It stated that as of April 24, 2020, no study had evaluated whether the presence of antibodies to SARS-CoV-2 conferred immunity to subsequent infections by the virus in humans or not.

The WHO was reacting to suggestions by some governments that the detection of antibodies to the SARS-CoV-2, the virus that causes COVID-19, could serve as the basis for the issuance of an "immunity passport" or "risk-free certificate".

This certificate they propose, would enable individuals to travel or to return to work assuming they were protected against re-infection.

However, the WHO in its scientific brief, released on Friday, April 24, said there was currently no evidence that people who had recovered from COVID-19 and had antibodies were protected from a second infection.

Considering the measurement of antibodies specific to COVID-19, the brief said the development of immunity to a pathogen through natural infection was a multi-step process that typically took place over one to two weeks.

 This it noted, could cause people who assumed that they were immune to a second infection - because they had received a positive test result - to ignore public health advice.

 The use of such certificates may, therefore, increase the risks of continued transmission of infections.

 The WHO said, it had, therefore, published guidance on adjusting public health and social measures for the next phase of the COVID-19 response.

 It was still reviewing the evidence on antibody response to SARS-CoV-2 infection, adding that most of these studies showed that people who had recovered from infection had antibodies to the virus.

 "However, some of these people had very low levels of neutralizing antibodies in their blood, suggesting that cellular immunity may also be critical for recovery," it said.

It also explained that the body responded to a viral infection immediately with a non-specific innate response in which macrophages, neutrophils, and dendritic cells slowed the progress of virus and may even prevent it from causing symptoms.  

This non-specific response, it said, was followed by an adaptive response where the body made antibodies, which were proteins called immunoglobulins that specifically bound to the virus.

   It explained that the body also made T-cells that recognised and eliminated other cells infected with the virus, and termed this as "cellular immunity".

 "This combined adaptive response may clear the virus from the body, and if the response is strong enough, may prevent progression to severe illness or re-infection by the same virus," it explained.

   Laboratory tests that detected antibodies to SARS-CoV-2 in people, including rapid immunodiagnostic tests, needed further validation to determine their accuracy and reliability, it explained.

"Inaccurate immunodiagnostic tests may falsely categorise people in two: ways - The first, may falsely label people who have been infected as negative, while those who have not been infected could falsely be labelled as positive, with both errors having serious consequences that would affect control efforts".

These tests also needed to accurately distinguish between past infections from SARS-CoV-2 and those caused by the known set of the six human coronaviruses, four of which caused the common cold and circulated widely.

The other two viruses are responsible for Middle East Respiratory Syndrome and Severe Acute Respiratory Syndrome.

The WHO said people infected by any one of these viruses may produce antibodies that cross-reacted with antibodies produced in response to infection with SARS-CoV-2.

 It said many countries were now testing for SARS-CoV-2 antibodies at the population level or in specific groups, such as health workers, close contacts of known cases, or within households.