Nearly 10 million older people are without full immunization. That not only endangers them, but provides opportunities for the coronavirus to keep mutating in the bodies of those with weak immune systems. It could also complicate the planned distribution of third shots…

Even older people who want the shots can face substantial obstacles. Early on, with local health authorities operating mass vaccination sites, “some older adults couldn’t manage the online registration, or couldn’t get to a center,” Dr. Grabowski said.

Months later, with vaccines widely available, people who are disabled, frail or cognitively impaired may still struggle to access the first or second shot.

That’s particularly true for the homebound, defined as people who leave their homes once a week or less. Their numbers have sharply increased during the pandemic, according to a survey in JAMA Internal Medicine published last month.