What We’ve Learned about COVID-19 Tests As We Enter The Fall Season

COVID test

The asymptomatic problem

The structure of SARS-CoV-2 allows it to stick to human cells with extreme efficiency. This makes COVID-19 more contagious than other coronaviruses and therefore hard to contain. COVID-19 has an incubation time of up to 17 days during which infected people do not show symptoms. Combine this with the ~40% of people who never develop symptoms at all (asymptomatics) and you can easily see how predominant the asymptomatic issue is and promises to be as we reopen the country. Yet, the current official safety guidelines are still largely based on detecting symptoms in an attempt to prevent contaminated people from infecting others. This approach has uncovered a major challenge for containing the spread of the virus, as businesses and schools are reopening and some states are allowing long-term care patients to have visitors.

asymptomatic spreader jpgThe rapid uptick in outbreaks that started just days after schools and summer camps reopened under strict guidelines has proven how crucial it is to be able to identify people who are not displaying symptoms such as fever and cough. Access to accurate, convenient and rapid testing is key to eliminating the silent spread of COVID-19 by asymptomatic viral carriers. One of the FDA’s roles is to determine whether the tests developed for use in the U.S. provide sufficiently accurate and reliable results and to help to provide timely access to such tests. As of October 7 2020, the FDA has approved 165 molecular assays for detecting SARS-CoV-2. ~90% of those tests utilize technologies based on the amplification of the virus’ genetic material (called PCR or RT-PCR). Other tests use techniques which despite enabling faster detection, often prove less reliable and less scalable than RT-PCR.


Making sense of COVID tests

There are two categories of tests that can enable the detection of asymptomatic carriers of COVID-19: diagnostic tests performed on individuals who do not display symptoms, and environmental tests.

Diagnostic tests. The FDA has been approving two types of tests for diagnosing patients who are currently infected by COVID-19: molecular and antigen tests. Both types of tests look for parts of the virus itself, so they are referred to as “viral tests”. Most viral tests are done on samples of cells and fluids from your respiratory system.

Molecular tests. They detect specific parts of the genetic material of the virus. A healthcare worker collects fluid from a nasal or throat swab or from saliva and the sample is mailed to a lab for testing. Results may be available in minutes if analyzed on-site or one to two days if sent to an outside lab. Molecular tests are considered very accurate when properly performed by a healthcare professional, but the rapid tests appear to miss some cases (higher false negatives). The FDA also approved certain COVID-19 at-home nasal swab or saliva test kits, which are available only with doctor approval. As of April 13, 2020 the FDA approved the use of saliva samples as a means of emergency testing for COVID-19. This test is less invasive than taking a nasopharyngeal sample and involves giving a saliva sample in a tube. The saliva tests remain scarcely available and are therefore less commonly used than the nasal swabs. However saliva tests are a powerful tool to decreasing the risk of contamination from healthcare professionals who previously had to come into close contact with patients in order to administer a test. “Though rapid saliva-based antigen test has shown a 30% false negative rate and does a poor job of diagnosing infection, it’s “likely the better tool for judging infectiousness” say researchers Drs. A. David Paltiel, Professor of Public Health at Yale School of Medicine, and Rochelle Walensky, M.D., Professor of Medicine at Harvard Medical School and a practicing Infectious Disease physician at Massachusetts General Hospital. Importantly, the FDA advises anyone who tests negative with the saliva-based test to confirm the results with a second testing method.

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Antigen tests. Those diagnostic tests emerged more recently. Instead of genetic material, they detect proteins that are part of the virus’s structure. Antigen tests can produce results in minutes. These tests are faster and less expensive than molecular tests, making them easier to use for large groups of people. A positive antigen test result is considered very accurate. However false negative results are more common, meaning that antigen tests are not as sensitive as molecular tests. To account for this, doctors may recommend an additional molecular test to confirm a negative antigen test result, which will add to the cost and timeline of the diagnosis. The authorized antigen tests produce results in about 15 minutes. Compared to molecular tests, antigen tests are useful due to their speed but are not as sensitive, meaning that there is a greater chance of a false negative result. However, they are very specific for the virus so positive results tend to be highly accurate. This means that if your antigen test comes back positive, it is very likely that you have the virus. If your test comes back negative, you’ll likely want to confirm your negative result using a molecular test.

Antibody tests. Antibody or serology tests have only recently appeared on the market. They are designed to determine whether an individual had a past infection. Antibody tests are done on blood samples, referred to as “serology tests”. It is important to note that the antibodies for SARS-CoV-2 may not be detectable for 1 to 3 weeks after the infection; and testing too early, when the immune response is still building up in the body, may result in a negative test, which is why the CDC and FDA do not recommend those kits for diagnosis purposes. Experts don’t fully know what the presence of COVID-19 antibodies means, how much protection the antibodies might provide or how long this protection might last. Importantly, previous exposure to COVID-19 does not necessarily protect from re-infection, as a recent study in The Lancet shows. In fact, in some cases, the second COVID-19 infection may lead to more severe symptoms and death.


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