Scientists have developed a new lab testing procedure for the detection of antibodies against SARS-CoV-2 that gives results more quickly than existing assays and specifically identifies so-called "neutralizing" antibodies - those that protect by blocking infection of cells.
"With many assays currently in use, we can detect antibodies, but that doesn't tell us if they're neutralizing antibodies. We only know the level of antibodies someone has," said study author Shan-Lu Liu from Ohio State University in the US.
"Some antibodies might be protective, some might not be protective, and some might even enhance infection - we know with this type of coronavirus and some other viruses, some antibodies can even do harm," he said.
"Our assay examines whether antibodies are potentially protective, which means they prevent a patient from reinfection and block viral replication. That's the outcome of infection that we want people to have," Liu added.
In analyses of blood samples from several different populations that had tested positive for Covid-19, the researchers found with this new assay that, overall, ICU patients had produced the highest concentration of neutralizing antibodies, and convalescent plasma donors and health care workers had the lowest antibody levels.
"Our assay could be used to tell whether antibodies have been developed in individuals who have had contacts with SARS-CoV-2," the researchers said in a paper published in the journal JCI Insight.
For the results, the researchers developed what is called a "pseudotype" virus-neutralizing antibody assay, in which an HIV vector and core is coated with the SARS-CoV-2 spike protein to detect antibodies against the coronavirus.
The team applied a new approach by selecting a different form of the light-producing enzyme that can be detected conveniently in culture media containing the virus-infected cells.
That choice saved several steps, and time, in the detection process without losing accuracy and sensitivity to the target virus.
The results showed that, in general, hospitalised patients - and ICU patients in particular - had the highest concentrations, or titers, of neutralizing antibodies in their systems.
However, over 14 per cent of those who had been hospitalized had no or very low levels of antibodies.
The assay detected no SARS-CoV-2 antibodies in the samples from people who had been sick with other types of respiratory diseases.
The test accuracy was further validated by verifying in a lab setting that the antibodies detected in the Covid-19 patient blood samples did in fact neutralize the authentic SARS-CoV-2 virus.