Antibodies are the soldiers of the immune defence system.
Recovery from any viral illness will be a result of an internal battle between the virus and the immune response. An individual who has experienced an illness previously, or has been immunised against that virus, is likely to already have antibodies against that infection.
If an individual is exposed to the same virus again, their natural immunity is able to win this battle relatively easily and the infection never takes hold. If they are exposed to an infection to which they have no immunity, the strength of the immune system will be impacted by some general factors such as age, fitness, nutritional status, other health conditions or drugs that they may be taking. The impact of the internal battle in this situation will be a balance of the ability and strength of the immune system in fighting the infection and the individual characteristics of the specific virus. In an ideal situation, the infection is recognised as a threat and the body is able to produce an effective response.
The first line of this response typically involves IGM antibodies. These are the forward response teams of our defence systems. We know that in COVID-19 they start to be produced within a few days. Later, as the battle is hopefully being won these antibodies are replaced by longer-lasting IGG antibodies. We now have good evidence that vaccination produces high levels of IGG antibodies in most people. Early studies in Hong Kong suggest that the antibody levels are higher with BioNTech than the Sinovac vaccination.
What are the benefits of testing for COVID-19 Antibodies?
Health authorities have indicated that they may start to take blood samples for arrivals in order to test for coronavirus antibodies. An individual who has experienced an illness previously, or has been immunised against that virus, is likely to already have antibodies against that infection. But what will we gain from knowing this?
Testing for Individual Immunity
Individuals who have a positive antibody test have immunity. For COVID-19, this immunity seems to be very good at preventing infection and even in situations in which infections occur they are likely to be mild. We are not yet certain how long this immunity will last for but it seems likely to be at least one year and possibly longer.
Antibody studies can also give information on how close any population is to Herd Immunity. Recently a study on blood donors in the UK showed that in randomly tested blood samples 75% had evidence of antibodies. The UK is at, or very close to Herd Immunity. Probably half of the immunity has come from disease and the rest from vaccination. In Hong Kong, we have very low levels of natural immunity so our antibody levels are still a long way away from the Herd Immunity threshold.
Written by Dr David Owens
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