“Vaccine passports," or vaccine certificates, are documents that show you were vaccinated against COVID-19 or recently tested negative for the virus. They could help you get into places such as stadiums or even countries that are looking to reopen safely.

The certificates are still being developed, and how and whether they’ll be used could vary widely around the world. Experts say they should be free and available on paper, not just on apps, since not everyone has a smartphone.

In the U.S., federal officials say there are no plans to make them broadly mandatory. In some states, Republican governors have issued orders barring businesses or state agencies from asking people to show proof of vaccination.

Objections revolve mostly around privacy and security - how people’s personal information will be stored - and fairness. Critics say the passports will benefit people and countries with more access to vaccines.

Supporters say they could make reopenings faster and easier. Proof of vaccination or a negative test could be a way for businesses and schools to reassure customers, students and parents that steps are being taken to limit transmission of the virus.

International travel bans by countries could also be eased if people are able to show proof they’re vaccinated. Some countries have long had requirements to prove vaccination against yellow fever.

Still, a challenge is creating certification systems that work across vaccine providers and businesses. More than a dozen initiatives are underway to develop a credential that could be stored on a smartphone or printed on paper, using a QR code.