Key to bringing the pandemic under control is testing: to find out where the coronavirus is flaring up, how it’s spreading and where it has already been. There’s been a global rush to ramp up two categories of tests -- those that diagnose people who are currently infected and those that show whether someone has already fought the virus -- and deploy them in various approaches.
1. What kind of coronavirus tests exist?
There are two main types. The first, for diagnosing active infections, typically looks for the presence of the virus in your nose, throat or mouth. The second tests whether you were infected with the virus in the past. Known as an antibody test, this second variety looks for evidence, generally from a blood sample, that your immune system has built up antibodies to fight off the virus.
2. How do diagnostic tests work?
So-called molecular tests look for bits of the virus’s nucleic acids, or its genetic materials, in a sample of secretions. These tests generally require several components, including chemical reagents, small plastic containers, and swabs if the sample is from the nose or throat. Shortages of components, or a lack of trained lab technicians, have limited the test’s availability in many places. Newer versions, using saliva, eliminate the need for a swab, though some studies suggest this approach picks up fewer cases. Molecular tests typically rely on sophisticated equipment, including instruments that perform a process called reverse-transcription polymerase chain reaction, which amplifies the virus’s genetic material enough to study it in detail.
3. How long does it take for results?
There are rapid molecular tests that can churn out results in 30 minutes or less, but they don’t scale very well. Roche Holding AG.’s more-industrialized instruments take a few hours to produce results, but can perform thousands of tests a day -- in optimal situations. However, given the logistics of collecting samples and sending them to labs, many of which are overwhelmed, it’s very common in some places for people to wait several days to more than a week to get their results.
4. Is there a simpler way to diagnose?
Yes. Many public health officials are calling for wider adoption of rapid antigen diagnostic tests, especially in low- and middle-income countries that have relatively few complex laboratories. These tests can be about as simple as those for pregnancy. They detect the presence of viral proteins -- which make up most of the virus’s physical structure -- in someone’s nose or mouth. While typically less reliable than molecular tests, rapid antigen tests can be a lot cheaper, faster and easier to use, allowing for quick scaling-up. Used strategically and often enough, these tests could become powerful tools both in handling individual infections and in guiding public health measures.
5. Why is diagnostic testing so important?
If everyone in the world stopped interacting with everyone else for a couple of weeks, the coronavirus would die out because it wouldn’t be able to infect anyone new. 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 their recent, close contacts can be identified, quarantined and tested as well. This “contact tracing” slows the spread of the virus, since it lets people know when they’re likely to be spreaders, even if they don’t feel symptoms.
6. How are the tests deployed?
When molecular tests are in very short supply, they should be used above all on people showing symptoms of a coronavirus infection and their close contacts, officials say. Beyond that, the tests can be used to conduct so-called surveillance testing -- when a sample of a community is tested to monitor the state of the outbreak in that population. One way to make tests go further is to use pool testing, in which samples are grouped into one batch and subjected to a single test. If it comes up negative, no further action is taken; only if it’s positive is each person individually tested. This method makes sense only if fewer than about 5% of people in a population are infected at a given time. Otherwise, too many pools test positive, creating even more work.
7. How accurate are diagnostic tests?
While no medical test is perfect, some of the biggest makers of coronavirus tests -- including Roche and Abbott Laboratories -- say theirs are nearly 100% reliable when properly used. But human error is always a possibility when getting a good sample from someone’s nose or throat, keeping it at the right temperature for perhaps a couple of days and using chemicals to extract the virus’s genetic material properly. Also, a false negative is possible if the patient is tested too early or too late in their infection. Early in the pandemic, some rushed-to-market molecular tests proved unreliable. 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, versus only 59% using molecular tests. More recently, independent organizations have confirmed the reliability of plenty of molecular tests.
8. How do antibody tests work?
A person infected by a virus summons antibodies specifically designed, like keys for a particular door, to latch onto and neutralize the proteins that form the bulk of the virus. Depending on the virus, these antibodies can linger for months or even years after you recuperate, in case you encounter the virus again. To make antibody tests, developers either grow in a lab -- or buy from a supplier -- copies of proteins that appear naturally on a particular virus. If a person has already fought off the virus, their blood should contain antibodies that will latch onto these proteins that are in the test. For these tests to be useful, they need to be highly specific, so a positive result isn’t triggered by similarly shaped antibodies that once fought off, say, the coronavirus that causes the common cold.
9. Why are antibody tests important?
For policy makers, antibody tests are a powerful tool to understand how widely the virus has spread in a region, which public health measures are working and which aren’t. Such surveillance testing found that in the U.K. as of July, an estimated 7.1% of people had coronavirus antibodies. The figure was measured at 23% in New York City in April. That knowledge could be useful once vaccines are widely available, since it’s believed that 55% to 82% of people in a community will need to have protection before so-called herd immunity can kick in -- the point at which infections eventually stop because so few people are vulnerable.
10. What are the challenges?
It’s still unknown how strong immunity is for people infected with the coronavirus, and early evidence suggests that antibody levels fall quickly. In contrast, 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. Yet neither of those studies demonstrated whether the presence of antibodies was protection against re-infection. When it comes to this pandemic, it isn’t clear what level of antibodies is needed for someone to be protected against Covid-19 again either. Nor is it clear how much T cells, another weapon in the immune system, can provide protection even if antibody levels drop.
11. Are antibody tests reliable?
While many of the earliest antibody tests were essentially worthless, raising big doubts about the technology, there are now plenty that are considered highly reliable. For any such test, a crucial task is to ensure that very few individuals receive a false positive result for coronavirus antibodies, since that might lead them to lower their guard on social distancing.
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