The FDA emphasized the uncertainty about antibody tests in a statement released on April 18.
Although the tests can identify people who have been exposed and who developed an immune response to the virus, the agency noted, “we don’t yet know that just because someone has developed antibodies, that they are fully protected from reinfection, or how long any immunity lasts.”
The FDA says that the role of these antibody tests, at present, lies in providing information to “help us track the spread of the virus nationwide and assess the impact of our public health efforts now, while also informing our COVID-19 response as we continue to move forward.”
The World Health Organization (WHO) also emphasized the current uncertainty over antibody tests at a press briefing on April 17. “Nobody is sure about the length of protection that antibodies may give and whether they fully protect against … the disease,” said Mike Ryan, MD, executive director of the WHO’s emergencies program. There is also a concern that such tests may give false assurance or be misused. “There is still a lot of work that needs to be done to validate these antibody tests,” he added.
“The WHO are right to highlight that any antibody test, if we get one, won’t be able to definitely say whether someone is immune to the infection, because we just don’t know enough yet about how immunity works with COVID-19,” commented Prof. Chris Dye, Oxford Martin School, University of Oxford, in reaction on the UK Science Media Center.
Expanding on this point on the same site, Andrew Easton PhD, professor of virology at the University of Warwick, noted that “a serology test does not discriminate between neutralising and non-neutralising antibodies; a discriminatory test is much more complex and slow.”
Only the neutralizing antibodies have the ability to inactivate the invading virus, he noted.
“When people are infected, the proportions of neutralising and non-neutralising antibodies can differ. It is not always understood what makes an antibody neutralising and another non-neutralising, or why an infection leads to production of more of one of these types of antibodies,” he explained. “The initial immune response immediately following infection sets the memory of the immune system, so if the person had generated mostly non-neutralising antibodies, the next time that person encounters the same virus, they may not be able to prevent an infection.”
So at present, the information from antibody testing is largely unhelpful to individuals, but it could be valuable to epidemiologists and policy makers.
“States are looking at ways they can integrate reliable serologic tests for surveillance,” explained APHL’s Blank.
Knowing how widespread the infection has been within a community could guide research and possibly public health decisions, Wroblewski said at the APHL press conference. But she’s hesitant here, too. “I know there has been a lot of talk about using this testing to ease restrictions, but I do think we need to be cautious on how quickly we move in that direction.” If people don’t have antibodies, it means they haven’t been exposed and that they’re still vulnerable, she noted. “If nothing else, that still informs policy decisions, even if they’re not the policy decisions we want.”