There are many headlines about contact tracing and other software apps, hoped to keep people safe as the world's coronavirus lockdowns are slowly lifted.
In Estonia, the government and businesses including Radisson hotels are cautiously testing what's called a "digital immunity passport."
Like a physical immunity passport, it records whether a person has caught the virus and whether they've recovered. It's hoped to enable safe travel, or allow employees to return to work.
But the science behind it is very uncertain.
Liis Narusk is with Back to Work, the NGO developing the software:
"We are currently operating with a prerequisite that immunity as such, regarding COVID-19, is currently, you know it's vague, like scientists haven't really agreed yet on when does it exactly develop, for how long. We are in that sense focusing on the future scenario when we do have more answers to that question. But currently we do know that immunity is developing. That they already agree on."
"So, we're a bit ahead of time, but we have agreed that we cannot really start developing something technologically when we have all the questions answered."
In fact, the World Health Organization has explicitly warned governments not to issue immunity passports. It says there is actually no evidence, so far, that a person cannot be infected twice.
It's a concern shared by Estonia's own board of health, despite the government tests.
Mari-ann Harma is head of the infectious disease department there.
"There is so much uncertainty, scientifically, that we cannot trust that passport, and it might pose a threat for the public health in sense that when a person sees that he or she is positive, has antibodies, then he or she might think that, yes, I am now protected. But we don't know if you are protected it might be that you're not protected and then you have the false sense of security."
If the science proves solid and the test successful, Estonia's government hopes to offer the digital passport to its fellow EU member states.