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Covid-19 Antigen & Antibody Testing

Updated: Sep 2, 2021

All you need to know to navigate Covid-19 testing including a deep-dive into antibody tests.

There are a number of diagnostic tests available to help medical professionals and individuals navigate the Covid-19 pandemic. Here we look further at how they work and the ins and outs of what you need to know if using one.

The Short Read:

  • Early studies are showing a positive relationship between antibody levels and protection from serious Covid-19 illness. We don’t yet have sufficient information to rely on them solely and confidently as a firm measure of ongoing immunity - i.e. to protect people from (re) infection, avoid infecting others, or be confident that protection will last for a defined period of time.

  • There's a vast range of antibody tests on the market and available with medical professionals. Not all antibodies are equal, and not all antibody tests are equal. An important consideration if measuring immunity post-vaccination is that the test used is suitable for the type of vaccine taken. In Hong Kong, Sinovac vaccine antibodies can be recognised by N-type and S-type antibody tests, while Pfizer will not be recognised on an N-type test, but only an S-type test. Tests also vary significantly in reliability, with clinics and labs typically offering more reliable tests than home-testing kits.

  • Hong Kong's recent announcement to reduce quarantine terms to 7 days reflects the stage at which we are in Covid-19 immunity confidence. A vaccination certificate is required as vaccination reduces infection risk; a quality lab-based antibody test is required as this gives greater confidence that vaccination has taken place and been effective; a negative PCR test is required to demonstrate the individual has not been infected post-vaccination/previous infection.

The Longer Read:

Antigens & the Immune Response

When a person is infected with a virus or bacteria, the immune system is able to detect that its protein structure is foreign to our bodies. This foreign protein structure is called an antigen and once recognised, allows our immune system to mount a response in order to rid the body of the infection. This occurs through a complex interplay of immune cells and organs, including T-cells, B-cells and antibodies [1]. A key component of this immune response is the ability of the immune system to remember this foreign protein structure or antigen such that should an individual be exposed to the same virus or bacteria again in the future, the immune system can quickly mount a response which can prevent or reduce the severity of the infection i.e. the person is “immune” to that particular bacteria or virus.

About Covid-19 Diagnostic Tests

The diagnostic tests that help us understand current and past infection fall into two main categories:

1. PCR Tests. These look for evidence of the virus in the form of its genetic material in the body, typically using samples taken from the nose, throat or a stool sample. PCR tests are used to diagnose an ‘active’ or current infection and can be used whether the person has symptoms or not. Generally once a person has recovered from the infection, the PCR test will no longer generate a positive result because the body has been able to rid itself of the virus.

PCR is the gold standard test for Covid-19, and is more reliable than rapid antigen tests particularly in settings where incidents of infection are low, hence remains the official test of choice in Hong Kong.

2. Antigen Tests: These tend to be rapid tests which look for evidence of foreign viral protein and can provide a quicker result than PCR tests as they do not require laboratory analysis. They are not as sensitive as PCR tests however, and so a positive antigen test would need to be confirmed with a PCR test.

3. Antibody Tests. These look for evidence of an immune response to the infection by measuring antibodies to the virus rather than looking for the virus itself in the blood. Antibodies will typically be produced while a person is fighting an infection, however, this may take up to a week and so antibody tests to diagnose early infection are of limited value. Antibodies do stay in a person’s blood for some time after they have recovered from the infection however, meaning that they are most useful for identifying an infection in the later stages or a past infection. We do know that some people who have had a previous covid infection, will not have detectable antibodies in their blood some time after their infection, particularly if the infection was mild or asymptomatic. However, they may still maintain some immunity to the virus from other components of the immune system such as T cells.

Importantly, it remains difficult to test cellular immunity (T-Cell and B-Cell behaviour) outside a research laboratory setting because of a lack of accessible diagnostic solutions[1].

How the tests work

Antigen/PCR tests look for evidence of the foreign protein in bodily fluid and matter, typically taken from the nose, throat or via a stool sample. Antibody tests typically look for the presence of antibodies in the blood (serum), and may involve a blood draw (needle used to extract blood) or a pin-prick test (finger prick device used to capture a smaller volume of blood), although advanced diagnostics may allow antibodies to be measured via a saliva test.

How Antibodies Behave

Figure 1 shows a typical response to a SARS-CoV-2 infection in terms of antigen and antibody behaviour. It’s important to emphasise that the human immune system is unique and individuals will exhibit different responses which may not reflect the pattern in Figure 1, but it helps to demonstrate a typical pattern of behaviour and explain the likely results of different test types to a typical response.

Figure 1: Graphical representation of typical antibody behaviour post-exposure

What Antibody Tests Can Tell You

An effective antibody test can tell you if it is likely that you have had COVID-19 before and/or if you have antibodies in response to Covid-19 vaccination. There's evidence that this provides protection against moderate to severe Covid-19 infection.

What Antibody Tests Can't Yet Tell You

A positive antibody test is not a guarantee that you won't catch Covid-19 or be reinfected. [3] Research is ongoing into the levels of antibody typically needed to provide protection, and the duration for which those antibodies typically endure after vaccination or infection. Similarly, a positive antibody test could still accompany a Covid-19 infection, and if infected, you may still be infectious to other people.

Types of Covid-19 Antibody Test

Not all antibodies are the same, and not all tests look for the same types of antibodies. There is also a significant range in quality and reliability of tests and all of these factors can influence the degree to which you can rely upon the results you receive.

Immunoglobulin Type: IgG/IgM/Dual Type Antibody Tests - as Figure 1 shows, different types of antibodies are produced at different stages of the immune response. IgM (Immunoglobulin M) are proteins that typically appear as a ‘first responder’ to infection and so rise in the earlier stages and they typically drop after infection. IgG antibodies are specific to an antigen, and typically take longer to produce, rising after IgM then stabilising. They tend to endure for longer and in doing so, can offer longer-term protection from the same antigen.

Antibody tests are available to measure only IgG, only IgM, or both, and also cover a broader range of immunoglobulin such as IgA [12]. An IgM-only test can help to determine whether an infection is current or recent; IgG tests are more readily used to evaluate longer-term protection. A dual-type test will look for either type of antibody and therefore offers fewer insights into the timing of a potential infection. As we are all unique, we can only take these results as an indication. But it’s important to use the test that best matches your needs.

Protein-Specific Tests - N-Protein -v- S-Protein As Figure 2 shows, the SARS-CoV-2 virus is made up of different proteins including Nucleocapsid (N), Spike (S), Membrane and Envelope[5]. If naturally infected with the virus, the immune system will make antibodies in response to all the proteins. The early antibody tests used in the Covid-19 pandemic focused on the N-protein, so they would typically return a positive result where a patient’s immune system has made traceable antibodies to the N-protein (which would happen under natural infection). However, the majority of Covid-19 vaccines are designed around only the S-protein (the Spike), therefore only an S-protein test will be able to identify those antibodies.

The two vaccines available in Hong Kong - the Fosun Pharma/BioNTech Comirnaty vaccine and Sinovac’s CoronaVac vaccine - are designed differently, with Sinovac’s product using the whole virus (both the N- and S-proteins), while Pfizer’s only uses the S-protein[6][7]. Therefore, an S-protein test would be needed to successfully test antibodies after vaccination with the Pfizer vaccine, while either type of test should work with the Sinovac vaccine.

It can be difficult to find out from manufacturers’ descriptions which antibody responses the tests are able to detect, particularly with home testing kits.

Figure 2 Graphical abstract - schematic representation of Novel Coronavirus showing target proteins[17]

Neutralising -v- Binding Antibody Tests

Binding antibodies can attach to a pathogen, such as a virus, but it’s not yet clear whether they prevent further cell infection and/or destruction. Neutralising antibodies have been observed in a laboratory setting to decrease SARS-CoV-2 viral infection of cells. Many vaccine clinical trials used neutralising antibody tests, however there can be significant variation in the tests and experts are still studying the implications for Covid-19 immunity. Neutralising antibody tests are typically performed in a lab setting.

Qualitative -v- Quantitative Antibody Tests

Qualitative tests give a simple ‘positive’ or ‘negative’ result, where quantitative tests provide a reading against a given scale, or a ‘titer’. As of today we have insufficient data to provide a confident ‘measure’ of immunity relating to Covid-19. Therefore, no results can provide a definitive picture of immunity. Qualitative tests are generally suited to identifying a past infection, where quantitative results will be instrumental in research into Covid-19. It’s likely that in time, experts will agree upon an antibody titer value or range which can provide a measure of immunity, as is the case for other vaccine-preventable diseases such as Hepatitis B.

Reliability of the Tests

No test is 100% reliable. Many high-quality laboratories will be using the most reliable products available, but it’s worth asking for details. In order to gain regulatory approval in major markets, diagnostic tests need to prove, and make public, reliability indicators associated with their products. Typically this is measured in sensitivity and specificity:

Sensitivity: the ability of a test to correctly identify patients with a disease.

Specificity: the ability of a test to correctly identify people without the disease.

True positive: the person has the disease and the test is positive.

True negative: the person does not have the disease and the test is negative.

There have been concerns in particular about the accuracy of pin-prick antibody tests. Researchers in Oxford, England, found out of six different at-home antibody tests, most were not sensitive enough to detect immune responses. [9][11].

Typically, laboratory tests require a larger sample of blood, use more accurate diagnostic techniques than home-test kits, and are operated and interpreted by people with more specialist knowledge of an often complex subject area, so are generally considered a more reliable source. They are usually therefore more expensive than home testing kits. It’s likely that in time, we’ll start to see more accessible and reliable test kit options available, but it’s important as a consumer to understand what to look for particularly if you don't have a medical professional to guide you through the process.


[1] T-Cells: The Missing Link in Immunity? The FT

[2] Department of Health and Social Care. (2021). Guidance Coronavirus (COVID-19): antibody testing. Gov.UK. Retrieved from:

[3] NHS. (2021). Antibody testing to check if you've had coronavirus (COVID-19). Retrieved from:

[4] Imperial College London. (2021). COVID-19 vaccine Q&A - latest news, antibody tests and tackling new variants. Retrieved from:

[5] A Visual Guide to the SARS-CoV-2 Coronavirus

[6] Imperial College London. (2021). COVID-19 vaccine Q&A - latest news, antibody tests and tackling new variants. Retrieved from:

[7] CDC. (2021). Antibody Testing Interim Guidelines. Centres for Disease Control and Prevention. Retrieved from:

[8] rthk. (2020). Expert casts doubt on home test kits for coronavirus. Retrieved from:

[9] Guardian. (2020). UK government urged to abandon 'poor' finger-prick antibody tests This article is more than 1 year. Retrieved from:

[10] Immunity & COVID-19: British Society for Immunology. Feb 3 2021 -

[11] rthk. (2020). Expert casts doubt on home test kits for coronavirus. Retrieved from:

[12] Confirm Biosciences. (2020). Coronavirus Testing Explained: Antibody (IgG and IgM) Testing, PCR Testing and Antigen Testing. Retrieved from:

[13] CDC. (2021). Antibody Testing Interim Guidelines. Centres for Disease Control and Prevention. Retrieved from:

[14] Nuffield Health. (2021). COVID-19 antibody testing explained. Retrieved from:

[15] CDC. (2021). Antibody Testing Interim Guidelines. Centres for Disease Control and Prevention. Retrieved from:

[16] BMJ: Antibody Protection

[17] Insights into SARS-CoV-2 genome, structure, evolution, pathogenesis and therapies

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