An Epidemiologist Explains the New CDC Steerage on 15 Minutes of Publicity and What it Means for You
By Ryan Malosh, University of Michigan
The Centers for Disease Control and Prevention have new guidelines that clarify what exactly “close contact” means when it comes to transmitting SARS-Cov-2, the virus that causes COVID-19.
The previous guidance indicated that close contact occurred when a person was within six feet of an infectious person for 15 minutes. The CDC now recognizes that a brief contact can lead to the transmission. In particular, the new guidelines suggest that those who spend a total of 15 minutes in contact with an infectious person over a 24 hour period should be in close contact.
Despite the change, most health professionals have realized for months that six feet is nothing magical. Likewise, 15 minutes is nothing magical. These should be used as rough estimates to indicate the types of contact where the risk is relatively higher.
This new guide is therefore an important acknowledgment of the ease with which this virus can spread. It is not a dramatic reversal of the CDC guidance as it relates to masks and the back and forth of testing asymptomatic individuals.
This change reflects new knowledge. This change is an example of how science works. As an epidemiologist studying respiratory virus transmission, I don’t think this change will have much of an impact on how we live our lives during the pandemic, but it is further evidence of how easily this virus is spreading.
Why the change?
The new advice follows an outbreak investigation published in the CDC’s weekly report on morbidity and mortality. The investigation found that a Vermont prison worker was infected, most likely during a series of brief contacts with infected but asymptomatic inmates.
The inmates waited in a quarantine unit for test results. The employee did not report close outside work contacts and had not traveled outside of the state. At the time, Vermont was experiencing low prevalence in the community. The outbreak investigation used video evidence from surveillance cameras in the prison to document the brief interactions. Each interaction lasted about a minute, and in total the operator was in close contact with the infected inmates for about 17 minutes during an eight-hour shift. For at least some of these interactions, the infected inmates did not wear masks.
Documenting infectious contact is difficult for respiratory viruses. After all, we can’t see the virus moving through the air. The video footage in this case is pretty solid evidence. Therefore, the CDC recognizes the possibility that shorter interactions carry some risk.
This change is also confirmation that the previous definition contains at least one explicit assumption that may not be true. The main assumption of the old rule is that there is a threshold effect of exposure. That is, once you’ve been exposed to a certain amount of virus (worth 15 minutes), your risk of disease increases. The downside of this assumption is that below this threshold your risk remains low. Because of this, some schools have mistakenly moved students at 14 minute intervals.
The new guidelines suggest that there is more of a dose-response relationship between virus exposure and disease risk. That said, the more viruses you’re exposed, the higher your risk, even if the exposure doesn’t happen all at once.
What does it mean?
While I don’t think this update will bring any major changes, it does add to the contact tracking pool. Ideally, this change could mean catching more cases early after exposure. These people can then start quarantining before they become contagious and spread it to others.
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Take the upcoming holidays for example. Having a family for Thanksgiving usually means sharing a meal and likely spending several hours in close contact with others. That’s still a risk, especially since those without symptoms can spread the disease.
The people attending this gathering would all have been considered close contacts before, and they still are. Now, brief interactions that add up over time – for example with a server in a restaurant – are considered close contacts.
This change by CDC suggests that we need to be more careful with brief interactions – for example in the office or at school. We shouldn’t think, “This will only take a minute, I don’t need my mask.” The importance of wearing masks at all times to protect others has never been so clear. We may not know we are infected and even a brief, unmasked encounter could spread the virus.
Ryan Malosh, Assistant Research Scientist, University of Michigan
This article is republished by The Conversation under a Creative Commons license. Read the original article.