How long does it take to get the coronavirus if you’re around someone who’s infected? Unfortunately, there’s no magic number of minutes.

The Centers for Disease Control and Prevention recently revised how it defines a close contact, the people contact tracers track down and advise to quarantine and get tested. Previously, a close contact was anyone who spent at least 15 minutes within six feet of a person confirmed to have COVID-19. Now, those 15 minutes don’t need to be consecutive. Instead, a close contact is anyone who was within six feet of an infected person for a minimum of 15 minutes within a 24-hour period.

Why did the rules change?

The guidelines follow the release of a study that showed how a correctional officer in Vermont tested positive for COVID-19 after multiple, brief interactions with six inmates who had no symptoms. In an eight-hour shift, the employee had 22 encounters with the inmates for a total of 17 minutes.

So, are you safe if you’ve spent less than 15 minutes, total, with someone who was infected?

Not necessarily.

Fifteen minutes is just a guide. “You want to give some kind of broad guideline that’s easy to remember but in reality it’s a continuum,” says Neal Goldstein, assistant research professor of epidemiology and biostatistics at Drexel University. “You could be at low risk for an extended period of time or you could be at high risk for a short period of time. There are so many variables that impact the dose [of virus] that someone receives, and that’s really what matters the most.”

Beyond exposure time, other factors at play include whether the infected person had symptoms, had no symptoms, or developed symptoms later, if you were inside or outside, what kind of ventilation there was, how many other people were there, and what kind of activity you were doing.

For example, if you were singing together inside a friend’s basement versus sitting together in the park, your risk level goes up and you may get infected in less time.

“There are so many different layers, so it’s tough to ascribe the percentage of risk you face, but you want to avoid multiple layers,” says Goldstein.

» READ MORE: Can I see my friends and family for Thanksgiving if I quarantine for two weeks?

What if you were wearing a mask?

The CDC’s guidelines for contact tracing don’t change if you were wearing a mask. You’re considered a close contact even if both you and the infected person were wearing one. But masks are still important to prevent getting sick.

“Masks make a big difference. I would never go, ‘Oh, I’m going to hang out with this person without masks, and it’s OK for 14 minutes but not 15 minutes,'” says Henry Raymond, associate professor and epidemiologist at Rutgers University. “These are guidelines to help facilitate contact tracing in a meaningful way, but they shouldn’t be applied as guidelines for how to protect yourself.”

The CDC says it doesn’t factor in masks for contact tracing because the general public hasn’t been formally trained on choosing a mask and wearing it properly. And wearing your mask wrong increases your risk of spreading and getting the coronavirus — as does longer exposure time and closer proximity to someone who’s infected.

» READ MORE: Around someone who’s recovered from the coronavirus? You should still wear a mask.

While 15 minutes isn’t a hard-and-fast guarantee that we’ll stay safe, neither is six feet, but both help contact tracers narrow down who it makes sense to quarantine and get tested after an exposure.

“You don’t want to have contact tracing go back to the moment you checked out with a cashier or passed someone on the street,” says Goldstein. “There’s never a situation with zero risk, but that’d be an unreasonable burden and a tremendous waste in resources.”

  • Neal Goldstein is an assistant research professor of epidemiology and biostatistics at Drexel University.

  • Henry Raymond is an associate professor and epidemiologist at Rutgers University.