CDC Revises Definition Of `Close Contact' With A COVID Case

 

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Often, when dealing with a newly emerging disease, the initial guidance offered by the CDC, the WHO, or other public health agencies is based on past experience with similar pathogens. Over time, as actual data is accumulated, recommendations may change.  

Early in the COVID-19 outbreak, it was assumed that SARS-CoV-2 was only transmissible after symptom appeared, since this was the pattern seen with the SARS coronavirus in 2003.  

But within a few short weeks we were seeing signs of asymptomatic or presymptomatic transmission (see MMWR: Asymptomatic & Presymptomatic SARS-CoV-2 Infections in Residents of a LTCF), necessitating changes in how risk exposures were defined.   

For the purposes of contact tracing, and recommendations on who should stay home for 14 days following a potential exposure, some assumptions as to what constitutes a `close contact' had to be made.  The CDC's guidance, up until yesterday, was:

Someone who was within 6 feet of an infected person for at least 15 minutes starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to specimen collection) until the time the patient is isolated.

This `15 minutes' exposure is, admittedly, somewhat arbitrary, and was based on the idea that a brief exposure was unlikely to result in disease transmission.  Unlikely, but not impossible, as the CDC has allowed:

Data are limited to precisely define the “prolonged exposure” to determine “close contact”, however 15 minutes of close exposure can be used as an operational definition for contact investigation.

Yesterday, however, a new report appeared in the CDC's MMWR that suggests multiple shorter exposures may result in COVID transmission, which has prompted a change in the CDC's guidance. 

In late July a correctional officer had at least 22 brief exposures ( < 60 seconds each) over an 8 hour shift with six detainees awaiting COVID test results (all positive), totaling roughly 17 minutes of exposure.  

As he did not meet the existing 15 minute continuous exposure criteria, he continue to work his shifts, until a week later when he became symptomatic and tested positive for SARS-CoV-2.  An investigation showed he was unlikely to have been exposed outside of his work environment. 

You can follow the link below to read the full report.

COVID-19 in a Correctional Facility Employee Following Multiple Brief Exposures to Persons with COVID-19 — Vermont, July–August 2020

Early Release / October 21, 2020 / 69

Julia C. Pringle, PhD1,2; Jillian Leikauskas, MPH2; Sue Ransom-Kelley3; Benjamin Webster3; Samuel Santos3; Heidi Fox, MSN3; Shannon Marcoux3; Patsy Kelso, PhD2; Natalie Kwit, DVM2 (View author affiliations)

As a result, as of Oct 21st, The CDC's new interim definition of a `close contact' now reads:

Close Contact

Someone who was within 6 feet of an infected person for a cumulative total of 15 minutes or more over a 24-hour period* starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to test specimen collection) until the time the patient is isolated.

 * Individual exposures added together over a 24-hour period (e.g., three 5-minute exposures for a total of 15 minutes). Data are limited, making it difficult to precisely define “close contact;” however, 15 cumulative minutes of exposure at a distance of 6 feet or less can be used as an operational definition for contact investigation. Factors to consider when defining close contact include proximity (closer distance likely increases exposure risk), the duration of exposure (longer exposure time likely increases exposure risk), whether the infected individual has symptoms (the period around onset of symptoms is associated with the highest levels of viral shedding), if the infected person was likely to generate respiratory aerosols (e.g., was coughing, singing, shouting), and other environmental factors (crowding, adequacy of ventilation, whether exposure was indoors or outdoors). Because the general public has not received training on proper selection and use of respiratory PPE, such as an N95, the determination of close contact should generally be made irrespective of whether the contact was wearing respiratory PPE. At this time, differential determination of close contact for those using fabric face coverings is not recommended.

This serves as a reminder that COVID-19 is easily transmitted, particularly in enclosed spaces, even with brief exposures. While face covers, eye protection, and hand hygiene may not guarantee protection against the virus, they can reduce your risks of being infected.