CDC changes definition of close contact with virus

Several short exposures add up to the same as one longer one, guidelines say

It has been a rule of thumb in a global pandemic that has everyone rethinking every interaction with anyone else: the 15minute mark.

The simple message from public health authorities has been that you could be infected coming within two metres or six feet of someone with COVID-19 for that period of time or longer.

But new guidance from the Centers for Disease Control and Prevention in the U.S. makes it clear that those minutes can add up. Several shorter exposures that combine for a quarter of an hour or more within 24-hours can also put you at risk, officials at the Atlanta-based health protection agency said Wednesday. So what does that mean? “It’s totally reasonable,” Dr. Isaac Bogoch, an infectious diseases specialist at the Toronto General Hospital Research Institute and the University of Toronto, said of the updated messaging.

“It really looks at cumulative exposure instead of a one-time exposure.”

Bogoch added he suspects many people have already been thinking of their possible exposures this way.

Researchers still don’t know what the infectious dose is for the virus that causes COVID-19, or how much of it a person must be exposed to before they get it. But, in general, the more time a person is in close contact with someone who has a contagious disease, the more at risk they are.

Canada’s national COVID Alert app, which sends out a randomized anonymous code to other phones that have been in close contact with someone who has reported testing positive, already takes into account cumulative exposures, Public Health Agency of Canada spokesperson Natalie Mohamed said in an email.

“The app counts cumulative exposures and notifies someone of a possible exposure if they have been in close contact with one or more confirmed cases for a cumulative total of15 minutes or more in a 24-hour period,” she said, adding it was designed that way from the beginning.

The tweaks to U.S. guidance were made after a Vermont prison guard tested positive for COVID-19 after having several brief interactions with six inmates who had it. The guard was wearing a mask and goggles, but the inmates weren’t always wearing their masks. After the guard, who lives in an area with low community transmission, became ill, researchers watched back security footage and counted how long he was exposed to the inmates over the course of the day. It added up to 17 minutes over his eight-hour shift.

CDC officials said in a statement the case shows how important it is to wear masks.

“As we get more data and understand this COVID we’re going to continue to incorporate that in our recommendations,” director Robert Redfield said at a news conference.