That’s the question of the moment.

Will it surprise you when I tell you that Rochelle Walensky didn’t have a good answer, a day after instituting a new policy that affects 160 million people?

“Right,” Walensky admitted. “So we’re asking everybody in those areas of orange and red to mask up. Here is the reason why — if you’re a vaccinated person and you’re in one of those areas, as you said, a sea of red, a sea of Covid, you have a reasonably high chance, if nobody is wearing a mask, to interact with people who may be infectious.

“Every 20 vaccinated people, one or two of them could get a breakthrough infection. They may only get mild disease, but we wanted them to know that they could bring that mild disease home,” she continued. “They could bring it to others.”

Yes, they could bring it to others — but most of those others have willingly assumed the risk of a bad outcome by not getting vaccinated. Not everyone, of course. Kids can’t get vaccinated, but kids suffer few ill effects from infection. Walensky herself said in this same interview that they have no evidence that the Delta variant is making kids sicker than previous strains were. So, sure, if you’re worried about your kids catching it, mask up. But why would you worry?

There’s also a chance that a vaccinated younger person could infect a vaccinated older person since older people have weaker immune systems. But a vax-to-vax infection should be especially mild, one would think, as vaccine immunity is working to reduce the virus’s strength on both ends. If the vaccines work to keep prevent severe illness in senior citizens, and there’s a mountain of evidence that they do, then we shouldn’t worry much about vaccinated people bringing the virus home to vaccinated elders.

We should worry about them bringing it to home to immunocompromised family members. But in that case, the new mask guidance could have been “Mask up if you’re in regular contact with someone whom you know is immunocompromised.”

The situation Walensky’s thinking of here, I suspect, is when a vaccinated adult is around an elderly parent who stubbornly refuses to get immunized for whatever political or cultural reason. There are a lot of them out there, believe it or not; more than 20 percent of seniors still aren’t fully vaccinated. The CDC wants people who got their shots to know that their unvaxxed parents are now at real risk from being around them, which wasn’t the case before Delta.

Although, again, in that case the guidance could have been more nuanced. “If you know for a fact that any member of your family hasn’t been vaccinated, be advised that you’re capable of infecting and potentially killing them even if you’re vaccinated. Take appropriate precautions.”

What we ended up with yesterday instead was a hash, with Americans left to try to deduce whether masking is recommended or not depending upon where they live:

Its announcement of the new guidelines was both vague and technical, making it hard for many nonexperts to understand. The agency did not make clear which parts of the country were affected or how that might change in coming days. Instead, officials used the phrase “high transmission” areas, as if it meant something to most Americans. President Biden, in a public statement, referred to “areas covered by the C.D.C. guidance,” leaving listeners to guess what they were…

All of which raises a question: Should Americans assume that the new mask guidelines will soon apply to almost the entire country — or will remain highly regional, focused on the south and other less vaccinated regions? I asked government officials yesterday, and they didn’t have a solid answer.

In fact, would it shock you to know that the CDC set the new policy despite not having smoking-gun evidence that the vaccinated are transmitting the virus now?

An administration official, speaking on the condition of anonymity, told STAT that health experts do not have studies proving that fully vaccinated people are transmitting the virus. Rather, the official said, the updated guidance is based on studies showing that vaccinated people who contract the Delta variant have similarly high levels of virus in their airways, which suggested that they may be infectious to others. With other variants, vaccinated people had substantially lower levels of virus in their noses and throats compared to unvaccinated people.

It was fair of the agency to assume that if the vaccinated have the same amount of virus in their air passageways as the unvaccinated then they’re equally contagious. (There’s also reason to believe that those particles are live virus, not dead, as often used to be the case when vaccinated people tested positive.) But if you’re of the opinion that they jumped the gun by implementing a sweeping, overboard new policy without waiting for foolproof data, there’s your hook.

On the other hand, with Delta spreading rapidly, how long should they have waited for that foolproof data before encouraging people to mask up? Time is of the essence.

Here’s Walensky this morning on CNN, less convincing than ever.

.@CDCDirector Dr. Rochelle Walensky on the new mask guidance: "With prior variances, when people had these rare breakthrough infections, we didn't see the capacity of them to spread the virus to others. But with the Delta variant … you can actually now pass it to somebody else" pic.twitter.com/K1qvfjNUrX

— New Day (@NewDay) July 28, 2021