Since March, we’ve all been forced to rethink our handling of personal space. Dictating our distance from other humans is a rule of thumb that defines “close contact”: If you’re within 6 feet of someone for more than 15 minutes, it’s potentially dangerous. This is the rule that federal and state health departments use to determine whom needs to be contacted after someone tests positive for COVID-19, and it’s a rule (especially the 6 feet part) that many Americans are using to determine their risk when going shopping or seeing friends. Restaurant servers adhere to it by not lingering at tables; news anchors adhere to it on TV, their seats carefully spaced out. But as we move indoors in increasing numbers—particularly into classrooms, dormitories, and offices—the underlying assumptions that made 6 feet and 15 minutes a rule to live by are no longer correct. To reflect the risk that comes with gathering indoors, and our evolving understanding of how COVID-19 can be spread, we need to rethink the formula for “close contact.”
We’ve been aware for a while that the main way you become infected with SARS-CoV-2 is through inhalation of virus-containing particles; this is why we have physical distancing. Those particles come from the airway of an infected person who coughs, sneezes, sings, yells, or even just speaks and breathes. If you are close enough to breathe those particles as they fall through the air—at the beginning of the pandemic, we thought most particles that conferred risk were relatively large—you stand a chance of becoming infected. For interacting with the general population, where we don’t know who is infected, prevention has centered on minimizing the ways one might be exposed to the virus. These include staying far enough away from one another such that most of the potentially virus-containing particles from one person are either dispersed (this is why the outdoors is considered safer) or fall to the ground before they reach someone nearby (this is the purpose of the 6-foot rule). Disinfecting surfaces where particles may have fallen can be a useful precaution.
For a disease as serious and as infectious as COVID-19, we take additional steps when we identify an infectious case. The cornerstone of pandemic response is isolation, contact tracing, and testing. When this works, it identifies an infected person quickly, through regular testing, then minimizes the number of people who come into contact with them, by isolating the infected person. It proactively identifies other people who had prior contact with the case, through contact tracing; these people are at greatest risk of infection. Who is categorized as having had “close contact” with the infected person is very important: It may affect if they are contacted in the first place, and if they are then counseled to quarantine to avoid further disease spread. In this process of identifying and informing “close contacts,” the finer details matter. Although it is informed by science, there is an art to whether someone qualifies as an instance of close contact. In the process of identifying close contacts, an investigator is likely to talk to many more people than will eventually be deemed “close contacts”—people who came into contact with an infected person only briefly or from a distance. The messaging in all of these discussions must be persuasive, informative, and accurate. That can’t happen if the definition of “close contact” isn’t based on good, up-to-date science.
The original definition for close contact was basically a good guess, made at the beginning of the pandemic, with the understanding that the virus spread via relatively large particles. Helpfully, our understanding of how the virus makes its way from one person to the next has evolved since. It turns out that large virus-containing particles, the kind that usually don’t travel more than a few feet and don’t linger in the air, aren’t the only particles that an infected person expels as they breathe, talk, and cough. They also emit smaller particles that remain airborne for minutes to hours. (This is what people who say that the virus is “airborne” mean). Outdoors, the smaller particles should be diluted and dispersed. But those smaller particles do not disperse as readily in an indoor environment. Those aerosols are also infectious; we call this route of exposure “airborne transmission.” Defining “close contacts” as just those “within 6 feet for longer than 15 minutes” ignores this important disease transmission pathway. Experts who study airborne viral transmission met in late August at a National Academies of Sciences, Engineering, and Medicine workshop and concluded that airborne transmission of SARS-CoV-2 is playing a role in the spread of the virus. Though the exact extent to which it is causing spread of the virus is still uncertain, we need to take it into account in our infection control strategy.
Currently, “close contact” ignores airborne transmission. Using the current rule, if you (or your child) sat several desks away from another student during a two-hour classroom lecture, no one would need to inform you if that student tested positive for the virus. This is true even if the room is not well ventilated. In fact, in shared spaces where desks, cribs, or mats are placed more than 6 feet apart, the current rule would tell us that the presence of an infected individual would not lead to any of the occupants of the room qualifying as a close contact. This means that other occupants, students, teachers, or caregivers would not, according to federal health guidelines, need to be notified that they had been exposed to an infected individual.
We believe that this failure can lead to unnecessary disease spread. We also believe that strict applications of the “6 feet, 15 minutes” rule is at odds with the expectation parents, students, and teachers have that they should be informed if there is an infection in the classroom. Protracted proximity, under circumstances where ventilation and filtering are substantially reduced relative to being outdoors, should override the fact a person was by-the-measuring-stick distanced from the infected individual. Failing to account for such a commonsense concern of increased risk of airborne transmission in a stuffy room is not just a poor reading of recent science but also bad public health policy. Communication and trust in public health is a cornerstone of disease prevention. Sharing indoor space with a group is inevitably risky. People should have information on how best to protect themselves and others. They also should have the information needed to make personal decisions following a potential exposure—particularly if they are not currently identified as “close contacts” but nonetheless shared a space for an extended period with someone who has tested positive.
A prudent approach moving forward would be to consider all classroom occupants close contacts of one another and, in the event of an infection, recommend quarantine as well as testing of those individuals. The testing data that results could then be used as evidence to refine future classroom quarantine measures, and to develop a formula for close contact that takes into account ventilation, longer exposure periods, and even mask-wearing. At minimum, we recommend that everyone in the classroom with a positive case be notified so that they can be instructed to quarantine or they can decide to quarantine in order to prevent additional community spread of COVID-19. This would be good public health policy. We may never have a magic rule for preventing spread, but an updated evidence-based formula can help us depend less on magic to protect human lives.