Governments should not issue so-called "immunity passports" or "risk-free certificates" as a way of easing lockdowns, the World Health Organization (WHO) says.
It said there was "no evidence" that people who had developed antibodies after recovering from the virus were protected against a second infection.
Such a move could actually increase virus transmission, it warned.
People who assumed they were immune might stop taking precautions, it said.
Some governments have considered permitting people who have recovered to travel or return to work.
Restrictions imposed on movement to stop the virus spreading have crippled economies around the world.
More than 2.8m cases of the virus have been confirmed worldwide and nearly 200,000 people have died.
What did the WHO say?
"There is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection," the WHO said in a briefing note.
Most studies carried out so far showed that people who had recovered from infection had antibodies in their blood – but some of these people had very low levels of antibodies.
This suggested that another part of the body's immune response – T-cells, which eliminate infected cells – may also be "critical" for recovery.
As of Friday no study had evaluated whether the presence of antibodies to the virus conferred immunity to subsequent infection by the virus in humans, the WHO said.
"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 said.
The organisation also said laboratory tests to detect antibodies needed further validation to determine their accuracy and also needed to distinguish between previous infection by the SARS-CoV-2 virus – which has caused the pandemic – and the six other known coronaviruses in circulation.
Passports too risky – for now
Analysis by Rachel Schraer, BBC health reporter
The WHO's guidance is based on evidence from researchers all around the world. But it could well change as we rapidly learn more about this virus.
There isn't currently any evidence to suggest having had the virus once protects you from getting it again. So the idea of an "immunity passport", allowing people who test positive for antibodies to have fewer restrictions, would be a very risky one.
Many countries including Germany, Italy and the UK are beginning to test samples of their populations for antibodies. In the UK, 25,000 people will be tested every month for the next year – both for antibodies, and to check if they currently have the virus.
This could provide more information about whether (and for how long) the disease confers immunity to those who have recovered. And that would give us a clearer idea about whether testing individuals and giving them some kind of immunity status might be an option in the future.
Where are 'immunity passports' being considered?
Last week Chile said it would begin issuing "health passports" to people deemed to have recovered from the illness.
Once screened for the presence of antibodies to make them immune to the virus, they could rejoin the workforce, officials said.
In Sweden, which has chosen to keep large parts of society open, some scientists believe people may end upRead More – Source