Among all the tools that health agencies have developed over the years to fight epidemics, at least one has remained a constant for more than a century: paper vaccination certificates.

In the 1880s, in response to smallpox outbreaks, some public schools began requiring students and teachers to show vaccination cards. In the 1960s, amid yellow fever epidemics, the World Health Organization introduced an international travel document, known informally as the yellow card. Even now, travelers from certain regions are required to show a version of the card at airports.

But now, just as the United States is preparing to distribute the first vaccines for the virus, the entry ticket to the nation’s reopening is set to come largely in the form of a digital health credential.

....

But the great leap forward to 21st-century digital health credentials from 19th-century paper vaccination certificates represents much more than a technological shift. Some civil liberties experts say the vaccine passport apps signal a troubling privatization of public health practices and caution that the technology comes with privacy risks.

“The corporate part of it, the ​extent to which this is going to be privatized is​ ​quite new​,” said Michael Willrich, a history professor at Brandeis University and the author of “Pox: An American History,” a book ​on health control during 20th-century smallpox epidemics. “It raises concerns about privacy, about the right to be free from everyday surveillance.”