Federal health officials issued new guidance on Wednesday that greatly expands the pool of people considered at risk for COVID-19 by changing the definition of who is a “close contact” of an infected individual.

The change by the Centers for Disease Control and Prevention is likely to have its biggest impact in schools, workplaces and other group settings where people are in contact with others for long periods of time. It also underscores the importance of mask-wearing to prevent spread of the virus.

The CDC had previously defined a “close contact” as someone who spent at least 15 consecutive minutes within six feet of a confirmed coronavirus case. The updated guidance, which health departments rely on to conduct contact tracing, now defines a close contact as someone who was within six feet of an infected individual for 15 minutes or more over a 24-hour period, according to a CDC statement Wednesday.

The updated guidance is based on new evidence about transmission of the novel coronavirus, which causes COVID-19. In a report published Wednesday, CDC and Vermont health officials discovered that the virus was spread to a 20-year-old prison employee who interacted with individuals who later tested positive for the virus after 22 interactions with them totaling 17 minutes during an eight-hour shift.

“Available data suggests that at least one of the asymptomatic [infectious detainees] transmitted” the virus during these brief encounters, the report said.

The prison employee wore a cloth mask, gown and eye protection during all of the interactions. The infected individuals wore masks during most of their interactions with him but not during several that took place in a cell doorway or prison recreation room, the report said.

“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,” the CDC said in a statement.

As many has half of people who have the disease do not show symptoms “so it’s critical to wear a mask because you could be carrying the virus and not know it,” the CDC said. “While a mask provides some limited protection to the wearer, each additional person who wears a mask increases the individual protection for everyone. When more people wear masks, more people are protected.”

Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security, called the updated guidance an important change.

“It’s easy to accumulate 15 minutes in small increments when you spend all day together – a few minutes at the water cooler, a few minutes in the elevator, and so on,” Rivers said. “I expect this will result in many more people being identified as close contacts.”

She added: “This change underscores the importance of vigilant social distancing – even multiple brief interactions can pose a risk.”

At the same time, Rivers said it’s not clear whether the brief encounters were the only explanation for how the prison employee became infected. Other potential pathways might have been airborne or surface transmission of the virus. She also noted that the new guidance “will be difficult for contact-tracing programs to implement, and schools and businesses will have a difficult time operating under this guidance.”

At the Vermont prison, the employee had brief encounters on July 28 with six prisoners while their coronavirus test results were pending. The next day, all six individuals tested positive. The Vermont health and correction authorities conducted a contact-tracing investigation and determined that the correctional officer did not meet the definition of a close contact, and he continued to work.

A week later, at the end of his Aug. 4 shift, the employee had symptoms of the disease, including loss of smell and taste, runny nose, cough, shortness of breath, and loss of appetite. He got tested the next day and on Aug. 11 found out that he was positive.

Vermont authorities reviewed July 28 video surveillance footage and determined that the employee never spent 15 consecutive minutes within six feet of any of the infected individuals. But the employee did have “numerous brief (approximately one-minute) encounters that cumulatively exceeded 15 minutes.” During his eight-hour shift that day, he was within six feet of an infected person an estimated 22 times, for a total of about 17 minutes of cumulative exposure, according to the CDC report.

The officer reported no other known close contact exposures to individuals with COVID-19 outside work, and did not travel outside Vermont during the 14 days before he got sick, the report said. Investigators said “his most likely exposures occurred in the correctional facility” through the brief encounters.

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