I have been getting a lot of questions lately about the Coronavirus antibody test so I wanted to take the time to explain it here. If you have gotten tested for the antibodies and it is positive, that’s good news but experts aren’t sure exactly how good it is and here’s why.
If you have antibodies it means you have had a past infection with the coronavirus that causes COVID-19. Antibodies are proteins that help fight off infections and can provide protection against getting that disease again (you have immunity). Antibodies are disease specific. For example, measles antibodies will protect you from getting measles if you are exposed to it again, but they won’t protect you from getting other bacteria or viruses.
When fighting an infection your white blood cells can produce these proteins to build immunity. For every infection the body could make two different kinds of antibodies. There are IgM antibodies, which happen early in an infection and IgG antibodies, which are more likely to show up later. But the body doesn’t always make antibodies when fighting an infection. It is possible to have gotten infected with the coronavirus and not make antibodies.
Most people have IgG antibodies about 14 days after symptoms start. They usually stay in your blood long after the infection goes away, but we don’t know how long that is for the new coronavirus.
Across the US, Americans are anxious to learn whether they may have been infected with the coronavirus – which they hope would offer some immunity. In New York, researchers found that 19.3%, which is well below the estimated 67% needed to achieve community immunity.
But while the tests may be helpful in understanding how the virus moves through a population, researchers say much remains unknown: not all tests produce consistently accurate results, and scientists are still researching whether and for how long antibodies offer immunity from reinfection. The recommendation is to not change any social distancing or other practices based on an antibody test results, simply because we don’t know how long or even if there is protective immunity against reinfection if you’re antibody-positive. There are still too many unknowns, both about the accuracy of the antibody tests that are available and about the nature of the virus itself.
The World Health Organization, in late April, released a scientific brief that said, "There is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection." While studies of blood samples taken from people who have recovered from Covid-19 do show an immune response to the virus, some have "very low levels of neutralizing antibodies in their blood," WHO said. It's possible that immunity may need to occur on a cellular level as well as via antibodies circulating in the blood. "The body also makes T-cells that recognize and eliminate other cells infected with the virus. This is called cellular immunity," WHO explained. Along with antibodies, the "combined adaptive response may clear the virus from the body, and if the response is strong enough, may prevent progression to severe illness or reinfection by the same virus," WHO said.
Antibody tests should not be used to diagnose a current COVID-19 infection. An antibody test may not show if you have a current COVID-19 infection because it can take 1–3 weeks after infection for your body to make antibodies. To see if you are currently infected, you need a viral test. Viral tests identify the virus in samples from your respiratory system, such as a swab from the inside of your nose.
There are plenty of antibody tests floating around that haven't been reviewed or validated by the US Food and Drug Administration. According to the FDA, there are currently over 130 unvetted antibody tests in use, along with the dozens of diagnostic tests. The agency recently announced it was tightening its policy to keep unproven and even fraudulent tests from entering the market by requiring commercial test makers to meet new standards of accuracy and submit information proving testing quality. The FDA maintains a public website with the performance rates of the 12 EUA tests, showing how likely a certain test is to give false positive confirmations of the antibodies, and sensitivity, how likely the test will give a true positive result when someone has the virus. But the numbers can be hard to interpret by consumers who don’t have medical background
To date, the antibody test that seems to be the most accurate is made by Roche but that could be changing soon. For more info on the Roche test, https://www.roche.com/media/releases/med-cor-2020-05-03.htm
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