CDC redefines COVID exposure time and isolation guidance is supported

Organisation: Position: Deadline Date: Location:

The US Centres for Disease Control and Prevention (CDC) has expanded how it defines a “close contact” with COVID-19 following new evidence, writes MedicalBrief. Meanwhile, a separate evidence review confirms CDC guidance about length of isolation following an infection.

Stat News reports previously, the CDC described a close contact as someone who spent 15 minutes or more within six feet of someone who was infectious. Now, the agency says it’s someone who spent a cumulative 15 minutes or more within six feet of someone who was infectious over 24 hours, even if the time isn’t consecutive, according to an agency spokesperson.

Close contacts are those who are tracked down during contact tracing and are recommended to quarantine.

The announcement from the CDC comes as scientists described in a new study how a correctional officer in Vermont appears to have contracted the coronavirus during “multiple brief encounters” with six incarcerated people who had COVID-19. The infected people were awaiting the results of their COVID-19 tests while the interactions happened.

In the study, the authors – including officials from the CDC and Vermont’s health and corrections departments – noted that the data for defining a close contact have been limited. “A primary purpose of contact tracing is to identify persons with higher risk exposures and therefore higher probabilities of developing infection, which can guide decisions on quarantining and work restrictions,” they wrote, adding that “public health officials should consider transmission-risk implications of cumulative exposure time within such settings.”

Experts have long noted that the 15-minute, within-six-feet rule was not some sort of threshold that needed to be hit for transmission to occur. So much about whether spread happens depends on how infectious a person is, how well-ventilated the room that people are in is, how the virus might move through the air in a particular setting, whether people are wearing masks, and more. The 15-minute window had just been used as a benchmark to prioritise who should be followed up with for contact tracing and quarantine.

One reason why the length of interactions might matter, experts think, is because people need to be exposed to a certain level of virus if they’re going to get infected.

Researchers still aren’t sure what that “infectious dose” is – and if a higher dose corresponds to how sick people are likely to get – but the thought is that the longer someone is around someone else who is infectious, the higher level of virus they will be subjected to, and the more likely they are to get COVID-19.

After the officer was diagnosed with COVID-19 in August, health officials and staff at the correctional facility reviewed surveillance footage of his interactions with the six incarcerated people. Though he never spent 15 straight minutes within six feet of any one of them, he was within six feet of them at least 22 times during one eight-hour shift, totalling at least 17 minutes of exposure. During their interactions, the incarcerated people were wearing masks most, but not all, of the time, while the officer always had a microfiber cloth mask, gown, and eye protection on.

The correctional officer had no known contact with anyone else with COVID-19 and coronavirus cases were low in his home county and in the rest of the correctional facility at the time, leading researchers to rule that his case most likely came over the brief encounters.

The new study “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,” the CDC spokesperson said.


A review of dozens of studies by researchers at Oregon Health & Science University and Oregon State University suggests that people may shed virus for prolonged periods, but those with mild or no symptoms may be infectious for no more than about 10 days. People who are severely ill from COVID-19 may be infectious for as long as 20 days.

That's in line with guidance provided by the US Centres for Disease Control and Prevention, confirming recommendations for the length of time people should isolate following infection with SARS-CoV-2.

"Detection of viral RNA may not correlate with infectivity since available viral culture data suggests shorter durations of shedding of viable virus," the authors write. "Additional data is needed to determine the duration of shedding of viable virus and the implications for risk of transmission."

Researchers decided to conduct the review to gain more information on transmission and to help inform infection control practices, said co-author Dr Monica Sikka, assistant professor of medicine (infectious diseases) in the OHSU School of Medicine.

"Even though people can shed virus for a prolonged period of time, the studies we reviewed indicated that live virus, which may predict infectiousness, was only detected up to nine days in people who had mild symptoms," Sikka said.

The researchers identified 77 studies worldwide, including 59 that had been peer-reviewed, and combed through the results. All studies reported assessments of viral shedding using standard methods to identify the virus by replicating it through a process called polymerase chain reaction, or PCR.

Although PCR positivity can be prolonged, culture data suggests that virus viability is typically shorter in duration," the authors write.

Authors include Dr Jessina McGregor, associate professor in the Oregon State University College of Pharmacy; Dr Angela Holly Villamagna, an instructor in infectious diseases in the OHSU School of Medicine; and Dr Lauren Fontana, formerly of OHSU but now an assistant professor at the University of Minnesota.

Objective: Transmission of SARS-CoV-2 has significant implications for hospital infection prevention and control, discharge management, and public health. We reviewed available literature to reach an evidenced-based consensus on the expected duration of viral shedding.
Design: We queried four scholarly repositories/search engines for studies reporting SARS-CoV-2 viral shedding dynamics by PCR and/or culture available through September 8, 2020. We calculated the pooled median duration of viral RNA shedding from respiratory and fecal sources.
Results: Seventy-seven studies on SARS-CoV-2 were included. All studies reported PCR-based testing and 12 also included viral culture data. The overall pooled median duration of RNA shedding from respiratory sources was 18.4 days (95% CI: 15.5 days – 21.3 days; I2=98.87%, p<0.01) among 28 studies. When stratified by disease severity, the pooled median duration of viral RNA shedding from respiratory sources was 19.8 days (95% CI: 16.2 days – 23.5 days; I2=96.42%, p<0.01) among severely ill patients and 17.2 days (95% CI: 14.0 days – 20.5 days; I2=95.64%, p<0.01) in mild/moderate illness. Viral RNA was detected up to 92 days after symptom onset. Viable virus was isolated by culture from -6 days to 20 days relative to symptom onset.
Conclusions: SARS-COV-2 RNA shedding can be prolonged, yet high heterogeneity exists. Detection of viral RNA may not correlate with infectivity since available viral culture data suggests shorter durations of shedding of viable virus. Additional data is needed to determine the duration of shedding of viable virus and the implications for risk of transmission.

Lauren Fontana, Angela Holly Villamagna, Monica K. Sikka, Jessina C McGregor


Full Stat News report


CDC Contract Tracing guidance


Oregon Health & Science University material


Infection Control and Hospital Epidemiology review abstract

Receive Medical Brief's free weekly e-newsletter

Related Posts

Thank you for subscribing to MedicalBrief

MedicalBrief is Africa’s premier medical news and research weekly newsletter. MedicalBrief is published every Thursday and delivered free of charge by email to over 33 000 health professionals.

Please consider completing the form below. The information you supply is optional and will only be used to compile a demographic profile of our subscribers. Your personal details will never be shared with a third party.

Thank you for taking the time to complete the form.