Testing for antibodies after COVID-19 vaccine

Normally when giving vaccinations we assume the vaccine to be effective. There are some situations in which we test the presence of immunity. For example, we test people who are considering pregnancy or who are newly pregnant to ensure they are immune to rubella or chickenpox and we test healthcare workers and at-risk individuals to make sure they are immune to Hepatitis B post-vaccination.

The government has just announced that quarantine from many locations will be reduced to 7 days for arrivals who are shown to have a positive antibody test in addition to a negative PCR test. This indicates that the government will be using antibody levels as one of the strategies to mitigate risk. People in Hong Kong who are hospitalised with Covid are kept in hospital until they reach a ‘safe threshold’ for discharge based upon a measure of viral load and positive antibody test. Hong Kong has so far been reluctant to treat vaccinated individuals as if they are immune. It seems that, at least in some circumstances, the backup of a positive antibody test may provide the extra reassurance needed to relax some restrictions.

Antibody Testing post-vaccination

There are a number of blood tests which measure antibodies against the SARS-Cov-2 virus. An IGG test becomes positive within the first few weeks of the illness or vaccination. We have an increasingly large dataset around the effectiveness of vaccination and the results are very impressive. At OT&P we are consistently seeing high antibody levels after vaccination. This is especially so for BioNTech vaccination. Well over 90% of OT&P patients are now vaccinated with 96% of our patients choosing BioNTech. We recently reviewed the first cohort of antibody results which showed a mean antibody level of 8675 AU/mL and a median of 7580 AU/mL with a value of >50 indicating immunity. These results are compatible with the recent data reported out of HKU.

Example of post-vaccine immunity. Over 50 is considered immune. Most cases we see are measured in the thousands and it is not uncommon to see cases, as above, beyond the upper measurable limit.

How long will protection last for?

It is too early to say but we are building increasing evidence of immunity 10 months after natural infection even in individuals without an antibody response at that point. This is due to the strong secondary immunity provided by T-cells and memory cells. Our early data suggests that antibody levels drop quite quickly at least over the first two months. This is similar to what we see with natural infection. However, the antibody response to BioNTech especially, is significantly higher than that we see with natural infection. It would be surprising, in view of the high initial antibody levels, if vaccination does not last for at least one year and quite possibly much longer.