(KXAN) — As debate over whether countries and airlines can or will require proof of vaccination for travelers — or “vaccine passports,” as they’re being called — many online are pointing out that such documents already exist.

Thankfully, while “yellow fever” trended on Twitter on Monday afternoon, it wasn’t due to a new outbreak. Many users explained that much of international travel already requires vaccinations — including for yellow fever.

The conversation was partly sparked by a tweet by TV health personality Dr. Drew, who said, in part: “These vaccine passports segregate people and strip them of their freedom to travel internationally.”

The “passport” to prove yellow fever vaccination is technically called a certificate of vaccination or prophylaxis (ICVP), according to Frommers. It’s been around since 1959 and is commonly referred to as a “Carte Jaune,” or “Yellow Card.”

The certificate is required to be shown at border crossings, especially when travelers are coming from places where yellow fever is common.

Travelers out of South America, Africa, India and Asia all likely have to show their ICVPs, Frommers says.

Arguments in favor of “vaccine passports” include ensuring travel safety and helping encourage travel to skittish customers. Arguments against include violations of privacy and of personal freedom.

But some countries are already requiring such documentation, including Israel, which requires a “green passport” — to verify that someone has been vaccinated and/or recovered from COVID-19.

A similar idea is expected to launch soon in Denmark.

Here in the U.S., however, the debate over personal liberty and responsibility surrounds the concept, with Florida Governor Ron DeSantis signing an executive order banning “vaccine passports” just last week.

The Republican DeSantis condemned their use, saying: “It’s completely unacceptable for either the government or the private sector to impose upon you the requirement that you show proof of vaccine to just simply be able to participate in normal society.”

Meanwhile, the Biden administration has so far said it won’t move to mandate the use of a vaccine passport.

“It’s not the role of the government to hold that data and to do that,” Andy Slavitt, a White House virus-response adviser said. “It needs to be private, the data should be secure, the access to it should be free, it should be available both digitally and in paper and in multiple languages.”

Additional concerns around passports include worries about equity: wealthier people in wealthier nations typically have more access to vaccinations. Requiring a health passport could limit lower income individuals and communities of color from necessary travel, some say.