It's also serves notice that the coronavirus can spread more easily than many people realise, he added.
The definition change was triggered by a study of a 20-year-old Vermont correctional officer, who was diagnosed with a coronavirus infection in August.
The guard, who wore a mask and goggles, had multiple brief encounters with six transferred prisoners before test results showed they were positive. At times, the prisoners wore masks, but there were encounters in cell doorways or in a recreational room where prisoners did not have them on, the report said.
An investigation that reviewed video footage concluded the guard's brief interactions totalled 17 minutes during an eight-hour shift.
In a statement, CDC officials said the case highlights again the importance of wearing masks to prevent transmission.
- AP