COVID-19 Testing: Genetic vs. Immune Response
The need for more widespread testing of COVID-19 is paramount. We need accurate tests, and a large number of them to determine who is infected and shedding the virus to others. The test that accomplishes this is a genetic test. Usually, a patient’s nasal cavity and/or throat are swabbed. The sample is then added to chemicals that breakdown the envelope and capsid of the virus, exposing its ssRNA. Reverse transcriptase and subsequently many rounds of PCR are performed. The resulting solution is run with a specific SARS-CoV-19 visible tag. The visibility of the tagged confirms the presence of active, productive infection. The individual is currently infectious for COVID-19.
But with the unprecedented financial sequelae of this pandemic, it is in everyone’s best interest to get the world up and running again, as soon as safely possible. For this to happen, it is important to know not only who is infected, but also who was infected and has since recovered. According to an article from Medical News Today, molecular, or genetic, tests are not able to do this. As we know, anyone who has recovered is at very low risk for contracting the virus again. To identify these people, more and more serology tests are being performed to detect the presence of IgM and/or IgM in a patient’s blood. These antibody tests involved taking a small amount of the patient’s blood and essentially mixing it with a number of SARS-CoV-19 antigens. If the patient has developed an immune response as a result of COVID-19 infection, he or she will have IgM and/or IgG that form complexes with the viral antigens. These complexes are then run over a membrane with separate fixed anti-IgM and anti-IgG antibodies. These regions will essentially grab on to the patient’s antibody-antigen complexes.
Results
We know that IgM is the first produced antibody in the acquired immune response to any primary infection, including COVID-19. Later, however, IgM production wanes and IgG production ramps up and hangs around for a while. Thus, results to a COVID-19 blood test are interpreted as follows:
-Primarily IgM: This patient is in the early stage of infection. He or she is likely contagious but may or may not be experiencing the signs or symptoms yet.
-Both IgM and IgG: This person is in the middle stage of infection. He or she is likely showing the signs and symptoms of the disease. If this patient is not showing signs or symptoms by now, he or she is likely an asymptomatic carrier.
-Primarily IgG: This person has likely contracted COVID-19 but has since recovered. At this point, the person is not infectious and is unlikely to get sick from COVID-19 again.
It is important to note that these antibody test can give false positives. While, antibodies are incredibly specific in what they binds, cross reactivity can occur. There are many different types of Coronaviruses and many people have been infected with them before. Some of these other Coronavirus may have caused a similar enough antibody response which caused the false positive to COVID-19. According to Med Page Today, “False positive results for IgM and IgG antibodies may occur due to cross-reactivity from pre-existing antibodies or other possible causes.”
Implications
Antibody testing is important for several different reasons. First off, it is a method by which we can know if someone currently infected with the virus. However, antibody testing can tell us who can re-enter the world. Patients exhibiting only IgG reactivity are immune and non-contagious so they can be allowed to go back to work. This is incredibly important because the economic side effects of this pandemic are putting some people in a worse situation than they would be if they got sick with COVID-19.
Another very important feature of the antibody test is that it allows us to identify those who could be serum donors. Sometimes, patients who are severely ill with COVID-19 can receive components of the blood of those who have recovered. While this person, may or may not be producing their own antibodies, the donor’s antibodies can help keep the infection in check. This is called passive immunity. According to an article from Scientific American, “Patients with severe cases of COVID-19 may be able to be treated with blood plasma from people who have developed antibodies and are immune to the disease in what is effectively an antibody-transfer operation.”
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