Epidemiologists explain new CDC guidance on 15 minutes of exposure and what that means to you

Ryan Marosh, University of Michigan

At the Centers for Disease Control and Prevention New guidance Clarify exactly what “close contact” means for infection with SARS-Cov-2, the virus that causes COVID-19.

Previous guidance suggested that close contact occurred when a person was within 6 feet of an infected person for 15 consecutive minutes. Currently, the CDC acknowledges that even short-term contact can lead to infection. Specifically, the new guidance suggests that anyone who has been in contact with an infected person for a total of 15 minutes during a 24-hour period should be considered in close contact.

Despite the changes, most public health professionals have been clear for months No magic about 6 feet.. Similarly, there is no magic for about 15 minutes. These should be used as an approximation to indicate the types of contacts that are at relatively high risk.

Therefore, this new guidance is important to recognize that the virus can spread easily. This is not a dramatic reversal, as it is related to CDC guidance. mask And before and after Asymptomatic individual test..

This change reflects new evidence that has emerged. This change is an example of how science works.As Epidemiologist studying respiratory viral infections, I don’t really think this change will have a big impact on our lives during a pandemic, but it represents continuous evidence of how easy it is for the virus to spread.

Why change?

New advice comes shortly after the outbreak survey published by the CDC Weekly morbidity and mortality reports.. Investigations revealed that Vermont prison personnel were infected. Probably during a series of short contacts with infected but asymptomatic prisoners.

The prisoner was waiting for the test results in the quarantine unit. Employees reported no close contact outside of work and no trips outside the state. At that time, Vermont had a low level of community expansion. The outbreak investigation used video evidence from surveillance cameras in prison to record short interactions. Each interaction lasted about one minute, and employees were in close contact with the infected prisoners for a total of about 17 minutes during the eight-hour shift. For at least some of those interactions, infected prisoners did not wear masks.

Recording infectious contacts is difficult for respiratory viruses. After all, you can’t see the virus moving in the air. The video footage in this case shows fairly strong evidence. As a result, the CDC recognizes that short interactions can pose risks.

This change is also an admission that the previous definition makes at least one explicit assumption that may not be true. The main assumption of the old rule is that it has a threshold effect on exposure. In other words, exposure to a certain amount of virus (equivalent to 15 minutes) increases the risk of illness. The back side of this assumption is that below that threshold, the risk remains low. As a result, I saw some schools accidentally move students. 14 minute intervals..

The new guidance suggests that there is a more dose-response relationship between viral exposure and risk of illness. In other words, even if you are not exposed to all the viruses at once, the risk is higher as you are exposed to the viruses.

What do you mean?

I don’t think this update will make a big difference, but one thing is to expand the pool of people for contact tracking. In an ideal scenario, this change could mean finding more cases early after exposure. These people can start quarantine before becoming infected and spread it to others.

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For example, consider the following holiday. Taking a family for Thanksgiving usually means sharing a meal and probably spending hours in close contact with others. It is still a risk, as asymptomatic people can spread the disease.

Everyone who attended the rally would have previously been thought to be closely related, and they are still. But now, for example, short interactions that increase over time with a restaurant’s server are considered intimate.

This change by the CDC suggests that more attention should be paid to simple interactions in offices, schools, etc. You shouldn’t think for yourself, “This is just a few minutes and you don’t need a mask.” The importance of always wearing a mask to protect others is clearer than ever. We may not know that we are infected, and even short unmasked encounters can spread the virus.

Ryan Marosh, Assistant Research Scientist, University of Michigan

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