Coronavirus antibody tests too unreliable to gauge if people can go back to work, US says
Antibody tests should not be used to gauge if people are safe to return to work because there is a high risk of false results, the US government's disease control agency has said.
New guidance from the Centres for Disease Control and Prevention also warned that while people did develop antibodies after infection by the new coronavirus, it was still not clear how much immunity this gave, or how long lasting it was.
The guidance said that because only a small proportion of people in many parts of America had been infected, there was a high risk of tests wrongly finding antibodies when there were none.
The findings are a blow to hopes that antibody testing, also known as serological testing, could quickly determine who had immunity to Covid-19 and therefore was safe to resume normal life.
Several countries have said they will pursue so-called immunity passports, potentially giving those who can prove they have had the virus access to work, travel, school and restaurants.
“Serologic test results should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities,” the CDC concluded.
It went on: “Serologic test results should not be used to make decisions about returning persons to the workplace.”
The agency warned that low rates of infection, even in regions that had been hard hit by the pandemic, meant there was a high chance of giving a false positive result.
It calculated that for example in a population where coronavirus prevalence is 5 per cent, a test with 90 per cent sensitivity and 95 per cent specificity would still produce results where less than half of those testing positive will truly have antibodies.
The guidance advises taking more than one test to improve the result.
Serological tests, which are typically carried out by a device that pricks your finger for blood, detect if a person's immune system has produced antibodies in response to coronavirus. They are a measure of whether the person has been infected in the past, rather than if they are infected now.
The CDC said that people appeared to develop antibodies two to three weeks after the onset of Covid-19.
Recurrence of the disease also “appears to be very uncommon, suggesting that the presence of antibodies could confer at least short-term immunity” to the coronavirus.
Yet is said too little was known about the immune response for antibody tests to be used as a guide to immunity.
“Serologic testing should not be used to determine immune status in individuals until the presence, durability, and duration of immunity is established,” the agency said.
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