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So-called “immunity passports” should not be used as a strategy to relax quarantines against coronavirus, the WHO said in a new technical report, released Friday night (24).
The document, which reviews 20 scientific studies, says “there is no evidence that people who have recovered from Covid-19 (a disease caused by the coronavirus) and have antibodies are protected against a second infection.”
The risk is that people with a positive result will ignore public health advice because they consider themselves immune to a second infection.
“The use of such certificates can, therefore, increase the risks of continuous transmission,” concludes the technical recommendation.
The “immunity passports” would be based on tests to measure antibodies (produced by the body to fight and defeat an invader) specific to the coronavirus.
The WHO points out that immunity to a pathogen is a process that involves several reactions of the organism, generally during one or two weeks.
When infected with a virus, the body responds immediately with cells called macrophages, neutrophils, and dendritic cells that slow the progress of the virus and may even prevent it from causing symptoms.
This response is non-specific, that is, it is a reaction to any invading virus. The body then produces proteins called immunoglobulins, the antibodies, that specifically bind to the virus.
The body also produces T cells that recognize and kill other cells infected by the virus. This is called cellular immunity.
This combined adaptive response can kill the virus and, if strong enough, can prevent progression to severe disease or reinfection with the virus itself.
This process is often measured by the presence of antibodies in the blood.
According to the WHO, most studies conducted during the pandemic show that people who have recovered from the infection have antibodies against Sars-Cov-2 (name of the new coronavirus).
However, some of these people have very low levels of antibodies, which may indicate that other nonspecific reactions may also be critical to recovery.
Until this Friday, according to the WHO, no study has evaluated whether the presence of antibodies against Sars-Cov-2 confers immunity to subsequent infections by this virus in humans.
Furthermore, laboratory tests that detect specific antibodies have not been shown to be accurate and reliable so far, the organization warns.
There is still no certainty that they can accurately distinguish Sars-Cov-2-transmitted infections from infections by any of the other six human coronaviruses (four from common colds and those that cause Mers and SARS – Respiratory Syndrome in the Middle East and severe acute respiratory syndrome).
According to the WHO report, “People infected with any of these viruses can produce antibodies that cross-react with antibodies produced in response to Sars-Cov-2 infection.”
Inaccuracy can lead to two errors: people who have been infected may have a false negative result and uninfected people may have a false positive result. “Both errors have serious consequences and will affect control efforts.”
The WHO supports antibody tests conducted in specific groups (health professionals, close contacts of known cases or within families) or in population studies, to understand the scope and risk factors associated with infection.
However, according to the organization, these studies were not designed to determine if these people are immune to secondary infections.
