drop in new reported cases to a real downward trend in actual cases, it was inconsistent to say that the cause was a sudden change in people’s attitude toward testing, since that would also not be likely to change abruptly. And he was right.

But that made me realize that it’s no longer true to say that more testing leads inevitably to more reported cases. What matters much more than the amount of testing is the test positive rate, which is an indicator of what percentage of people are actually infected – the higher the rate, the more positive tests. Nationally, the 7-day moving average of that rate peaked at about 8.4% in late July, and is now around 6.5%. That’s a good sign, of course, since in order to say the virus is under control it needs to be around 1% (it’s now .8% in New York). But we obviously have a long way to go as a country, even if some states are now doing better.

  

The numbers

These numbers are updated every day, based on reported US Covid-19 deaths the day before (taken from the Worldometers.info site, where I’ve been getting my numbers all along). No other variables go into the projected numbers – they are all projections based on yesterday’s 7-day rate of increase in total Covid-19 deaths, which was 4%.

Note that the “accuracy” of the projected numbers diminishes greatly after 3-4 weeks. This is because, up until 3-4 weeks, deaths could in theory be predicted very accurately, if one knew the real number of cases. In other words, the people who are going to die in the next 3-4 weeks of Covid-19 are already sick with the disease, even though they may not know it yet. But this means that the trend in deaths should be some indicator of the level of infection 3-4 weeks previous.

However, once we get beyond 3-4 weeks, deaths become more and more dependent on policies and practices that are put in place – or removed, as is more the case nowadays - after today (as well as other factors like the widespread availability of an effective treatment, if not a real “cure”). Yet I still think there’s value in just trending out the current rate of increase in deaths, since it gives some indication of what will happen in the near term if there are no significant intervening changes.

Week ending	Deaths reported during week/month	Avg. deaths per day during week/month	Deaths as percentage of previous month’s
March 7	18	3
March 14	38	5
March 21	244	35
March 28	1,928	275
Month of March	4,058	131
April 4	6,225	889
April 11	12,126	1,732
April 18	18,434	2,633
April 25	15,251	2,179
Month of April	59,812	1,994	1,474%
May 2	13,183	1,883
May 9	12,592	1,799
May 16	10,073	1,439
May 23	8,570	1,224
May 30	6,874	982
Month of May	42,327	1,365	71%
June 6	6,544	935
June 13	5,427	775
June 20	4,457	637
June 27	6,167	881
Month of June	23,925	798	57%
July 4	4,166	595
July 11	5,087	727
July 18	5,476	782
July 25	6,971	996
Month of July	26,649	860	111%
August 1	8,069	1,153
August 8	7,153	1,022
August 15	7,556	1,079
August 22	7,544	1,078
August 29	7,873	1,125
Month of August	32,519	1,049	122%
September 5	8,217	1,174
September 12	8,577	1,225
September 19	8,951	1,279
September 26	9,343	1,335
Month of Sept.	37,442	1,248	115%
Total March – September	226,732

Red = projected numbers

I. Total deaths

Total US deaths as of yesterday: 175,092

Deaths reported yesterday: 1,320

Yesterday’s 7-day rate of increase in total deaths: 4% (This number is used to project deaths in the table above; it was 5% two days ago. There is a 7-day cycle in the reported deaths numbers, caused by lack of reporting over the weekends from closed state offices. So this is the only reliable indicator of a trend in deaths, not the three-day percent increase I used to focus on, and certainly not the one-day percent increase, which mainly reflects where we are in the 7-day cycle).

II. Total reported cases

Total US reported cases: 5,656,204

Increase in reported cases since previous day: 43,021

Percent increase in reported cases since 7 days previous: 7%  

III. Deaths as a percentage of closed cases so far in the US:

Total Recoveries in US as of yesterday: 3,011,577

Total Deaths as of yesterday: 175,092

Deaths so far as percentage of closed cases (=deaths + recoveries): 5%

For a discussion of what this number means – and why it’s so important – see this post. Short answer: If this percentage declines, that’s good. It’s been steadily declining since a high of 41% at the end of March. But a good number would be 2%, like South Korea’s. An OK number would be 4%, like China’s.

I would love to hear any comments or questions you have on this post. Drop me an email at tom@tomalrich.com