SARS-CoV-2, the virus that causes COVID-19, has been called an “invisible enemy.” It has spread through communities and across the globe, thanks in part to its ability to shed off of infected individuals for days before and after they experience symptoms, and even off of people who do not experience symptoms at all. While we cannot see the virus, molecular biology can make this invisible threat visible through diagnostic testing. In the face of the global pandemic, biopharmaceutical companies are rapidly developing, distributing and processing diagnostic tests to meet the urgent clinical need.
Widespread testing for COVID-19 is a vital component of fighting the pandemic. Testing allows infected individuals, and anyone who may have been exposed to them, to be isolated from the rest of the population. Aggressive testing and quarantining of the infected is how nations such as South Korea and Singapore contained the spread of the virus early on. Testing is also necessary in order to understand the epidemiology of the disease. It is impossible to know key features like the true transmission rate and fatality rate of COVID-19 without knowing how much of the population actually has the virus.
There are two main ways of testing for the virus: either directly by detecting the presence of viral RNA, or indirectly by detecting antibodies that a patient’s immune system has generated to fight the virus. Testing for RNA is acutely important because it reveals who has an ongoing infection, while antibodies may be detectable only late in the course of an infection or after it has resolved. However, each type of test has an important role to play in the public health response, and each is a focus of intense research and manufacturing by the biopharmaceutical industry. Tests for COVID-19 are built on technologies already used to test for other diseases, so companies with relevant expertise are quickly adapting their resources for detection of this new virus.
Testing for viral RNA is done through a common molecular biology technique called real-time reverse transcription polymerase chain reaction (rRT-PCR, also called “quantitative” or qRT-PCR). Samples can be taken by inserting a nasopharyngeal swab into a patient’s nose and collecting fluid where the nasal passage meets the throat. The sample is physically and chemically processed to separate out any RNA molecules. The isolated RNA is then reverse transcribed into DNA, using a type of protein called reverse transcriptase (which was itself originally discovered in RNA viruses). DNA can be rapidly replicated in a tube using a type of protein called DNA polymerase, employing the same basic molecular mechanism that allows any living cell to copy its DNA. Importantly, this process involves designating only a small region of interest in the DNA to amplify, allowing for the identification of specific sequences that only come from SARS-CoV-2. Tests designed by different manufacturers target different regions of the SARS-CoV-2 genome, leading to diversity in the effectiveness of the tests. Finally, the amount of amplified DNA — a readout for the presence of viral RNA in the original sample — is quantified using fluorescent probes.
PCR tests for SARS-CoV-2 have been independently developed in many affected countries and are being created and distributed for use in the U.S. by technology and health care companies including Quest Diagnostics, LabCorp, Thermo Fisher, Roche Diagnostics, Hologic and Cepheid. One notable test for SARS-CoV-2 RNA is Abbott’s ID NOW COVID-19 test, which runs on existing equipment used to detect pathogens including the influenza virus and streptococcus bacteria. It can return test results for SARS-CoV-2 in as few as five minutes using an isothermal process in which all biochemical reactions take place at a single temperature as opposed to the repeated heating and cooling cycles of conventional PCR.
SARS-CoV-2 tests that run on equipment such as ID NOW, or Mesa Biotech’s Accula, are designed for point-of-care diagnosis in clinical settings and test one sample at a time. Other tests, such as Roche’s, require more specialized laboratories but can test hundreds of samples at once. A combination of large-scale, centralized laboratory testing and small-scale, rapid clinical testing will ultimately be used to meet existing needs.
Tests for SARS-CoV-2 antibodies are quite different but are also quickly being developed. They too are based on well-established molecular biology techniques. A test called the enzyme-linked immunosorbent assay (ELISA) is commonly used to identify proteins of interest — in this case the antibodies against SARS-CoV-2. Antibodies are proteins created by the immune system to identify and combat invading pathogens, with structurally distinct antibodies produced to identify distinct targets, such as proteins on the surface of SARS-CoV-2. Antibodies are generated over the course of an infection as the pathogen is identified and fought off by the immune system, and they continue to circulate in the blood afterward, offering some degree of immunity to that specific pathogen. Early response antibodies of a type called IgM are detectable a few days after infection, while IgG-type antibodies are detectable later and provide a longer-term form of immunity. This means that a person with IgM antibodies against SARS-CoV-2 likely has an ongoing or recently resolved infection, whereas IgG antibodies indicate a past infection.
So far in the United States, the FDA has authorized one antibody test: a rapid diagnostic test by Cellex Inc. Rapid diagnostic tests are simplified versions of ELISAs that give fast results but are less precise. FDA granted Cellex an emergency use authorization for its antibody test. The FDA also issued a policy allowing companies to begin to distribute and use their own antibody tests without prior FDA review. All of the antibody tests are required to state that they cannot be used as the basis of confirming or ruling out an infection, unlike RNA tests for the virus.
Though testing for antibodies against SARS-CoV-2 is in its early stages, it will have a pivotal role to play in combating the pandemic. Identifying people who have contracted COVID-19 and may have been unaware due to having no or mild symptoms will help establish which individuals are protected and who is still vulnerable. And isolating effective antibodies against SARS-CoV-2 may be helpful therapeutically, both in terms of providing guidance in the production of antibody-based treatments and for direct use in convalescent plasma transfusion.
For the time being, tests for COVID-19 are still extremely limited. But testing in the United States has ramped up, from a total of 4,000 people in the U.S. having been tested by the end of February, to over 1 million a month later, to over 2 million total tests today. Increasing testing capacity continues to be a priority for the industry. Abbott, for example, is now producing 50,000 of its widely anticipated ID NOW COVID-19 tests per day and increasing that capacity. And the development and deployment of antibody tests are accelerating as we enter a new phase of the pandemic in which measuring immunity will be crucial.
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