Although the new vaccines have been shown to be highly effective at preventing people from developing symptomatic cases of COVID-19, little data exists on how well they can stop the spread of the virus, raising the possibility that vaccinated people, despite being much safer individually, could still pose a threat to those they love.

For that reason, "we're still going to be taking all the same precautions," said Kristen Bell, whose husband, Dr. Taison Bell, 37, received a dose of Pfizer's vaccine. "I feel good that he's not going to get sick."

The rest of the critical care physician's family — his wife and their children, Alain and Ruby — are unlikely to be vaccinated before the spring or summer. They, like many others, will soon live in a home divided by the splinter-thin prick of a needle — one person vaccinated, three not. They represent a liminal state that will persist for months nationwide, as the first people to be injected navigate a new coexistence with the vulnerable at home.

Public health experts have estimated that a majority of Americans, perhaps 70% to 80%, will need to have some degree of immunity to the virus for its spread to sputter and slow.

Even as vaccines find their way into more and more arms, scientists will continue to study their effects on the population at large, searching for signs of unexpected or rare side effects and monitoring whether the vaccine might curb the coronavirus' ability to pass from person to person.

Bell, of UVA Health, is treading cautiously with the unknowns. He said he suspected that the vaccine would have at least some impact on contagiousness. But he plans to maintain his routine of discarding his work clothes and showering before engaging with his children.

Dr. Krutika Kuppalli, 42, an infectious-disease physician at the Medical University of South Carolina, where she received her first dose of Pfizer's vaccine on Dec. 15, expressed some discomfort at being first in line for the vaccine, while so many others in the U.S. and beyond line up for their own shot at safety. "I don't think guilt is the right word," she said.

After nearly a year on the front lines fighting the coronavirus, health workers are finally receiving a long-awaited coat of armor. It feels strange to wear it, they said, amid the many millions still left without their own chain mail.

Manevone Philavong, 46, who has worked in environmental services at the University of Pittsburgh Medical Center Passavant for 21 years, was among the first in the nation to be vaccinated on the morning of Dec. 14.

He long ago became accustomed to the risks posed by his line of work, which involves cleaning just about "every aspect of the hospital," he said. When he arrives home from work, he enters through the garage and disrobes in the basement before heading inside.

When Philavong told his parents about his injection, they were thrilled. "They said, 'Now you can spend more time with us,' " he said. "I said, 'Not quite yet.' "

The vaccine provides "a layer of hope," Philavong said. "But I'm still going to use every precaution I can."