(CNN) -- The US Centers for Disease Control and Prevention has updated its definition of a close contact with a Covid-19 patient to include multiple, brief exposures, after a Vermont prison worker appears to have been infected that way, CDC Director Dr. Robert Redfield said Wednesday.
The new definition includes exposures adding up to a total of 15 minutes spent six feet or closer to an infected person. Previously, the CDC defined a close contact as 15 minutes of continuous exposure to an infected individual.
The agency changed the definition after a report from Vermont of a corrections officer who became infected after several brief interactions with coronavirus-positive inmates -- none of them lasting 15 minutes, but adding up over time.
The corrections officer never spent much time with any particular inmate, but opened and closed cell doors, collected soiled linen, opened doors to shower and recreation rooms for inmates, conducted health checks and dispensed medication, Julia Pringle, a CDC officer working with the Vermont Department of Health, and colleagues reported.
The six inmates had no symptoms and had traveled from out-of-state facilities while they were awaiting coronavirus test results, Pringle's team reported in the CDC's weekly report, the MMWR.
His 22 short encounters added up to about 17 minutes of total exposure, the team calculated.
The data suggests at least one of the six inmates transmitted the virus to the officer during one of these brief encounters. The six inmates wore microfiber cloth masks for some, but not all interactions with the officer. "During all interactions, the correctional officer wore a microfiber cloth mask, gown, and eye protection (goggles)," the team wrote.
Redfield said it's an example of real-world science informing policy. The CDC has now updated its definition of what constitutes a close contact.
"As we get more data and understand the science of Covid, we are going to incorporate that in our recommendations," Redfield said at a news conference held at CDC headquarters in Atlanta.
"Originally, contact that was considered to be high risk for potential exposure to Covid was someone within six feet for more than 15 minutes," Redfield added.
The new data is being incorporated into recommendations, he said.
"In an MMWR published today, CDC and Vermont health officials found that multiple, short and non-consecutive exposures to persons confirmed to have COVID-19 led to transmission," the CDC said in a statement.
"The CDC website now defines a close contact as someone who was within 6 feet of an infected person for a total of 15 minutes or more over a 24-hour period. Previous language defined a close contact as someone who spent at least 15 minutes within 6 feet of a confirmed case."
The website notes that this is not an exact science
"Factors to consider when defining close contact include proximity (closer distance likely increases exposure risk), the duration of exposure (longer exposure time likely increases exposure risk), whether the infected individual has symptoms (the period around onset of symptoms is associated with the highest levels of viral shedding), if the infected person was likely to generate respiratory aerosols (e.g., was coughing, singing, shouting), and other environmental factors (crowding, adequacy of ventilation, whether exposure was indoors or outdoors)," it says.
And it's clear what people should do, the CDC says.
"Wearing a mask is one of the most effective steps you can take to help stop the spread of COVID-19," the CDC said in the statement.
"A mask can protect other people from the virus-containing particles exhaled by someone who has COVID-19. As many as half of all people who have COVID-19 don't show symptoms, so it's critical to wear a mask because you could be carrying the virus and not know it," it added.
"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."
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