Others are moving forward. Universities like Rutgers, Brown and Cornell have already said they will require proof of vaccination for students this fall. The Miami Heat this week became the first team in the N.B.A. to open special “vaccinated only” sections.

And though businesses have yet to announce outright bans on unvaccinated clientele, some states and technology firms are preparing: At least 17 companies or nonprofits are developing websites or apps that might be used by sporting venues, restaurants and other businesses seeking to keep their customers and employees safe, according to Joel White, the executive director of the Health Innovation Alliance, a broad coalition of health providers, tech companies, employers and insurers.

Airlines including JetBlue and United are also testing the “CommonPass” app, developed by The Commons Project, a nonprofit trust dedicated to using technology to help people control their personal information. Airlines for America, the trade group for the nation’s major carriers, opposes making proof of vaccination mandatory for air travel but would like a clean, easy way for travelers to show their status. Other countries may require proof of vaccination, and the apps can also be used to prove negative coronavirus test results, which the United States requires for international travelers.