The World Health Organisation has warned governments against basing their post-lockdown policies on the outcome of Covid-19 antibody test results.
Such tests are seen by some as the best hope of ending movement restrictions before the emergence of vaccines – which still remain many months away at least.
The WHO’s concern focuses on so-called “immunity passports” granted to people whose test results indicate past infection by the Covid-19 virus, and are then allowed to return to normal life.
Such schemes have been considered as a means of ending lockdowns by various countries – including the UK and United States – but have so far only been launched in Chile.
In a briefing note on April 24, the WHO warned that there was "currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection".
But in a statement on Sunday, apparently to allay "concern” about its initial remarks, the WHO made clear it was not saying that those who recover from Covid-19 have no protection against becoming infected again.
“We expect that most people who are infected with Covid-19 will develop an antibody response that will provide some level of protection," the organisation said.
"What we don't yet know is the level of protection or how long it will last.
"We are working with scientists around the world to better understand the body's response to Codid-19 infection."
Far from being a U-turn, the important clarification bridges the gap between the common perception of how the body responds to viral infections and the reality.
It is widely believed that once exposed to a virus by either infection or vaccination, the disease-fighting immune system develops antibodies that will defeat any future attack.
But the truth is far more complex. While the body can acquire long-term protection against some viruses like measles, polio and tetanus, this is not always the case.
The coronaviruses – to which the Covid-19 virus belongs – are a case in point.
Studies have shown that protection against some types declines in a matter of months, allowing people to be re-infected.
There have already been reports from China, South Korea and Japan of people who seem to have recovered from Covid-19 and then become re-infected with the virus.
Scientists have so far been cautious about such claims. The sheer speed with which the virus re-appeared has led to other explanations being put forward.
For example, such patients may never have been truly free of the disease, or their positive test results may have been false alarms.
It is also possible that – as with the coronavirus 229E responsible for colds – the patients may have become re-infected, but now pose little risk to either themselves or others.
What is clear is that there remain big gaps in the science of how the immune system deals with the Covid-19 virus.
It is also not unreasonable to suggest these unknowns could prove catastrophic for any post-lockdown policy such the immunity passport that puts misplaced faith in test results.
As the WHO statement makes clear, while most people may have some immunity following recovery from Covid-19, it is unknown how long such immunity lasts, and thus how long an immunity passport should be valid for.
Even the reliability of antibody tests remains a matter of controversy. As the WHO points out, these may be fooled by past infections with far more common coronaviruses that cause colds, and therefore wrongly indicate a previous infection with Covid-19.
As a result, people granted immunity passports could return to normal life with a false sense of security, only to fall victim again.
Even if such issues affect just a few per cent of people, the consequences could be dire when rolled out across an entire nation, overwhelming its health-care system.
There is also concern that the value of having an immunity passport could lead to corruption and faked test results.
According to the WHO: “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.””
In the meantime, the scientific consensus remains that there are no short cuts to permanently lifting lockdowns.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK