No 10 has pointed to immunity passports as a way to end the lockdown. How would the policy work in practice? What is an immunity passport? An immunity passport is a certificate declaring that someone is no longer at risk of contracting coronavirus because they have already had it. It would exempt holders from restrictions on activity imposed to contain the spread of the virus. The concept has been given momentum by the German public health body the Robert Koch Institute, which is organising a mass study into how many people are already immune to Covid-19. The German government has not officially commented on where it stands on immunity passports. How is immunity determined? Given that the virus can be asymptomatic, a test is required to show whether coronavirus antibodies are present in the blood of the individual. A positive test would indicate that they have already had the virus and therefore would be likely to have some degree of immunity. It would also mean that they were not infectious to others. What are the benefits of immunity passports? Healthcare workers would be able to return to work from isolation more quickly. They would also allow clinicians who have tested positive to be allocated to highest risk areas and enable personal protective equipment, which has been in short supply, to be distributed to those who need it the most. Other key workers, such as teachers, could also be prioritised, allowing schools to be reopened. How reliable are antibody tests? The tests, which can already be purchased on the internet, vary widely in reliability. So far, the best are 90% sensitive to Covid-19 and 90% specific. Last week, Prof Sharon Peacock, director of infections at Public Health England, told MPs that millions of home-testing kits would be available within days, only for the chief medical officer, Prof Chris Whitty, to quash the idea hours later, citing concerns about accuracy. Another issue is that the tests are ideally done 28 days after an infection. Assuming the test is accurate, can you be certain of immunity? One of the problems of combating a new virus is that there can be no certainties about how it will act. The idea of immunity passports is based on the hope that people who have recovered and test positive for coronavirus antibodies will be protected for a lengthy period of time - but, at present, it remains just a premise. Patients with Sars, which is also a coronavirus, did not have long-term immunity, with potentially only up to a year after the infection. In February, a Japanese tour guide tested positive for coronavirus for a second time after being given the all-clear, in what was believed to be the first case of its kind. Are there any other potential drawbacks? To ascertain the immunity of large portions of the population, home tests would likely be necessary, but that would open up the prospect of fraud. Paul Hunter, a professor in medicine at the University of East Anglia, said if they were not limited to key workers, they could even lead to people going out to deliberately get infected so they could return to work. Can it really end the lockdown? The antibody test looks to have potential for helping clinicians get back to work and assess their risk, but only if it is sufficiently accurate. The fact that the immunity period is unknown is another problem, which together with the difficulty of testing so many people – without relying on home kits – would seem to scupper the chances of it ending the lockdown for the wider population.