"As of 24 April 2020, no study has evaluated whether the presence of antibodies to SARS-CoV-2 confers immunity to subsequent infection by this virus in humans."
Some governments, wanting a gradual return to work and the resumption of economic activity, have put forward the idea of issuing documents attesting to the immunity of people on the basis of serological tests revealing the presence of antibodies in the blood.
But the effectiveness of an immunisation thanks to antibodies has still not been established and the available scientific data do not justify the granting of an "immune passport" or a "certificate of absence of risk", warns the WHO.
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."
"People who assume that they are immune to a second infection because they have received a positive test result may ignore public health advice," the WHO said.
"The use of such certificates may therefore increase the risks of continued transmission."
WHO also believes that the serological tests currently used "need additional validation to determine their accuracy and reliability".
In particular, the tests need to make it possible to distinguish the immune response to the novel coronavirus from the antibodies produced during an infection by another of the six known human coronaviruses, four of which are widespread, causing mild colds.
The other two are responsible for MERS (Middle East Respiratory Syndrome) and SARS (Severe Acute Respiratory Syndrome).
"People infected with one or the other of these viruses are capable of producing antibodies which interact with antibodies produced in response to infection caused by SARS-CoV-2", said WHO, empasising the importance of being able to identify them.