The power of the coronavirus is not that it’s particularly lethal or especially easy to catch from someone else, but rather that it’s stealthy. Infected people can pass it on to others before they realize they’re sick, or even if they never develop symptoms at all, making them unwitting spreaders. The defense against the virus’s insidiousness is thorough testing, to reveal where it is and might be in order to stop its spread, and to determine where it has been, which may help restart economies.

1. What kind of coronavirus tests exist?

There are two main types. The first tests whether you are actively infected with the virus. Known as an antigen test, it looks for the presence of the virus, typically in your nose or throat. The second type tests whether you were infected with the virus and beat it. Known as an antibody test, it looks for evidence, generally from a blood sample, that your immune system has built up antibodies to fight off the virus.

2. How do antigen tests work?

While there are a couple of different types, most people are familiar with so-called molecular tests. These became available for the novel coronavirus in the middle of January after Chinese researchers published the genome sequence of the virus. A molecular test looks for bits of the virus’s nucleic acids, or its genetic materials, in a sample of secretions from the nose or sputum from the throat. The test generally requires several components, from swabs to chemical reagents to small plastic containers -- shortages of which have led to a shortfall of tests in many places, including the U.S. and the U.K. It also relies on trained lab technicians and sophisticated equipment, including instruments that perform a process called reverse-transcription polymerase chain reaction, a method to amplify the virus’s genetic material enough to study it in detail.

3. Is there an easier way?

Yes. Another type of antigen test that is just starting to appear looks for bits of the virus’s protein armor in a sample from a person’s respiratory tract. This technique is more difficult to get right initially but could offer a way to test people much more quickly and cheaply. No one has yet shown that a do-it-yourself test for coronavirus, akin to a home pregnancy test, is effective. U.S. regulators in mid-April gave emergency authorization to a test that people take at home but have to send to a lab for processing.

4. Why are antigen tests important?

If everyone in the world stopped interacting with anybody else for two weeks, the coronavirus would die out because it wouldn’t be able to infect anyone new. But since that’s not possible, the goal is to reduce spread from person to person as much as possible. To do that effectively, it’s necessary to know who is infected, so that they can be quarantined and so that all their recent, close contacts can be identified, tested and quarantined as well. This “contact tracing” slows the spread of the virus, since it lets people know when they’re most likely to be spreaders, even if they don’t feel symptoms.

5. How accurate are antigen tests?

They’re certainly imperfect. Tests have been rolled out quickly, without undergoing rigorous validation for accuracy. In a study published in February of 1,014 suspected coronavirus cases in a hospital in Wuhan, China, 88% tested positive using CT scans of the chest whereas only 59% did so using antigen tests. Factors potentially undermining accuracy include a shortage of supplies and material for the tests, long incubation times for the infection and the challenge of getting an adequate sample from a patient.

6. How do antibody tests work?

When a virus infects you, your body summons antibodies specifically designed to latch onto and neutralize the proteins that form the bulk of the virus itself -- like keys designed to unlock a particular door. Those antibodies typically linger for months or years after you recuperate, in case you encounter the virus again. To make the tests, developers either grow in a lab -- or buy from a supplier -- copies of a particular virus’s proteins. The proteins for the coronavirus can flag whether a blood sample contains antibodies designed to fight the virus. To work well, the tests have to be highly specific, so they don’t say “positive” when, in fact, they have found similarly shaped antibodies that once fought off, say, a different coronavirus that causes the common cold.

7. Why are antibody tests important?

In theory, those who have defeated the coronavirus and have built up immunity should be free to re-enter society for some period of time with little or no risk of getting sick with it again or spreading the virus. There are growing calls for antibody tests to be rolled out on a wide scale to identify these individuals and let them get back to work to open up the economy as much as possible.

8. What are the challenges?

It’s unknown how strong immunity will be for different people infected with the coronavirus and how long it will last. A 2007 study showed antibodies to the coronavirus responsible for severe acute respiratory syndrome (SARS) appear to dissipate after an average of two years. Another report found those generated by infection with the coronavirus behind Middle East respiratory syndrome (MERS) tend to stick around a bit longer, but not in everyone. And neither study demonstrated whether the presence of antibodies was protection against re-infection.

9. Are antibody tests reliable?

That’s another caveat. Antibody tests for the novel coronavirus are too new to have been proven reliable. A danger is that individuals who receive a false positive for coronavirus antibodies and stop social distancing will become infected and carry the virus back into their households.

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©2020 Bloomberg L.P.