• The race to get shots to millions of homebound Americans,

A group of health care workers hurried out of a Boston hospital on a recent morning, clutching small red coolers filled with COVID-19 vaccines.

Their challenge: Beat traffic, a looming snowstorm and the clock. They had to get shots in the arms of homebound patients before the vaccines expired in a few hours.

“That clock is in the back of my mind the whole time,” said Dr. Won Lee, a home care specialist at Boston Medical Center.

Millions of Americans will need COVID-19 vaccines brought to them because they rarely or never leave home. Doctors and nurses who specialize in home care are leading this push and starting to get help from state and local governments.

But they face several challenges. Researchers say many homebound people don’t receive regular medical care, which makes it hard to identify everyone who needs a vaccine.

Supplies also are limited, and both the Pfizer and Moderna vaccines expire a few hours after syringes pull the vaccine from vials.

That makes it tough for one doctor to see many patients when they must also stay in someone’s home for at least 15 minutes after the shot in case an allergic reaction develops.

Even so, health care providers report progress. Dr. Lee figures she can deliver five or six doses in an average day.

One of her recent stops was at the apartment of a regular patient, 106-year-old Domingas Pina, who hasn’t left home in about a year.

Dr. Lee sat with Ms. Pina at her table, swabbed her shoulder and swiftly gave the shot as Ms. Pina looked away.

Ms. Pina smiled beneath her surgical mask and flashed a thumbs-up. She will get her second dose next month, right around her 107th birthday.

Harvard Medical School professor Dr. Christine Ritchie estimates about 2 million U.S. residents are homebound. Another 5 million have trouble leaving home or need help doing so; many may need vaccines brought to them as well.