In order to diagnose someone with an active COVID-19 infection, we take a nasal swab and run an RT-PCR test to identify if there is elevated coronavirus RNA in the specimen. But how do we tell if someone is mounting an immune response to a virus? How do we tell if someone contracted the virus, recovered, and now immune? How do we tell if someone has antibodies in plasma that can be donated to someone else for treatment?
A serological antibody test can answer all those questions. There have been many small biotechnology companies developing these serological tests. In this post, I’ll outline a few of the ones I found online.
When someone first contracts an infection from a virus, there is first an asymptomatic phase of about one week before he or she has her first symptom like fever or cough. Generally, this coincides with increased virus replication and increased viral RNA, which can be identified through a polymerase chain reaction test. However, our body then starts to mount an immune response, first with IgM antibodies which last for months and then with IgG antibodies which last for years.
We do not know the exact length of time it takes someone to build these IgM/IgG antibodies. An early report from Guo et al, suggested it takes 5 days for IgM to be detected and 14 days for IgG to be detected after symptom onset. If this current SARS-CoV-2 virus is similar to SARS-CoV1, then we should expect it to take a few weeks after the onset of symptoms to build a robust immune response (Li et al. NEJM, 2003).
Serological Tests in Literature
There have been a few wonderful research reports detailing the application of serological tests.
For example, Amanat et al. developed an assay that detects antibodies to the spike protein of SARS-CoV2. Reagents, plasmids, and proteins can be requested from their website. More details of their protocols are available here.
Li et al. also developed a rapid IgM-IgG combined antibody test with a sensitivty of 88% and specificity of 90%.
There are now many companies building serological tests for detecting antibodies against COVID-19. Two that I found online are from Pinnacle BioLabs and BD. To be honest, these reports are a little confusing because they note the FDA approved their products, but no serologic tests were granted Emergency Use Authorization, as of March 31. More details here and here.
The FDA made the following recommendation for use of serological tests:
Serologic Testing Moving Forward
Different countries are using serologic testing for various purposes.
In Spain, serological tests were ordered from China, but withdrawn when discovered to have a 30% detection rate.
Germany will issue coronavirus antibody certificates to allow people to re-enter society based on their serology tests.
It seems like the CDC is developing serology tests to identify the full scope of the coronavirus outbreak. It is still unclear what is the United States strategy. I feel like we are still trying to figure out who is actively infected and recommending social distancing measures to make sure our healthcare system is not totally overwhelmed.
In my opinion, the biggest benefits of serology tests are for healthcare workers to allow them to return to work if they have developed immunity and to identify a cohort of people for which we can retrieve convalescent plasma.