In the U.K., for example, there's talk of immunity wristbands. In Italy, similar immunity "licenses" are being planned in some regions, reports Reuters.

"Everyone's waiting for serologic testing, the whole world," said France's Health Minister Olivier Véran, as reported by AFP. He believes mass production of the tests could start within weeks and would be important in reducing confinement.

Even the European Commission’s roadmap to exiting containment measures relies on these tests.

They "will provide complementary data on the share of the population that has successfully overcome the disease and eventually measure the acquired immunity,” it reads.

In Germany, the Robert Koch Institute (RKI), which monitors infectious diseases, announced last week it plans to carry out three different types of antibody test studies among different groups, in different regions and with different sample sizes. One aim is to find out who's immune, including among those who might not be aware they were even infected.

There's one major caveat. Merely having antibodies doesn’t signal immunity.

“You can’t say just because someone has antibodies that they are immune,” said David Cavanagh, senior lecturer at the University of Edinburgh’s Institute of Immunology and Infection Research. “We just honestly have not enough idea of whether everyone will be immune after infection or not at the moment.”

Experts in the U.S. are also at pains to emphasize this.

Antibodies can be made against any individual component of a virus. Some of these antibodies confer immunity, others may not, explained Cavanagh.

“When you make an immune response, you can make very effective immune responses, or you can make ineffective immune responses,” said Cavanagh.

He explained that these antibodies could neutralize the virus by blocking its entry into cells, or they could aggregate the virus together so it can be destroyed by the body's immune cells.

Other antibodies, however, could bind to the virus but not actually stop it from invading the cells. Instead, they would promote infection.

Another issue is the timing of when tests are carried out. Lothar Wieler, the RKI's head, warned last week that despite the tests' many advantages, some might also "raise false hopes." For instance, in most cases, antibodies can only be detected at the soonest one to two weeks after an infection — so tests carried out at an earlier stage might be misleading, according to his institute.

A broader difficulty: Scientists just don’t know enough about the coronavirus yet to say which antibodies are likely to be effective, as it's a novel virus.

For example, preliminary findings looking at the presence of antibodies in residents in Gangelt, a town in a hard-hit region in western Germany, indicated that 14 percent of the population had been infected.





Some media reports called this figure a representative finding for the broader population because 400 households across the town were included. But others criticized the preliminary results for being published too early.

Simon Clarke, associate professor in cellular microbiology at the University of Reading, said that the study "does not in any way prove that 14 percent of the population are immune."

"While it's undeniably a good sign, merely having some antibodies does not necessarily prove immunity, as is often casually asserted," he said. We don't know what the correlates of protection are, and we don't know how long any immunity would last, he added.

In particular, what Clarke and other scientists are cautious about is overinterpreting the results, and the possibility of those tests delivering false positives.

Indeed, other similar studies also show that the picture isn’t clear cut.

David Heymann, an epidemiologist who headed the global response to SARS in 2003 and is now at the U.K.’s Chatham House, has pointed to Chinese data indicating that, in some parts of the country, people have developed either very low levels of antibodies or none at all.

The question remains open whether milder infections produce fewer or no antibodies, or whether the virus is less easily transmitted than previously thought, he noted at a Chatham House webinar April 15.

There’s another big problem. Plenty of tests found on the European market at the moment aren’t working very well.

For instance, scientists worry about the sensitivity of at-home versions of antibody tests, which aren't as precise as lab tests and can't tell you how much of an antibody response you had.

The head virologist of Germany’s Charité clinics, Christian Drosten, warned in a video recorded for the German health ministry that those home tests "come in very different quality."

"There are a lot of companies, especially from Asia, which offer these types of tests," Drosten said, noting they would look “like a pregnancy test."

But some of those versions aren't validated yet, he explained, which means they can't be compared to standardized lab tests.

"We are experiencing surprises" when it comes to the results of those home versions, he added. "Some work very well. Others are really useless and you really shouldn't use them, because you have a very high risk of not detecting the infection."

Meanwhile, some EU countries — including Belgium, Finland, Sweden, Ireland, Germany and the Netherlands — have warned against or even banned self-tests for coronavirus at this stage, according to a European Commission communication. That may be due to the possible difficulty that non-experts would have in interpreting results and implications correctly, the Commission cautioned.

In the U.K., meanwhile, the government ordered over three million tests, none of which worked as they should have. The New York Times reported that the government paid $20 million for two million of these tests. Similarly, the majority of tests ordered by the Czech Republic were unreliable, according to Czech news organization Seznam Zprávy.

In addition, governments need to set up a way for the results to be fed back into health systems, rather than get thrown out.

Some governments are taking action to ensure that only accurate tests get to market. For example, France’s health technology assessment body issued guidance on April 16 on the required sensitivity and specificity of the tests.





That doesn’t mean that antibody tests aren’t useful.

Where they can help — even if they aren’t particularly sensitive — is in population-level surveillance.

Germany’s mass studies, for example, aren’t just looking for immunity. They also aim to get a better picture of just how widespread the infections have been.

The results of these studies are key in assessing "the course and severity of the pandemic more accurately and to better evaluate the effectiveness of the measures taken," said the RKI's Wieler.

These studies also help governments calculate more accurate death rates.

And there’s another area where antibody testing is really important — developing a vaccine.

If antibody testing tells us that some people have a relatively robust immune response and recovered from mild forms of the disease, it will also help to determine whether there's any possibility of producing a vaccine, and what component of the virus the vaccine needs to target, Cavanagh explained.

In other words, antibody testing is one part of the broader ultimate exit strategy that rests on finding a vaccine. Until then, life can’t go back to normal, epidemiologist Neil Ferguson told BBC Radio 4 on April 16.

"We will have to maintain some level of social distancing, a significant level of social distancing, probably indefinitely until we have a vaccine available," he said.

Carmen Paun contributed reporting.

This article is part of POLITICO’s premium policy service: Pro Health Care. From drug pricing, EMA, vaccines, pharma and more, our specialized journalists keep you on top of the topics driving the health care policy agenda. Email [email protected] for a complimentary trial.