WHO warned against the idea of "immunity passports". It said there is currently no evidence that people who have recovered from COVID-19 are protected against a second infection.
The concept of "immunity passports" or "risk-free certificates" comes as a way of allowing recovered people to return to work.
But the Geneva-based UN health agency said, in a scientific brief on Saturday, that there is a need to more research.
"At this point in the pandemic; there is not enough evidence about the effectiveness of antibody-mediated immunity to guarantee the accuracy of an 'immunity passport' or 'risk-free certificate,'" it added.
It argued that people who assume they are immune to reinfection may ignore public health advice. Such certificates could also raise the risks of continued virus transmission.
WHO added that tests for antibodies of the new coronavirus also "need further validation to determine their accuracy and reliability."