SARS-CoV-2 coronavirus – what types of antibody tests are there?

SARS-CoV-2 coronavirus - what types of antibody tests are there?

Despite the development of a vaccine against SARS-CoV-2, the pandemic is not going away and there is still a high demand for testing patients. Molecular tests based on reverse transcription polymerase chain reaction (RT-PCR) are tests recommended by the World Health Organization (WHO) that can directly confirm active coronavirus infection. Another approach is to evaluate the concentration in the blood of antibodies directed against the SARS-CoV-2 coronavirus.

What are antibodies and how are they formed?

Antibodies are immune proteins produced by cells of the immune system after contact with agents that pose a health risk, such as SARS-CoV-2. It is worth knowing that there is a whole pool of different antibodies in our body, which add up to antibodies produced against specific proteins of viruses, bacteria, parasites and even food particles. Therefore, each antibody is said to be produced specifically against a specific agent, such as SARS-CoV-2 proteins. The main task of antibodies is to inactivate the molecules against which they are produced.

Specific antibodies are also produced in response to vaccination and can then be an indicator of its effectiveness.

What are the types of antibodies?

There are several types of antibodies, or classes, in the body. Among them, they differ in structure, concentration in body fluids (e.g., blood, saliva), biological properties, among others. There are the following classes of antibodies:

  • IgG
  • IgM
  • IgA
  • IgE
  • IgD

The first three classes are used in the laboratory diagnosis of SARS-CoV-2.

Each antibody is specifically produced against a specific fragment of the coronavirus protein. The most common are the spike protein (abbreviated S from the word spike) and the nucleocapsid protein (abbreviated N from the word nucleocapsid). Both the S protein and the N protein induce an intense immune response against SARS-CoV-2. For this reason, available diagnostic tests examine specific IgM, IgG and IgA antibodies specifically against these coronavirus proteins.

When and in whom to perform the antibody test?

Testing antibodies to SARS-CoV-2 will answer the question of whether our immune system has been in contact with the virus. Whether we had typical symptoms of infection (such as cough and fever) or were asymptomatic. Coronavirus antibody testing can be performed at any time in people without symptoms of COVID-19. This will allow us to confirm a history of coronavirus infection and whether we are recovered and can be plasma donors for COVID-19 patients. Note that antibodies can also be detected in people who are infected with SARS-CoV-2 at the time of testing. Their negative result does not exclude a pending infection.

Antibody testing can also be performed by people who have recently had a flu-like infection, a cold, or have had contact with a person infected with SARS-CoV-2 and have not had a molecular test performed. For this purpose, it is best to perform an antibody test approximately 7-21 days after the first symptoms indicating infection or contact with an infected person.

In addition, since a vaccine against SARS-CoV-2 is available, it is possible to check whether the body has produced adequate levels of protective antibodies. And it is also possible to distinguish whether the antibodies were formed as a result of contact with the coronavirus or under the influence of vaccination.

Remember, however, that the antibody level test should not be performed in people with symptoms to confirm or exclude SARS-CoV-2 infection, because the antibody test will not clearly answer the question of whether you are currently infected. For this purpose, an antigen or molecular test should be performed.

What does the antibody test involve?

To identify antibodies to SARS-CoV-2, so-called serological methods are used, which indirectly measure the body’s response to infection. The most common tests used in medical laboratories are based on the immunoenzymatic method ELISA and its variants (e.g. CLIA), which allow the capture of specific antibodies against coronavirus from the patient’s blood. For the test, venous blood is drawn from the elbow flexure, and the result is available within 24-48 hours.

So-called rapid cassette tests, which are usually not tests performed in a laboratory, but at medical outlets, use immunochromatographic methods. The principle of cassette tests resembles pregnancy tests, which are widely available in pharmacies. The test requires a few drops of blood taken from the finger. The result is usually obtained within several minutes after the blood is drawn.

Read: Coronavirus antibody tests – what are the differences and which ones are worth taking?

How to interpret the result of an antibody test?

To correctly interpret the result of an antibody test against the SARS-CoV-2 coronavirus, it is necessary to understand how our immune response works. After the coronavirus enters the body at the initial stage, there is an intense production of IgM antibodies against the virus proteins. Therefore, their presence in the blood may indicate a recent or currently ongoing infection.

As the infection continues, IgM antibodies disappear and are replaced by more persistent IgG antibodies. Therefore, during the transitional period of infection, one may have IgM and IgG antibodies to coronavirus in the blood at the same time. In contrast, when the virus is eradicated and we recover, IgG antibodies will persist in our body. How long they persist is an individual matter, and studies show that on average it is about 6 months. The presence of only IgG antibodies (without IgM) against coronavirus indicates a history of infection.

Having antibodies to coronavirus (IgG positive) can probably provide us with protection against reinfection. However, we currently do not know how long such protection may last and whether it protects against newly emerging coronavirus variants. In addition, although we may not get sick again ourselves, we can probably passively transmit the virus, so the presence of antibodies to coronavirus does not exempt us from wearing a mask and keeping a social distance.

Can antibody testing show the effectiveness of vaccination?

Currently available tests also allow us to assess to some extent our response to the SARS-CoV-2 vaccination and distinguish whether the antibodies in our body were formed after contact with the coronavirus or after vaccination.

In people who have undergone COVID-19, IgG antibodies are directed against various proteins of the virus. In contrast, in vaccinated individuals, IgG antibodies are directed only against the S1 protein. Therefore, evaluating IgG antibodies against S1 and N coronavirus proteins separately will allow differentiation, and the test result should be interpreted as follows:

  • IgG anti-N (-) and IgG anti-S1 (+) – vaccinated person who was not previously infected with SARS-CoV-2
  • IgG anti-N (+) and IgG anti-S1 (+) – a vaccinated person who was previously infected with SARS-CoV-2.

It is worth noting that the development of immunity against coronavirus is much more complex, and not only the measurement of antibodies alone is an indicator of it.


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