The United Kingdom has ordered 3.5
million antibody tests, which would show whether someone has been exposed to COVID-19.
Such tests, which just take a drop of blood, could help reveal people who have
been exposed to the virus and are now likely immune, meaning they could go back
to work and resume their normal lives.

Antibodies are proteins that the body’s
white blood cells produce to fight an infection. They bind to a virus,
preventing it from infecting a cell, and can remain in blood long after the
infection clears. Antibody tests are commonly used to test for exposure to
other viruses.

Science News
spoke with David Weiner, director of the Vaccine and Immunotherapy Center at the
Wistar Institute in Philadelphia, and Charles Cairns, dean of the Drexel University
College of Medicine, about how antibody tests work and what are some of the
challenges of developing the tests.

The
following responses have been edited for brevity and clarity.

SN: What does
an antibody test do?  

Cairns:  Antibody tests look to see if someone has
been exposed to a specific antigen, like a virus. The British tests are designed to work in one of two ways. They
either detect human antibodies in blood using an antigen designed to be similar
to a feature of the virus. Or conversely, the test detects the virus in blood
using a [human-made] antibody designed to trap the virus.

SN: How is it
different from tests that diagnose an infection?

Cairns:  Diagnostic tests are
using RT-PCR tests (SN:
3/6/20
). You take a nasal swab (or sputum sample) that identifies the
specific viral RNA from the COVID-19 virus. 
It’s the gold standard to see if you are actively infected.

The
antibody tests are quick — a prick of blood and you get a yes/no answer. You’ve
had COVID-19 or you haven’t.

SN: Why do antibody
tests matter?

Weiner:  People
who have recovered won’t have RT-PCR positive tests, as they’ve already cleared
the virus. Those who are recovered, those antibodies protect them from
reinfection. (It’s still unclear, however, how long that immunity might last.)

We’re now screening to find people who are positive [and have a current infection]. But it’s important to find people who have recovered and who are unlikely to be reinfected, so they can go out and be the buffers for the rest of us. That’s how herd immunity develops (SN: 3/24/20).  Antibody tests also give us a better idea of how many people have been infected and how many have recovered, so we can start to move forward.

SN: Who
should be prioritized for getting this test?

Weiner: I think the first responders and health care workers
should be screened first, because it’s very important for them [to feel
confident] to get back out on the front line.  

SN: What is
the status of providing widespread antibody testing in the United States?

Cairns: There are lots of people working on [antibody] tests [in
the United States]. But you’re trying to replicate a gold standard — the RT-PCR
test — with a test that uses smaller amounts of sample, is faster and is available
outside the lab. 

The big question is: Does a positive response for the antibodies mean that person is actively infected, or that they have been infected in the past? The tests need to be accurate, and avoid both false positives and false negatives. That’s the challenge.