Antigens and antibodies – Testing for COVID-19

The different tests for Covid-19 belong to one of two categories: tests detecting ongoing infection or tests detecting if a person has previously been infected.
The different tests for Covid-19 belong to one of two categories: tests detecting ongoing infection or tests detecting if a person has previously been infected.

Testing programmes have throughout the Covid-19 pandemic been a widely debated topic. What almost all discussions have in common is the conclusion – testing programmes are vital to successfully deal with the pandemic. There are different paths that lead to the same destination: screening for Covid-19.

PCR and antibody testing have throughout the pandemic been the most widely used screening methods. These complement each other since one test for ongoing infections and the other detects if a person has already been infected with the SARS-CoV-2 virus. As the crisis continues to unfold researchers are looking into alternative ways to screen for the virus that has turned the world upside down. As more alternatives appear, the more complex the testing landscape for Covid-19 becomes.

This article explains the basics of antibody tests, which has been commonly used during the Corona pandemic, and a more recent alternative – the antigen tests.

Definition of antigens

The different tests for Covid-19 belong to one of two categories: tests detecting ongoing infection or tests detecting if a person has previously been infected. An antigen test belongs to the first testing category – those that can tell if a patient is currently infected with Covid-19 or not.

Antigens are part of the virus structure and are detectable when the virus is still present in the blood. It should therefore be taken within the first days after the outbreak of symptoms. An antigen is a molecule capable of stimulating an immune response. It could be proteins, nucleic acids, lipids or polysaccharides. Regardless of which, they all have a specific surface known by the immune system.

For the SARS-CoV-2 virus causing Covid-19 several antigens are known. The antigen test distributed by Noviral aims for the nucleocapsid protein, others detect the presence or absence of spike glycoprotein. Once the immune system recognizes the presence of such molecules, it initiates the production of antibodies.

Antigen testing for Covid-19

Antigen tests usually take longer to develop than its alternative – the PCR tests. This is due to the fact that antigen tests need suitable antibodies to be identified and produced in order to make the assay. This process can take long and explains why antigen tests have not been as widely used as the PCR tests throughout the pandemic. Researchers do however expect the testing landscape for Covid-19 to change as the antigen tests become widely available. They believe so as the antigen tests have several advantages to PCR tests. Among them are that antigen tests can provide test results much quicker, does not require any lab capacity and are cheaper to perform.

As more and more manufacturers and distributors offer antigen tests, we are likely to see a shift from the logistically demanding PCR tests to easy-to-use antigen tests.

Definition of antibodies

Antibody testing belongs to the second category of Covid-19 tests: they tell whether a person has already been infected by recognizing the presence or absence of specific SARS-CoV-2 antibodies. Different tests test for different antibodies and the one provided by Noviral can detect both the early developed IgM antibody and the later produced IgG antibody. A positive result for IgM but a negative result for IgG can therefore imply that the patient was recently infected and have not yet initiated the production of IgG antibodies.

In contrast to antigens which are part of the virus, antibodies are part of the immune response. The process of producing antibodies is initiated as the body recognizes the presence of antigen. The Y-shaped antibodies, which is equipped with antigen binding sites on the tip of each arm, has the task of eliminating the virus. It performs its task by binding a specific part of the antigens surface to its arms. This binding process can be harnessed to develop antibody and antigen diagnostic tests.

Antibody testing for Covid-19

Antibody testing is usually performed using whole blood, serum or plasma. These tests can be laboratory-based or point-of-care based but do in general detect the absence or presence of SARS-CoV-2 specific antibodies.

An antibody test should be taken at least 14 days after the first outbreak of symptoms to provide best value. Research indicates that the first developed IgM antibody response peaks after two weeks (14 days) and the then developed IgG antibody peaks after three weeks (21 days).

If a person believes he or she has been infected with Covid-19 in the past, antibody testing provides valuable information. In contrast to PCR and antigen tests, which are great in detecting ongoing infections in patients experiencing symptoms, antibody testing can also detect asymptomatic patients. On a larger scale, the antibody test results can help track the spread and determine how many people have already been infected – thereby telling us how likely herd immunity is.

The test results provided by an antibody test can further help prioritize whom can return to work or whom to first give a potential vaccine to. Therefore, antibody testing is valuable for both individuals and larger groups.

Difference between antibody tests and antigen tests

The testing methods for Covid-19 are many and all together, they form a complex landscape hard for most of us to navigate. In this article we have deep dived into two tests – the antigen and antibody test. By comparing these two it becomes clear that the purpose of the testing programme needs to be identified previous to deciding what tests to use. In most cases, more than one test needs to be included in order to form a meaningful and diverse screening programme.

The differences between an antigen and an antibody test are many. The most appearing difference is the stage in which each test should be used – antigen tests should be taken within 5 days after the first outbreak of symptoms and antibody tests after at least 14 days. This fact implies that they together could form a comprehensive and meaningful combination of tests for companies, groups or a society wanting to screen for Covid-19. We’ll explain why:

If a patient believes he or she is currently infected with Covid-19 an antigen test could quickly and with high accuracy provide valuable information. If a patient however did not have access to a diagnostic test (PCR or antigen tests) when he or she experienced symptoms, an antibody test can confirm or reject if the symptoms were caused by SARS-CoV-2.

Further, a disturbing aspect with Covid-19 is that infection carriers don’t necessarily experience any symptoms. While antigen testing can’t determine if a person is currently infected unless the patient experiences symptoms, an antibody test can identify if an asymptomatic person has been infected in the past. The antibody test is therefore valuable in mapping out the spread of the disease even if it can’t eliminate the risk of asymptomatic persons infecting others as he or she is actively carrying the virus.

Another difference between the tests is that the test results provide different information. While an antigen test result facilitates patient treatment decisions, an antibody test result informs how much risk a patient is exposed to. On a larger scale the antibody results can tell whether herd immunity is likely to put in or not.

The fact that antibody and antigen have different areas of use implies that they should not be considered substitutes but rather complementarities. Together, they could form a comprehensive testing programme useful for different levels of the society.

Noviral is happy to help sorting out the complex Covid-19 testing landscape and is available for questions at info@noviral.se.