COVID-19 Antibody Testing – Which antibodies to test for and when?

COVID-19 antibody IgG testing has allowed patients who believe they have been infected with COVID-19 to confirm their infection status. It must be said however that some people do not develop IgG antibodies after having COVID-19. This test looks for detectable Nucleocapsid (N) antigens following infection.

In recent weeks, a new test has become available that enables the testing of antibodies following a COVID-19 vaccination for patients with no history of infection. This test is a quantitative test which means that it not only gives the patient a positive or negative antibody result, but also tells the patient the level of antibodies they have if they get a positive result. This test looks for detectable Receptor Binding Domain (RDB) Spike antigens following vaccination. Measuring these antibody levels could play an important role in establishing vaccine efficacy.

It is important to mention that if an individual has been infected with COVID-19, they may have detectable antibodies to both the N and RBD Spike antigens. Antibody tests do not detect COVID-19 infection.

Testing for antibodies after COVID-19 infection

There are a number of antibody tests and methods available to test previous exposure to COVID-19 infection.

IgM antibodies are usually the first antibody produced by the immune system when a virus attacks. A positive IgM test indicates that you may have been infected and that your immune system has started responding to the virus. When IgM is detected you may still be infected, or you may have recently recovered from a COVID-19 infection.

IgG antibodies often develop in most patients within seven to 10 days after symptoms of COVID-19 begin. IgG antibodies remain in the blood after an infection has passed. These antibodies indicate that you may have had COVID-19 in the recent past and have developed antibodies that may protect you from future infection. It is unknown at this point how much protection antibodies might provide against reinfection.

Test: Roche Elecsys Anti-SARS-CoV-2 (IgG & IgM Total Antibody)

Reporting ranges: Positive/Negative

Collection method: Serum – venous OR Capillary self-collection

Sensitivity: 97.4%

Specificity: 100%

Test: Abbott Anti-SARS-CoV-2 (IgG)

Reporting ranges: Positive/Negative

Collection method: Serum - venous

Sensitivity: 99.1%

Specificity: 100%

When should you test after infection?

Testing should not be considered any less that 14 days from the presence of symptoms or a positive PCR test in the case of asymptomatic patients.

If you have had a confirmed case of COVID-19, is it possible to test negative for antibodies?

Not everyone who is infected with COVID-19 develops IgG antibodies. This depends on the how much of the virus you were exposed to when infected (the viral load), the strength of your immune response and other physiological factors.

How long will antibodies last after having COVID-19?

The immune systems of more than 95% of people who recovered from COVID-19 showed evidence of antibody presence up to eight months after infection. These results provide hope that people receiving COVID-19 vaccines will develop similar lasting immune memories after vaccination.

Other research suggests that T cells (one of the important white blood cells of the immune system) also play an important part in protection, and T cell memory may continue after IgG antibodies fade.

Testing for antibodies after COVID-19 vaccination

A receptor-binding domain (RBD) is a key part of a virus located on its ‘spike’ domain that allows it to dock to body receptors to gain entry into cells and lead to infection. These are also the primary targets in the prevention and treatment of viral infections; therefore, the spike protein is the target of many COVID-19 vaccines in development.

Following vaccination, the new RBD antigen test, will determine not only the presence of antibodies, but if a positive result is returned, it also gives the level of antibodies to the spike protein. 

Test: Roche Elecsys Anti-SARS-CoV-2 S (SPIKE)

Reporting ranges: Positive with value reported in U/ml, Negative

Sample type: Serum – venous. Capillary self-collection AVAILABLE SOON

Sensitivity: 99.8%

Specificity: 98.8% in samples taken 14 days or later after positive PCR

 

When should you test after vaccination?

The recommended time to test is 28 days after your second dose of the vaccination.

The UK government has delayed the roll-out of the second vaccine dose until 12 weeks after the first. As yet, we don’t know what protection the first dose offers, however we understand that patients may be keen to test their antibody levels at this stage to check. The recommendation from the testing laboratory is not to test less than 14 day following the first dose of vaccination. We are happy to carry out these tests at this stage, but there is a chance that antibody levels will not be detected.

What is considered a successful level of antibodies for each vaccine?

At present, there is no standardised measurement process for what constitutes a successful level of antibody protection following infection or vaccination. The test will advise you if you have COVID-19 antibodies following your vaccination and also give you the level of antibodies, but there is no benchmark for what constitutes success. (any antibody presence is a positive sign?)

There are other factors such as the aforementioned T memory cells, that play a significant role in immune response following vaccination. We are still learning about COVID-19 and what effective immune responses look like, both post infection and post vaccination. As scientists continue to collect data following the vaccine roll out, this will help to determine what levels of immune response help to reduce both the number and severity of infections, but at this stage, it is too early to say. Additionally, new variants of the virus may respond to immunity in different ways.

With the RBD Spike antigen test, is it possible to determine whether antibodies are present because of a previous COVID-19 infection or because of recent vaccination?

No, unfortunately the test cannot differentiate between the antibodies developed due to past infection and those developed as a response to a recent vaccination.

Is it possible to still not detect antibodies after having had both vaccination doses?

No vaccine is 100% effective, and much is still being learnt about the way in which this new vaccine works. It is possible that some patients just don’t mount an antibody response to the vaccine. 

Does being vaccinated mean that I don’t need to be tested anymore?

If you are currently being tested as part of a regime by your employer, you should continue with this testing regime to protect not only yourself, but your colleagues around you. As yet, we don’t know how quickly or effectively the vaccine protects you from contracting COVID-19 – whether you have had a single of full dose. Evidence is still being gathered as to whether the vaccine prevents you from transmitting the virus to others, so you should also continue to practice the hands, face, space safety protocols.

Antibody testing is conducted by our clinical partners Your Doctor