How can we tell we’re ready to reopen?

Let me rephrase this in terms that you can use with your high school science students: If you were in charge of deciding whether or not to ease social distancing, what would you like to know?

I believe, and hope, that most students would include some variation of these two questions: How much coronavirus was circulating before social distancing restrictions went into force? How many cases are still out there? The answer to the first question would tell you how big a problem you were up against before you started social distancing. The answer to the second would tell you how much progress you made.

Here are the possible extremes in response to the second question. Maybe social distancing has worked so well that the virus has basically burned itself out because most infected individuals did not pass it on to anyone else. That would mean that the number of infected persons is now substantially lower than before social distancing. On the other hand, what if, in spite of social distancing, everyone infected with the virus has managed to transmit it to at least one person? That would mean that there are at least as many cases in the community now as there were before social distancing.

What do those two scenarios mean for reopening? If the number of cases has substantially declined, reopening will not lead to a dramatic uptick in cases. If the number of new cases is small, and rapid testing is widely available, each newly infected individual can be quickly isolated, their contacts traced and isolated, and a new outbreak prevented. (Much more about contact tracing in an upcoming article in our series.)

On the other hand, if the number of cases in the community is still large, any relaxation of restrictions will pretty quickly lead to a rapid increase, with too many cases to trace and contain, leading to another wave of the outbreak. ("Pretty quickly" isn't very scientific-sounding, is it? Can you estimate how long it would take? Hint: what's the lag between being exposed and displaying symptoms? Answer: eight days on average. So you would expect to start seeing an uptick in cases about eight days after easing restrictions.)

So how would your students answer the question "How many cases are still out there?" One way to find out (and this is the approach that most states are choosing) is to just go ahead and ease the restrictions and see what happens. In less than two short weeks, you will know whether the virus had more or less gone away or whether it was still present at high levels.

It looks as though some states will ease up quickly and others will take a much slower approach. Some cities or counties may try to open up more slowly than surrounding suburban or rural areas. Either way, the answer to the question of how much virus is out there will be answered by the number of sick people showing up at hospitals. At least fifty experiments will be done, and given that communities are different from each other in so many ways (initial infection rate, kinds of restrictions, degree of compliance, density of population, etc.), it seems unlikely that it will be possible to tease out which factors are most important in tamping down the spread of COVID-19.

There is another way, though, to find out how many cases are still out there before easing restrictions. And that way is: Testing. Testing. Testing.

Although the number of tests being administered in the US is rapidly rising, it is still mostly restricted to people who are displaying symptoms. Indeed, in most places, it is still mostly restricted to people sick enough to go to the hospital. Ideally, you would test every person with any symptoms at all and even plenty of people who aren't displaying any symptoms. You wouldn't have to test everybody (although if I had a magic wand, that would be awesome), but you'd need to test a big enough sample to get a robust answer to the question of how much virus is still out there.

National Public Radio does a nice job explaining one way we could judge whether our level of testing is high enough to be reasonably certain we have a good handle on how widely the virus is spreading. Right now states are reporting that they're conducting a combined 200,000 tests per day. (The COVID Tracking Project releases new results daily and is an invaluable source of data compiled from all 50 states. This would be a great site to send your students to—there are so many questions you could have them explore the answers to there.) According to experts cited in this Vox article, estimates of how many daily tests would be required to gauge the prevalence of cases in a community range from 2-3 million per week (we’re not too far from that) to 100s of millions per week. But the number is certainly higher than our current rate of testing. (Note: This article discusses a lot of political choices and capacity estimates that roam far beyond purely scientific considerations that I am not in a position to evaluate or endorse. But as a discussion of how many tests are necessary and what it would take to reach that capacity, it seems sound.)

Given how great a toll social distancing has taken on the U.S. economy, it is perhaps not surprising that taking the route of "let's just open up slowly and hope for the best" has become so tempting—even irresistible in many places. But it’s worth noting that without adequate testing, we'll never know how big an impact social distancing had, and whether we could have eased back into normal activities without risking a surge in cases and deaths.