“There is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection,” the United Nations agency said in an April 24 statement.

The WHO guidance came after some governments suggested that people who have antibodies to the coronavirus could be issued an “immunity passport” or “risk-free certificate” that would allow them to travel or return to work, based on the assumption that they were safe from re-infection, according to the statement. People issued such a certificate could ignore public-health guidance, increasing the risk of the disease spreading further.

Chile was the first country to announce plans to issue immunity cards based partly on antibody tests. This has raised concerns because the tests have proven unreliable elsewhere, and some people may get deliberately ill in order to obtain the card. The U.S. and others have nonetheless said they’re looking into the option.