The World Health Organization-approved proof of vaccination form is used these days for yellow fever.

The World Health Organization-approved proof of vaccination form is used these days for yellow fever.

This isn't the first time the world has been engaged in a conversation about "vaccine passports." And there even is a version of a passport currently in use – the World Health Organization-approved yellow card, which since 1969 has been a document for travelers to certain countries to show proof of vaccination for yellow fever and other shots.

Once Haffkine's vaccine was put into use in British India, discussions started about asking for proof of vaccination in certain circumstances, according to Sanjoy Bhattacharya, a professor of history at the University of York in the U.K.

"When there were outbreaks in South Asia people from there were not allowed to board ships, for instance, to Aden or Great Britain, or Mecca for the Hajj, without government-issued smallpox vaccination certificates," Bhattacharya says.

"These matters were considered even more urgent in the second half of the 20th century after the introduction of air travel," says Bhattacharya.

Currently, yellow fever is the only disease specified in the International Health Regulations for which countries may require proof of vaccination as a condition of entry.

But any type of vaccine proof is not a blanket rule all must follow, says Bhattacharya.

"No entity has a mandate to create universal certification, and the situation is tricky because it could be at least a couple of years before everyone in the world is vaccinated," says Jen Kates, senior vice president and director of Global Health & HIV Policy at the Kaiser Family Foundation in Washington, D.C.

In February, WHO published a position paper on the scientific, ethical, legal and technological issues that may arise when requiring international travelers provide proof of a COVID-19 vaccination.

"At the present time, it is WHO's position that national authorities and travel operators should not introduce requirements of proof of COVID-19 vaccination for international travel as a condition for departure or entry for a number of reasons.

at least for now is content with issuing the same kind of proof of vaccination as from the era of the plague vaccine: a paper certificate with the vaccine information added in ink