Many businesses are worried about the impracticality of enforcing a vaccination requirement, particularly at theaters, concert halls, restaurants, hair salons, and any other business that relies on indoor gatherings. Doing so, some said, could alienate some customers, hurt revenue, and even lead to safety concerns.

“With some of the mask mandates, businesses had to increase security,” said Xiomara Peña, the vice president for engagement at Small Business Majority, a nonpartisan, business-focused advocacy group. “That was a little bit easier to enforce. But how could this impact the bottom line of businesses, ultimately, I think it’s still to be determined.”

Many public health experts have also expressed ambivalence or outright opposition, though their reasoning is usually different.

Asking for proof of vaccination could politicize the immunization effort more broadly, Benjamin said, even if the people requiring it aren’t government officials. He and others have also argued the system could also worsen racial and economic gaps in who has received shots so far, further punishing people for being unable to access vaccines.