TORONTO -- A case study of a correctional officer who tested positive for COVID-19 despite having no sustained exposure of at least 15 minutes with any infected individual is providing new evidence that the virus can be transmitted in brief encounters.

The study, released Wednesday by the U.S. Centers for Disease Control (CDC) and Prevention, has prompted them to expand their definition of a “close contact.”

On July 28, the study states, six incarcerated or detained individuals who had not yet received their COVID-19 results arrived to a Vermont correctional facility from out of state, and were transported to a quarantine unit. In the process, all six -- who were not displaying any COVID-19 symptoms -- briefly interacted with a 20-year-old correctional officer.

The six tested positive for COVID-19 on July 29. In the contact tracing process, officials looked at every interaction the six had while they would have been infectious, and determined that the 20-year-old correctional officer was not a close contact who needed to be quarantined, according to the Vermont Department of Health’s rules, since he had never been within two metres of any of them for 15 minutes.

Thus, the correctional officer continued working — until August 4, when, at the end of his shift, he started feeling the symptoms of COVID-19, including a loss of smell and taste, a cough, a headache and shortness of breath, among other symptoms.

He tested positive for COVID-19 on August 11.

In order to find out how the correctional officer had contracted the virus, officials observed video surveillance on July 28 to tally up the time the officer had spent within two metres of any of the six individuals who had COVID-19.

“Although the correctional officer never spent 15 consecutive minutes within 6 feet of an [incarcerated person] with COVID-19, numerous brief (approximately one-minute) encounters that cumulatively exceeded 15 minutes did occur,” the case study stated. “During his eight-hour shift on July 28, the correctional officer was within six feet of an infectious [incarcerated person] an estimated 22 times while the cell door was open, for an estimated 17 total minutes of cumulative exposure.”

The study added that while the six incarcerated or detained people wore cloth face masks during some of these interactions, there were a few interactions in a cell doorway or the recreation room where they did not wear a mask.

The correctional officer was wearing a face mask and eye goggles at all times.

Since the officer had no travel-related exposure or any other known close contact exposures, officials surmised that he’d contracted the virus during one of his interactions on July 28.

A “close contact” according to the CDC is someone who was “within six feet of an infected person for at least 15 minutes starting from 2 days before illness onset.”

In the wake of this study, they are adding to the definition anyone who spends 15 minutes cumulatively within six feet (two metres) of an infected person, even if those 15 minutes are the result of numerous brief interactions, and not in one go.

Fifteen minutes has never been the make-or-break length of time that is necessary for an exposure -- there is no magic number detailing exactly how long it takes the virus to successfully make the jump from one body to another.

There are numerous factors that contribute to the risk of contracting COVID-19, and the 15-minute mark is merely a benchmark to allow officials to categorize at what point exposure is most likely, in order to know how to prioritize resources for contact tracing.

In Canada, the official COVID Alert app also uses the 15-minute rule, only alerting those who were closer than two metres for more than 15 minutes to a person who tested positive.

The risk of exposure can be minimized or increased by a number of things, including physical proximity, whether they’re in an enclosed space, whether there is adequate ventilation and air flow, as well as whether both individuals are wearing masks, among other factors.

The case study concluded by advising that public health officials consider the extra risk of cumulative exposure due to brief interactions in settings where frequent interaction within two metres of a person is necessary, such as within a correctional facility.