There is no evidence to suggest that people who have recovered from the coronavirus then have immunity, the World Health Organisation (WHO) has said.
Senior WHO epidemiologists warned despite the hopes governments across the world are putting on antibody tests, there is no proof those who have been infected cannot catch the virus again.
The British Government has bought 3.5 million serology tests, which measure levels of antibodies in blood plasma.
Many tests being developed are pin prick blood tests similar to widely used instant HIV tests, that 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 Kerhove 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.”
Her colleague Dr Michael 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.
“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.
The news casts doubt on how effective antibody tests will be as a measure that enables Governments to ease their lockdown restrictions.
Earlier this month, Health Secretary Matt Hancock revealed the government was considering handing out documents to allow people to "get back, as much as possible, to normal life” amid a nationwide lockdown.
But experts were quick to criticise the so-called "immunity passports" as it could lead to people deliberately trying to get infected so they could obtain one of the documents.
On Thursday the Foreign Secretary Dominic Raab extended the nation-wide lockdown by another three weeks and said the Government needed to be satisfied of five things before any adjustments could be made in the future.
One of the five factors was for testing capacity to be in hand and at a level that could meet future demands.
The WHO is due to issue updated guidance on the antibody testing issue this weekend.
