Here is an intuitive diagnosis of the danger. The airport screening as actually carried out is either unnecessary or counterproductive. For there are two possibilities: either the population screened contains no one carrying the virus, or some of the passengers waiting in line are already infected. In the former case, testing achieves nothing. In the latter, there are carriers in close proximity to others who were previously virus-free. As the result of the short-range interactions, the virus has a chance to spread. Assuming it can be expected to infect at least one new person, even if the detection is perfect there will be no gain. One current carrier will be screened out and at least one person who passes the test will bring the virus into the country. So, in this case, the screening proves counterproductive.
That’s only a rough analysis of the problem. It would be more exact to think in terms of the chances of transmission and the likely number of carriers. Suppose that, in a population of 1000 returning travelers, 5 people are infected. Assume that they are scattered through the long line of those awaiting screening. Each of them is in the proximity of 10 others for an extended period of time. Since infected people are rare, those other people are originally likely to be virus-free. Let’s set the individual rate of transmission at 0.2: that is, if Sick Sam is standing close to Healthy Holly, there’s a one in five chance of her becoming infected as a result of their encounter. Out of Sam’s 10 contacts, the expected number of the newly sick is 2. Since there are four other people just like Sam, the forced crowding will add 10 new carriers. They will all pass screening, since, even with a six hour wait, no identifiable symptoms will develop. Even if the screening goes perfectly, so that Sam and his friends are all detected, the net result of the screening will be to let 10 infected people loose on the general population instead of the 5 who would have entered in the absence of screening. As implemented, the screening policy has made the situation twice as bad as it would otherwise have been.
Screening is highly unlikely to be perfect. If one thing is clear about the coronavirus, it’s that many people who carry it have not yet developed symptoms (and a significant proportion of them never will.) Of course, if the overworked people assigned to do the checking only catch 3 out of the 5 actual carriers, the problem is exacerbated. Instead of 5 newly arrived carriers, the screening will have yielded 12.
Are the numbers I’ve chosen realistic? Nobody knows. The actual rate of transmission under the kinds of conditions experienced in the arrivals hall can only be estimated. Presumably, though, experts take there to be an excellent chance for the disease to spread in this kind of environment. Surely the worry about Disneyland is that the lines are long when you wait for a ride; but the wait times have been longer at the airport. Going to the theater or to the movies provides less opportunity for you to breathe on others or for them to breathe on you. If our policies of closing museums and amusement parks and cinemas and gatherings with significant numbers of people are justified, it must be because, in these environments, carriers have an appreciable chance of transmitting the disease. Setting the individual transmission rate at 0.2 might well be a serious underestimate.
How low would it have to be for screening to be a net plus? Suppose the individual transmission rate is r, the average number of individuals with whom each carrier has contact is c and the chance of a false negative is f. Simple arithmetic shows that screening (as currently practiced) only has advantages if rc + f < 1. Consequently, when it is hard to detect carriers using the kinds of (necessarily cursory) examinations currently available, even a low rate of transmission makes screening counterproductive.
Attempts to decide just how good — or bad — the policy is can refine this style of analysis in many ways. They can, for example, take into account the effects across time of screeners who become infected and then go on to infect the people whom they examine. Or they can consider the debilitating effects on the immune systems of tired travelers who must stand in lines after a long flight. Yet, given the worries about crowds that have already led (rightly) to many kinds of restrictions, the risks of the screening currently implemented making the pandemic worse are obviously severe. An apology for the inconvenience is not what the public needs. Nothing less than an immediate cancellation of the policy (and an admission of gross irresponsibility) will do. Instead of continuing to act in ways that put American citizens at risk, the administration would do better to implement the approach other nations have tried: to insist on a fourteen-day quarantine for all returning travelers.
Philip Kitcher is John Dewey Professor of Philosophy at Columbia University.