Antibody testing has been widely discussed these days as the Hong Kong government is allowing arrivals with a positive antibody result and a negative PCR test to reduce their mandatory quarantine from 14 to 7 days. This indicates the government is using antibody levels as a strategy to differentiate the high and low risk individuals. But what does a positive antibody result mean and does a positive result mean you cannot catch Covid-19?
What can this test result tell us?
A positive Covid-19 antibody level means that as of the timing of the test, the patient’s immune system had responded either to a natural covid infection or to a covid vaccine. However, false positives can happen - it means that a person tests positive for Covid-19 antibodies when they do not actually have those specific antibodies.
What do the different units of measurement mean?
Sample test result from PathLabs in Hong Kong using the Abbott Alinity test
Different methods have different units, values, cut-offs and dynamic ranges. In this particular antibody test, the unit of measurement used to define test readings is ‘AU’ which stands for ‘Arbitrary Units’. (Abbott Alinity)
For different antibody test results to be more easily compared, the WHO has introduced BAU, which stands for ‘Binding Antibody Units’, as an International Standard unit of measurement for anti-SARS-CoV-2 immunoglobulin (NIBSC code 20/136). Some tests may refer to this as ‘IAUs’.
Does a reading between 0 and 49 AU/mL mean I have no protection from Covid-19?
For this particular test, a patient with a test result under 50 AU/mL would generate a negative antibody test result. The reason a level of 50 AU/mL is a significant cut-off is because this is the necessary level of antibodies for this particular test to be reliable to its published levels of sensitivity and specificity (definition below). No test is 100% accurate, and each test will require a particular sample size to achieve a given level of reliability. A patient may in reality carry a positive level of antibodies below 50 and still have a level of protection from Covid-19, but the test would not be able to reliably confirm that and so a level below 50 AU/mL would be considered negative to avoid potentially-misleading results.
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.
Does a test reading above 50 AU/mL mean I’m immune?
It depends what we mean by ‘immune’ and definitions do vary - ‘one cannot catch something’ or ‘one has protection against something’?
For now, we should work with the latter - we have protection. We do know individuals have a level of protection from infection/re-infection if they’ve been vaccinated or had a previous Covid-19 infection, based on clinical trials and real-world data; we know the majority of people develop antibodies specific to Covid-19, and those antibody levels can vary quite significantly between people. For people who have received vaccines such as Pfizer-BioNTech, antibody levels can be significant (e.g. up to 40,000 AU/mL) three weeks after completion of the full course vaccination. This is all good news.
“Variables that can affect your antibody levels include your age, general health, timing of vaccination and of course, the brand of vaccine itself.” - Charles Brantly, Central Health
What do we not yet know?
It might not feel like it but Covid-19 hasn’t been around for that long relative to the time it takes to really understand the behaviour of a virus, particularly when we account for the different Covid-19 policies being applied globally, the range of vaccines being distributed, and the impact of variants amongst other factors.
With time and data, we need to more confidently understand the following:
1. How long our protection lasts from vaccination and previous infection. There’s evidence of people getting re-infected and infected by Covid-19 post-vaccination even with positive antibody levels, but so far, symptoms are usually mild. OT&P says that there is increasing evidence of immunity 10 months after natural infection even in individuals without an antibody response at that point. In addition, the BioNTech vaccine shows a promising initial antibody response and with that, it is estimated that the vaccine can offer protection for at least one year.
2. What our immunity levels mean for different Covid-19 outcomes upon infection. In managing a virus like Covid-19, we ideally want to understand the likely risk of different outcomes, including:
Risk of being infected but being asymptomatic
Risk of being infected but being symptomatic
Risk of being infected and transmitting to others
Risk of being hospitalised
Risk of being admitted to ICU
Risk of death
Risk of acquiring Long Covid
The good news is there are studies & real-world data exercises ongoing around the world and we are learning about Covid-19 at a rapid pace relative to anything that’s gone before in the field of infectious disease. We’ll undoubtedly see diagnostic and antibody tests evolve alongside medical protocol as we treat and predict the risk of Covid-19.
Case #1: Real life antibody data example
This individual received their first dose of Pfizer-BioNTech Vaccine on March 22 and second dose on April 12. They have done three antibody tests to illustrate the level of antibody (AU/ml) at different points of time after receiving the vaccine. Please note each individual reacts differently to vaccines thus antibody levels vary between individuals.
 Testing for Antibodies after Covid-19. 2021. Retrieved from: https://www.otandp.com/blog/testing-for-antibodies-after-covid-19-vaccine