Does More Testing Explain The Rise In COVID-19 Cases?

The number of new daily COVID-19 cases in the US has been rising for the last few days, and several states have set new records. Is the rise in case numbers mostly due to more testing, as several governors in states with rising numbers have claimed? Or do the rising numbers indicate a "true" increase in transmissions due to state re-openings?

Here is a look at the recent trends for several states that have shown increases in COVID-19 cases since the beginning of May, using Johns Hopkins data:

For comparison, here is a graph of states that have seen a consistent drop in COVID-19 cases:

Note that the second graph uses a logarithmic scale, since some of the drops are quite large. The drops in cases are relatively constant on the log scale, whereas the growth in cases shown in the first graph is restricted to the last 2-3 weeks for most states. Let's have a look at a couple of states in detail, comparing the number of tests to the number of COVID-19 cases (data are from the COVID tracking project). First, Arizona:

Data are smoothed using 7-day trailing averages to remove most of the day-to-day variation. For Arizona, we can see that both test numbers and positive cases increased in the period shown. However, the number of tests (in red) increased only by about 50%, while the number of COVID-19 cases increased by about 300%. So while the increase in testing contributed to the increase in confirmed cases, the primary cause of the observed increase in cases was an increase in transmissions; increased testing alone would only have resulting in a rise of daily cases to about 200, not to more than 400.

Next, Florida:

The number of daily new COVID-19 cases in Florida more than doubled over the last 2 weeks. While there also was a small increase in testing between 6/3 and 6/10, the steepest increase in cases happened after 6/10, when the number of tests dropped. Furthermore, the number of tests per day remained lower than it had been around 5/25. Clearly, the simple equation "more tests = more cases" does not explain the observed trend in Florida.

For comparison, let's have a look at New York:

For New York, the number of tests per day almost doubled to more than 60,000 between 5/20 and 6/14, but the number of confirmed COVID-19 cases dropped from about 2,000 to about 800. Clearly, the effect of continued social distancing and similar government restrictions by far outweighed any  increase from more testing.

The results for many other states are similar: numbers decreased in many northeastern and central states despite more testing, while the increases in many southern and western states were much higher than what can be explained by increased testing.

One factor that comes into play here is that increased test availability changes who can get a test. When tests are in short supply, stringent criteria are used to limit testing to those most likely to be positive; typical restrictions include the presence of symptoms and contact with a confirmed case. During the height of the COVID-19 epidemic in New York City, the positive rates exceeded 50%, and fewer than 10% of infected persons were tested. As tests become more widely available, restrictions who can get tested are relaxed, and the positive rate drops. When convenient, cost-free tests that do not require doctor's referrals or the presence of symptoms are available, some people get a test "just to be safe", or because they had unspecific symptoms so long ago that the chance for a positive test are very small. As a result, a sudden doubling in test numbers does typically not lead to a doubling in confirmed cases, but rather to a significantly smaller increase - which is exactly what we are seeing.

Looking at the graph of daily cases in the US on Worldometers,  very little seems to have changed since the beginning of May - aside from small fluctuations, mostly within each week, the number of daily cases now seems very similar to the number a month ago. What the graph does not show, however, are fundamental differences between the states, with many states showing consistent decreases while others show rapid increases. If we look at the states shown in the first two graphs, but omit the two most heavily hit states, New York and New Jersey, then the drop in one group is pretty much canceled out by the rise in the other group:

What we are seeing now is that the case numbers in the states that used to have by far the highest numbers are so low that further decreases do not change the overall total much; at the same time, the numbers for the "second riser" states like Arizona and Florida have reached levels that do make a difference. Worse, the growth in these states appears to be accelerating, with observed doubling times of two weeks or less. While this growth is not as rapid as during the early growth phase in March, it is fast enough to lead to significant underestimates of the severity of the epidemic.

The states that show rapid COVID-19 have several things in common, which include earlier re-opening and higher average temperatures, compared to the states that show declining case numbers. As described in this article, this dashes the hopes that the virus will "disappear" due to higher temperatures; instead, the higher temperatures cause people to spend more time indoors, where the risk of transmission is higher than outdoors. Right now, states like Arizona show only one fourth the number of COVID-19 cases per million population that New York or New Jersey have, and the actual factor may be even larger since testing was in short supply during the height of the epidemic in the Northeast. But with observed doubling times of two weeks and governors who clearly indicated that they will not re-institute restrictions before hospitals overflow, these states may "catch up" before the summer is over.