The World Health Organisation has cast doubt on the use of coronavirus tests to show whether people are immune or protected from infection.
Expert epidemiologist Maria Van Kerkhove told a briefing on Friday that the presence of antibodies in the blood did not mean that person was no longer at risk from the disease.
“We have no evidence that the use of a serologic test can show that an individual is immune or protected from infection,” she added.
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And Mike Ryan, who leads the WHO’s Covid-19 team, said there was so far little evidence of so-called “herd immunity” among populations exposed to the disease.
“A lot of preliminary information coming to us right now would suggest quite a low percentage of population have actually seroconverted to produce antibodies,” he said.
“There’s been an expectation that maybe herd immunity may have been achieved, the majority in society may already have developed antibodies.
“I think the general evidence is pointing against that, so it may not solve the problem governments are trying to solve.”
Dr Ryan said there was still ”a lot of uncertainty” about the effectiveness of antibody tests. “Nobody is sure whether someone with antibodies is fully protected against having the disease or being exposed again,” he added.
“Plus some of the tests may have issues of sensitivity. They may give a false result.”
It follows reports that the UK government paid £16m for antibody tests from two companies in China last month – only to discover they did not work.
Despite that setback, health secretary Matt Hancock said on 2 April that people who tested positive for coronavirus antibodies would be given “immunity certificates” to allow them to leave lockdown and return to normal life,
And on Friday the pharmaceutical giant Roche said it had devised a new antibody test which it hoped to make available in the UK and other European countries early next month.
The WHO said it hoped to issue guidance on antibody tests this weekend. “It’s a good thing that these things are being developed and available but they need to be validated,” said Dr Van Kerkhove.
Dr Ryan added that there were “serious ethical issues around the use of such an approach and we need to address it very carefully”.
