Brief but frequent interactions can spread Covid-19, says CDC as it changes guidance on who is a close contact
Even brief exposures, a minute or less, could spread the coronavirus if those exposures happen multiple times in a day, suggests new analysis by the US Centers for Disease Control and Prevention (CDC) and the Vermont Department of Health (VDH). It describes a correctional officer in Vermont who got Covid-19 after exposure to infected prisoners, even though he was never around the infected people for more than a minute at a time. He had several brief interactions with the inmates and together they were enough for him to catch the virus, say experts.
The incident has prompted the CDC to change its guidelines on what qualifies as close contact with a person infected with Covid-19. Previously, the agency described a close contact as being within 6 feet of an infected person for at least 15 minutes continuously. The updated guidance changes the definition of a close contact of a Covid-19 case to being within 6 feet of an infected person for a total of at least 15 minutes in a day. That includes multiple, but brief, encounters, one or two minutes at a time.
“The agency defines a close contact as 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 (for example, three 5-minute exposures for a total of 15 minutes),” explains the guidelines. It adds, “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), and whether the infected individual has symptoms (the period around onset of symptoms is associated with the highest levels of viral shedding). Other parameters include if the infected person was likely to generate respiratory aerosols (was coughing, singing, shouting), and other environmental factors (crowding, adequacy of ventilation, whether exposure was indoors or outdoors).
The study
On August 11, a confirmed Covid-19 case in a male correctional officer aged 20 years was reported to the VDH. On July 28, the correctional officer had multiple brief encounters with six incarcerated or detained persons (IDPs) while their coronavirus test results were pending. The six asymptomatic IDPs arrived from an out-of-state correctional facility on July 28 and were housed in a quarantine unit.
On July 29, all six inmates received positive test results. Video surveillance footage showed that the officer did not meet the definition of close contact -- that is, being within 6 feet of infectious persons for 15 or more consecutive minutes -- therefore, he continued to work. At the end of his shift on August 4, the officer experienced loss of smell and taste, muscle pain, runny nose, cough, shortness of breath, headache, loss of appetite, and gastrointestinal symptoms, and beginning August 5, he stayed home from work. He subsequently tested positive for Covid-19.
As a result of this positive test, officials once again reviewed the video surveillance footage from July 28. They found that the correctional officer never spent 15 consecutive minutes within 6 feet of a prisoner with Covid-19, but had numerous brief (approximately 1-minute) encounters that cumulatively exceeded 15 minutes. Specifically, during the 8-hour shift on July 28, the correctional officer was within 6 feet of an infectious prisoner an estimated 22 times while the cell door was open, for an estimated 17 total minutes of cumulative exposure.
The prisoners wore microfiber cloth masks during most interactions with the correctional officer that occurred outside a cell. However, during several encounters in a cell doorway or the recreation room, the inmates did not wear masks. During all interactions, the correctional officer wore a microfiber cloth mask, gown, and eye protection (goggles). The correctional officer wore gloves during most interactions. The correctional officer reported no other known close contact exposures to persons with Covid-19 outside work and no travel outside Vermont during the 14 days preceding illness onset.
The findings suggest that at least one of the asymptomatic prisoners transmitted Covid-19 to the officer during these brief encounters, write authors. “Data are limited to precisely define “close contact,” however, 15 minutes of close exposure is used as an operational definition for contact tracing investigations in many settings. Additional factors to consider when defining close contact include proximity, the duration of exposure, whether the infected person has symptoms, whether the infected person was likely to generate respiratory aerosols, and environmental factors such as adequacy of ventilation and crowding,” they explain.
According to the researchers, a primary purpose of contact tracing is to identify persons with higher risk exposures and therefore, higher probabilities of developing an infection, which can guide decisions on quarantining and work restrictions. “Although the initial assessment did not suggest that the officer had close contact exposures, detailed review of video footage identified that the cumulative duration of exposures exceeded 15 minutes. In correctional settings, frequent encounters of 6 feet or less between IDPs and facility staff members are necessary. Public health officials should consider transmission-risk implications of cumulative exposure time within such settings,” they recommend.
Based on the report, the CDC has reiterated the importance of wearing face masks and calls it one of the most effective steps one can take to help stop the spread of Covid-19. “This article adds to the scientific knowledge of the risk to contacts of those with Covid-19 and highlights again the importance of wearing face masks to prevent transmission,” emphasizes the agency.