(NBC) WASHINGTON, DC–October 21st,2020–New CDC guidelines on Covid-19 exposure now expand the pool of people now considered at-risk for infection.

A small outbreak of Covid-19 in a Vermont prison has prompted a change in the Centers for Disease Control and Prevention’s guidelines for what qualifies as close contact with an infected person — and illustrates the importance of wearing a mask.

The findings could affect how people gather as the weather grows cooler.

The updated guidance changes the definition of a “close contact” of a Covid-19 case to a person who has been 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.

Previously, a close contact was defined as spending 15 consecutive minutes with a Covid-19 patient. Those changes were expected to be updated Wednesday on the CDC’s website.

“Cumulative exposures can be as hazardous as 15 sustained continuous minutes of exposure,” Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville, Tennessee, said.

The CDC made the change after an investigation into a Covid-19 outbreak over the summer in a Vermont correctional facility. A report on the outbreak was published Wednesday in the CDC’s Morbidity and Morality Weekly Report.

At the end of July, six new prisoners arrived at the facility and were placed in a quarantine unit while awaiting Covid-19 test results. None had any symptoms, but the next day, all six tested positive.