Senior WHO epidemiologists warned despite the hopes governments across the world have piled on antibody tests, there is no proof those who have been infected cannot be infected again.
Many tests being developed are pinprick blood tests similar to widely used instant HIV tests and measure for raised levels of the antibodies the body uses to fight the virus.
Speaking at a press conference in Geneva, Dr Maria van Kerkhove said: “There are a lot of countries that are suggesting using rapid diagnostic serological tests to be able to capture what they think will be a measure of immunity.
“Right now, we have no evidence that the use of a serological test can show that an individual has immunity or is protected from reinfection.”
She added: “These antibody tests will be able to measure that level of seroprevalence – that level of antibodies but that does not mean that somebody with antibodies means that they are immune.”
Dr van Kerkhove said it was “a good thing” that so many tests are being developed.
But she cautioned: “We need to ensure that they are validated so that we know what they say they attempt to measure they are actually measuring.”
The WHO’s top emergency expert Mike Ryan said that even if antibodies were effective there was little sign that large numbers of people had developed them and were beginning to offer so-called “herd immunity” to the broader population.
“A lot of preliminary information coming to us right now would suggest quite a low percentage of population have seroconverted (to produce antibodies),” Mr Ryan said.
“The expectation that … the majority in society may have developed antibodies, the general evidence is pointing against that, so it may not solve the problem of governments.”
Dr Ryan said the antibody tests also raised ethical questions.
“There are serious ethical issues around the use of such an approach and we need to address it very carefully, we also need to look at the length of protection that antibodies might give,” he said.
WHO officials have said there is no evidence to suggest that having had the coronavirus would guarantee people immunity.
“You might have someone who believes they are seropositive (have been infected) and protected in a situation where they may be exposed and in fact they are susceptible to the disease.”
Dr Ryan said the tests had to be used as part of a coherent public health policy.
Maria van Kerkhove also said at the briefing that a sharp upward revision in China’s coronavirus death toll on Friday was “an attempt to leave no case undocumented”.
Dr Kerkhove said the Chinese authorities had gone back over data from funeral services, care homes, fever clinics, hospitals and detention centres, as well as the number of patients who had died at home, in the city of Wuhan, source of the coronavirus outbreak.
The WHO is due to issue updated guidance on the issue this weekend.