The Centers for Disease Control and Prevention (CDC) raised questions after the agency said clinically significant COVID-19 close-contact exposures could occur in intervals over time. For example, three five-minute exposures over a 24-hour period would roughly equal a 15-minute exposure for that day.

Data are limited, making it difficult to precisely define “close contact,” the CDC stated. “However, 15 cumulative minutes of exposure at a distance of six feet or less can be used as an operational definition for contact investigation.”1

Factors to consider when defining close contact include proximity, the duration of exposure, and whether the infected individual is exhibiting symptoms. The situation is further aggravated if the source case generates respiratory aerosols by coughing, singing, or shouting. Indoor crowding with poor ventilation would likely worsen these effects as opposed to an outdoor gathering with spacing.

Michael Bell, MD, deputy director of the CDC’s Division of Healthcare Quality Promotion, recently addressed this issue in some detail in a webinar by the American Medical Association.2 Bell’s comments have been edited for length and clarity:

Bell: The risk of infection is a combination of how much infectious material is being generated. This is a reflection of whether the individual who is ill is manifesting symptoms. We have data some people generate a lot more droplets and aerosols even when they are speaking quietly — not even from coughing. We have all seen the range of sneeze and cough behaviors. That’s one factor — people generating a lot of [viral] material. Related to that is the pathophysiology of the infected individual themselves. Is there a large enough infectious virus being generated? Are they making a lot and projecting a lot? That’s the source piece.

The environment piece is the second step. Are you in a very ventilated outdoor location, or are you in a small space with very little air exchange? Those are the two extremes. In the former, the risk is much lower; in the latter category, the risk is much higher. What we are seeing is that in enclosed places with poor air exchange, we are much more likely to see transmission from cases to multiple individuals.

The examples we have seen so far include exercise classes that were in small, not well-ventilated locations, where one person who was shouting and breathing hard during exercise managed to infect a large proportion of people in that small space. We don’t see that systematically, but we do when the conditions are right. Similarly, in a choir practice — we have seen this in a couple of examples now — in places without great air exchange, somebody who is aggressively generating aerosols by singing was able to infect a large number [of people] who were nearby.

The third factor is: What sort of mitigation actions are being taken? If the patient who is the source is wearing a mask, that vastly reduces the efficiency of [viral] release. Also, if you are wearing a mask — even if it is not a fit-tested N95 respirator — there is some effect.

We are starting to see data that goes from really great [protection] with a respirator to something like 40% to 50% with surgical masks. There is some benefit to wearing protection as well as keeping from exposing others to your own secretions. There are other factors, like inherent susceptibility based on your genetic makeup, but those are the factors that I think about.

With these in mind, the time that you spend in that context also is related to your likelihood of receiving enough of a dose to initiate an infection. The longer you are in that exposed setting, the more likely you are to be infected. We use the number 15 minutes, and this was originally proposed as a 15-minute [exposure] time. There is this question now — and we will be saying more about this in the coming weeks — that is related to an outbreak that occurred at a prison setting. What we saw there was a relationship with time, but it wasn’t just once — it was cumulative. It makes sense that cumulative series of exposures to shorter periods would add up to be a greater risk. It’s not as if you breathe for 14.99 minutes and only once you cross that threshold you become infected — that’s not how it works. Basically, if you think about it from a probability perspective, if you are spending two minutes with eight patients, there is a possibility that one of those people are going to be shedding coronavirus and you might be unlucky enough to be infected in that two-minute segment. A lot of segments are probably as bad as having one big one.2


  1. Centers for Disease Control and Prevention. Coronavirus Disease 2019 (COVID-19) Appendices: Close contact. Updated Oct. 21, 2020.
  2. American Medical Association. Episode 3: Project Firstline: CDC’s new national training collaborative for infection control. Oct. 29, 2020.