With 50 million Americans immunized against the coronavirus, and millions more joining the ranks every day, the urgent question on many minds is: When can I throw away my mask?

It’s a deeper question than it seems — about a return to normalcy, about how soon vaccinated Americans can hug loved ones, get together with friends, and go to concerts, shopping malls and restaurants without feeling threatened by the coronavirus.

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It seems clear that small groups of vaccinated people can get together without much worry about infecting one another. The Centers for Disease Control and Prevention is expected shortly to issue new guidelines that will touch on small gatherings of vaccinated Americans.

But when vaccinated people can ditch the masks in public spaces will depend on how quickly the rates of disease drop and what percentage of people remain unvaccinated in the surrounding community.

Why? Scientists do not know whether vaccinated people spread the virus to those who are unvaccinated. While all of the COVID-19 vaccines are spectacularly good at shielding people from severe illness and death, the research is unclear on exactly how well they stop the virus from taking root in an immunized person’s nose and then spreading to others.

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It’s not uncommon for a vaccine to forestall severe disease but not infection. Inoculations against the flu, rotavirus, polio and pertussis are all imperfect in this way.

And now coronavirus variants that dodge the immune system are changing the calculus. Some vaccines are less effective at preventing infections with certain variants, and in theory could allow more virus to spread.

At the moment, many experts believe that what’s permissible will depend to a large extent on the number of cases in the surrounding community.

The higher the number of cases, the greater the likelihood of transmission — and the more effective vaccines must be in order to stop the spread.

“If the case numbers are zero, it doesn’t matter whether it’s 70% or 100%,” said Zoe McLaren, a health policy expert at the University of Maryland, referring to vaccine effectiveness.

(Jennifer Luxton / The Seattle Times)

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