The World Health Organization has pushed back against the theory that individuals can only catch the coronavirus once, as well as proposals for reopening society that are based on this supposed immunity.

In a scientific brief dated Friday, the United Nations agency said the idea that one-time infection can lead to immunity remains unproven and is thus unreliable as a foundation for the next phase of the world's response to the pandemic.

"Some governments have suggested that the detection of antibodies to the SARS-CoV-2, the virus that causes COVID-19, could serve as the basis for an 'immunity passport' or 'risk-free certificate' that would enable individuals to travel or to return to work assuming that they are protected against re-infection," the WHO wrote. "There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection."

The statement comes days after Chile announced it would begin issuing immunity cards that effectively act as passports, allowing travelers to clear security at airports with a document that purportedly shows they have recovered from the virus. Authorities and researchers in other countries — such as France and the United Kingdom — have expressed interest in similar ideas, while some officials in the U.S., such as Los Angeles Mayor Eric Garcetti, have mentioned it as one possible facet of a reopening strategy.

The concept for such a card is largely based on the premise that an individual can only contract the coronavirus once before developing the necessary antibodies to fight it off. That premise undergirds another common theory: the concept, known as herd immunity, that if enough people have been infected with the coronavirus — and are therefore immune — its transmission will slow and the risks of infection will diminish even for those who haven't caught it yet.

But these ideas depend to a large degree on the supposition that one cannot catch the coronavirus a second time — an idea that world health authorities said leaders should not count on right now. As of Friday, the WHO said, "No study has evaluated whether the presence of antibodies to SARS-CoV-2 confers immunity to subsequent infection by this virus in humans."

What's more, data reported from the world's early COVID-19 hot spots, such as South Korea and China, have shown that a growing number of recovered patients appear to have suffered a relapse of the disease.

By mid-April, Korean health authorities said that just over 2% of the country's recovered patients were in isolation again after testing positive a second time. And in Wuhan, China, data from several quarantine facilities in the city, which house patients for observation after their discharge from hospitals, show that about 5% to 10% of patients pronounced "recovered" have tested positive again.

It remains unclear why this is occurring — whether it is a sign of a second infection, a reactivation of the remaining virus in the body or the result of an inaccurate antibody test.

Dozens of antibody tests for the novel coronavirus are already on the market, with varying degrees of reliability and accuracy. House Democrats have launched an investigation into the antibody tests and whether the Food and Drug Administration should increase its enforcement of them, according to CNN.

"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,' " the WHO warned.

"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 use of such certificates may therefore increase the risks of continued transmission."

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