As government leaders consider how to reopen society, officials in many countries have latched onto the idea of issuing “immunity passports” to people who have recovered from Covid-19.

It is a tantalising and controversial concept. If tests can prove that people are no longer susceptible to the coronavirus, those individuals could be free to return to work and go about their daily lives – helping to revive the economy and assist others who are still quarantined at home.

“We are looking at an immunity certificate – how people who have had the disease, have got the antibodies and therefore have the immunity, can show that and so get back, as much as possible, to normal life,” Matt Hancock, the United Kingdom’s health secretary, said earlier this month.

This week, Chile announced it would begin issuing physical or digital immunity cards to thousands of people who have recovered from the virus – the first country in the world to do so. Scientists and government officials in Germany, France, Italy, Spain, and other countries are considering the idea as well.

The mayor of Paris included immunity passports on a list of proposals for lifting lockdown restrictions in the city. Officials in the northern Italian region of Lombardy — the epicentre of the coronavirus outbreak in Europe — have begun conducting antibody testing, with the hope of eventually issuing immunity certificates. Catalan authorities in Spain are also evaluating the concept.

In the United States, immunity passports could “have some merit under certain circumstances,” Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told CNN this month. “It’s one of those things that we talk about when we want to make sure that we know who the vulnerable people are and not.”

Many countries are working to ramp up their coronavirus testing capacity. Diagnostic swab tests can detect whether an individual is currently infected with coronavirus, while blood tests can determine whether a person has developed antibodies from exposure to the virus.

Together, the tests can help public health officials identify outbreaks and calculate how much of the population has been infected, how deadly the disease truly is, and whether people may have developed some form of immunity. 

Scientists have cautioned that some early antibody tests have proven unreliable, however. And because Covid-19 is a new disease, it’s too early to know how long any immunity might last – or even whether people infected by the virus develop immunity at all. 

“We know that the current tests are not 100% effective,” Isabella Eckerle, a virologist at the Center for Emerging Viral Diseases in Geneva, told HuffPost France.

Two recent antibody studies in California have been heavily criticised by scientists, who have raised concerns about their methodologies and the fact that the tests are prone to false positives. The UK government recently paid £16 million for antibody tests from two Chinese companies, only to discover that the tests didn’t work.

Even if accurate and reliable tests are developed, the mere presence of antibodies doesn’t necessarily mean that a person is immune. The number of antibodies needs to be sufficient to stop the virus from multiplying.

“We have a vague idea that the more antibodies you have, the better protected you are, in the same way we know that a more powerful car engine allows you to go faster, but that doesn’t give us the exact speed,” Marc Eloit, the head of the infectious disease lab at the Pasteur Institute in Paris, told HuffPost France.

Eventually, scientists may learn this information. By regularly measuring the status of high-risk individuals, like health care workers, for example, researchers could potentially identify the antibody level at which people can be considered to be immune. But this kind of monitoring would take “two to three months,” Eloit said.

The concept of immunity passports raises other concerns as well. They could divide society into two groups: those who have immunity and can therefore resume normal life, and those who are required to remain under lockdown.

Employers could potentially require workers to obtain immunity passports as a condition of employment, and anyone without immunity could be discriminated against.

In the United States, laws protect people with disabilities and genetic conditions against workplace discrimination, but the Equal Employment Opportunity Commission recently issued guidance suggesting that discrimination on the basis of Covid-19 immunity may be acceptable.

Immunity passports could also become a condition for traveling on an airplane or attending concerts or sporting events. 

“We will make whatever changes to the business model that will be necessary,” Delta Airlines CEO Ed Bastian said this week. “If it turns out immunity passports will be a new form … You think about everything that came out of 9/11 with TSA and Homeland Security and new public agencies. Could there be a new public health agency coming out that requires a new passport to travel? We’ll be on the forefront of all those advances.”

In Spain, hotels and tourist boards are considering requiring travellers to certify that they are “Covid-19 free” before visiting.

Oriol Mitjà, an infectious disease expert in Spain who is helping to advise authorities in Catalonia, suggested that immunity passports could be required to attend large events — at least as an intermediary step until a vaccine is developed.

Do we want to “have concerts quickly, in six to 10 months, or do we prefer the alternative of not having concerts for two years?” Mitjà said recently.

However, if obtaining an immunity passport becomes a prerequisite for participating in normal life, it could encourage people to take risks with their health, scientists warn.

David Heymann, professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine, told HuffPost UK there is “always a concern” that people could try to infect themselves deliberately, so that they could obtain a certificate allowing them to return to work and move about freely.

“There was chickenpox before,” Heymann said. “Many times people try and expose their children to measles to get them sick early on so they would be immune later on.”

He added: “It’s not a good idea. This disease is infectious, and it affects all people and can cause illness in all people.”

The professor, who is currently advising the World Health Organization on coronavirus, said public officials must emphasize how dangerous such actions can be.

“I think if there is a passport rolled out where there are people with antibodies who are then able to go back to work, there has to be clear instructions to them as to what that means and the risk of trying to become infected themselves,” Heymann said.

With reporting from HuffPost UK and HuffPost France.