The Centers for Disease Control updated its definition of what it means to be in "close contact" with someone infected with Covid-19 on Wednesday, after publishing a report on Covid-19 cases that took place at a Vermont correctional facility.
Previously, the agency said that "close contact" meant being within six feet of an infected person for 15 consecutive minutes. Now, it's considered "close contact" if you spend a total of 15 minutes within six feet of an infected person over the course of a 24-hour period, starting two days before the onset of illness. For example, if you had three five-minute exposures to the person over the course of a day, that would count as close contact.
The designation is used to determine what contacts of the infected person qualify to be included in contact tracing and who should quarantine, but the Covid-19 case that prompted the change also shows that even brief interactions can be enough to spread the virus.
A study published on Wednesday outlines how a mask-wearing corrections officer in Vermont contracted Covid-19 after having several brief interactions with Covid-positive individuals over the course of an eight-hour shift.
After reviewing surveillance footage, public health officials noted that a 20-year-old corrections officer had 22 interactions (lasting about a minute each) within six feet of six incarcerated or detained individuals who were asymptomatic and awaiting Covid-19 test results. In total, the study authors estimate the officer had 17 minutes of exposure to infected people over the course of the day.
A day later, the six individuals tested positive for Covid-19. But since the corrections officer wasn't technically deemed a "close contact," he continued to go to work. Seven days after the brief exposures, he developed symptoms of Covid-19.
It's worth noting that the officer was wearing a microfiber cloth mask, gown, goggles for eye protection and in most interactions, gloves. The incarcerated or detained people also wore microfiber cloth masks most of the time, but the CDC says "during several encounters in a cell doorway or in the recreation room, [they] did not wear masks."
Based on this data, the CDC suggests that the officer contracted Covid-19 during the brief encounters — he didn't report any other close contact exposures and hadn't traveled from Vermont in the 14 days prior.
While experts are still discovering details about how Covid-19 is spread as the pandemic goes on, the updated information on short interactions "highlights again the importance of wearing face masks to prevent transmission," a CDC spokesperson said in a statement.
You should wear a mask anytime you're around people from outside your household, regardless of whether you have symptoms, because even short mask-free interactions like the ones in the report can lead to spread. The more people wear masks, the more protection we all have. Of course, staying away from people is also another way to stop the spread.
Broadly speaking, there could also be more people considered in close contact to an infected person, which could make the contact-tracing process more difficult. Close contacts also need to quarantine for 14 days and monitor symptoms.
Although it's just one report, the new guidance could have the most impact on places where people spend all day with others, such as offices and schools, Caitlin Rivers, a senior scholar at the Johns Hopkins Bloomberg School of Public Health, told NPR Thursday. It also adds a layer to consider when evaluating the risks of various activities and gatherings.
While 15 minutes is considered the current definition of close contact, it's important to remember that there are other factors that can impact transmission, besides the amount of time you're spending with someone. For example, how close you are to a person, whether the infected person is symptomatic (and therefore has the highest level of viral shedding) and if they are releasing respiratory droplets and aerosols can all impact your risk, according to the CDC. Other environmental factors, such as the number of people in a space and the ventilation indoors, are also relevant.