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All About Covid Tests and How Omicron Impacts Them

A One Medical Group Inc. nurse practitioner places a swab inside a test tube after swabbing a patient at a Covid-19 testing center at Medgar Evers Collage in the Brooklyn borough of New York, U.S., on Monday, April 20, 2020. New York Governor Andrew Cuomo said data suggest the state outbreak has begun a descent, with deaths falling for a sixth straight day. Photographer: Michael Nagle/Bloomberg
A One Medical Group Inc. nurse practitioner places a swab inside a test tube after swabbing a patient at a Covid-19 testing center at Medgar Evers Collage in the Brooklyn borough of New York, U.S., on Monday, April 20, 2020. New York Governor Andrew Cuomo said data suggest the state outbreak has begun a descent, with deaths falling for a sixth straight day. Photographer: Michael Nagle/Bloomberg (Bloomberg)
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The rise of the omicron variant of the coronavirus has refocused attention on the importance of testing as a tool for bringing the pandemic under control -- and keeping it that way. At the same time, the rapid tests that are widely used to identify infections may be less accurate detecting omicron cases, the U.S. Food and Drug Administration said Dec. 28. The terminology around coronavirus testing can be confusing. Here’s help sorting it out.

1. How do tests detect active infections?

Typically, they look for the presence of the coronavirus in your nose, throat or mouth. There are two basic types: molecular tests and antigen tests.

2. What are molecular tests? 

They are the gold standard for diagnostic testing -- at least in terms of accuracy. They work by looking for bits of the virus’s nucleic acids, or its genetic material, in a sample of secretions. 

3. What are the downsides?

Molecular tests typically require several components, including chemical reagents, small plastic containers and swabs. One widely used type relies on trained lab technicians to perform a complicated process called reverse transcription polymerase chain reaction (RT-PCR), which amplifies the virus’s genetic material so that it can be studied in detail. A lack of components, machinery or technicians can limit the availability of these tests. Testing saliva eliminates the need for a nasal or throat swab, though some studies suggest this approach picks up fewer cases. While some molecular tests can churn out results in 30 minutes or less, they don’t scale very well. Thus, countries are increasingly embracing antigen tests. 

4. What are antigen tests? 

They are faster, cheaper and more accessible tests, some of which people can perform at home. Often, these are lateral flow tools, which are similar to home pregnancy tests that run a liquid sample over a reactive surface. A kit, which can cost less than $5, includes a swab, a tiny bit of solution, and a small plastic tray containing antibodies resembling those that a person’s immune system would activate to fight off the coronavirus. The test-taker gets a sample from his or her nose or throat, dips it in the solution, and puts a few drops in the tray. If the person is infected, the antibodies will bind to proteins in the virus that are present in the sample. This will prompt a line to appear on the plastic tray within about 15 minutes -- or some other indication, depending on the test -- signifying an infection. As part of a strategy to confront a resurgence of new infections announced by President Joe Biden Dec. 21, the U.S. plans to distribute as many as 500 million at-home tests for free to Americans who request them.

5. How accurate are rapid antigen tests?

A review of 64 studies by the Cochrane Library, a medical database, published in March found antigen tests on average correctly identified 72% of people who had symptoms as being infected with the coronavirus. In people without symptoms, on average, the antigen tests correctly identified 58% of those who were infected. An analysis by the U.K. government published in May found fewer than one false positive in every 1,000 tests carried out. Antigen tests are generally less likely to pick up very early infections than molecular tests. According to an initial evaluation by the U.K. published Dec. 17, rapid antigen tests were as effective detecting omicron infections as for the previous strains. But the FDA said Dec. 28 that early data suggest that while they do detect the omicron variant, they may have reduced sensitivity.

6. What if I get a negative result with an antigen test? 

If you’re experiencing symptoms of Covid or have a high likelihood of infection due to exposure to the coronavirus, the FDA recommends following up with a molecular test. 

7. Has omicron impacted molecular tests?

Yes. The FDA has notified health care professionals of a handful of molecular tests that, in the absence of modifications, are expected to fail to detect the variant, as well as those that were modified in order to pick it up. 

8. When should I get tested?

The U.S. Centers for Disease Control and Prevention recommends testing, without delay, for anyone with Covid symptoms. It says those who are vaccinated and have come into close contact with someone with Covid are urged to test 5 to 7 days after their exposure; those who aren’t fully vaccinated should test immediately and if the result is negative, again 5 to 7 days later. If you’re using a home test to check your status before a social event, Irene Petersen, a professor of epidemiology at University College London, recommends taking the test right before leaving home. 

9. How do we keep track of variants?

The best way to identify new variants of concern and monitor their spread is to take samples from infected people and sequence the genome of the entire virus or at least the spike protein that gives it its crown-like appearance. That, however, can be expensive and time-consuming. As a shortcut, researchers use other techniques, such as subjecting samples to molecular tests that have been slightly tweaked to identify specific mutations. That helps get a faster and fuller picture of what virus strains are fueling a region’s outbreak, though it’s important for researchers to confirm some of those results with genetic sequencing. 

10. What other type of coronavirus tests exist? 

Another type, generally a blood test, detects whether you have any immunity to the virus, either from a previous infection or from a vaccination. A person infected by a virus or vaccinated against it summons antibodies designed specifically, like keys for a particular door, to latch onto and neutralize the proteins that form the bulk of that particular virus. These antibodies can linger for months -- or even years in some cases. 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 or been inoculated against it, their blood should contain antibodies that will latch onto these proteins in the test. To be useful, the tests need to be highly specific, so a positive result isn’t triggered by similarly shaped antibodies that once fought off, say, a related but different coronavirus that causes the common cold.

11. Why are antibody tests important?

To end the pandemic, we need to build up our collective immunity. It’s unclear how long protection against reinfection lasts for people who’ve recovered from a coronavirus infection or how long the immunity provided by Covid vaccines will last, especially in the face of new viral variants. While the earliest coronavirus antibody tests indicated whether someone had any protection against the virus, more recent versions actually quantify the level of antibodies -- and, used repeatedly, can show if levels are dropping. That could help determine who needs to receive a booster shot of a vaccine at some point. For scientists and policy makers, antibody tests are also a powerful tool to understand how widely the virus has spread in a region, which can shine light on which public health measures have worked and which haven’t. And for individuals who never tested positive for the virus but think they may have had it, these tests provide a way to look back to see if they may have some protection -- even if they haven’t been vaccinated.

12. What are the limitations?

Scientists are still investigating what level of antibodies is needed for someone to be protected against future coronavirus infection -- a factor that could play into how often people receive booster shots. Nor is it clear how much T cells, another weapon in the immune system, can provide protection if antibody levels drop. But that could begin to change, as tests designed to detect T cells against the coronavirus become more widely available.

(Updates with FDA caution on omicron’s effect on antigen tests)

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