Issue #118
October 2021
Antibodies: vaccine vs infection

Dr Linda Hueston is the Chief Serologist at NSW Health Pathology – ICPMR and was the first to develop serology assays to detect the COVID-19 virus in Australia. These assays have been used since February 2020 to detect the antibodies the body creates to fight the virus. Now that vaccines have been rolled out widely across Australia and New Zealand, Dr Hueston explains that detecting antibodies in a patient is not as straightforward as it once was.

“Early in the pandemic, COVID-19 serology tests were generally designed to detect antibodies generated by natural infection. Once vaccines became available for COVID-19, it became important to be able to distinguish antibody generated by vaccination from antibody generated by infection. At the beginning of the pandemic, if someone had antibodies present, it was because they were infected with COVID-19. Now, if antibodies are present in a patient, this could be due to vaccination or infection.

“Because of this, I developed a combination of assays which allow us to tell whether antibodies are derived from a vaccine or whether they have been naturally acquired through exposure to the virus. Depending on the levels of antibodies present in a person, it is possible to give guidance on whether it is an old infection, which can be helpful for contact tracing and for making decisions about whether to release someone from quarantine. Using these assays, it is also possible to tell the efficacy of the vaccines, however more studies are still required in this area,” said Dr Hueston.

As part of its vaccination strategy, Australia rolled out COVID-19 vaccines which utilise the coronavirus spike protein. This spike protein is located on the outside of a coronavirus and is how SARS-CoV-2, the virus which causes COVID-19, enters human cells. The particular spike protein is unique to SARS-CoV-2 therefore the immune system recognises it as ‘foreign’ and starts producing long-lasting immune cells and antibodies.

“Some of the commercial antibody tests that are available use spike antigens, whilst some use nucleoprotein antigens. As the vaccines used in Australia only produce antibody to spike antigens, it means that assays which use nucleoprotein antigens can be used to assess vaccine success. Not all laboratories test for both spike and nucleoprotein antigens so results will vary depending on where the sample is sent and what test system is used. Clinicians aren’t necessarily aware that there is a difference in the testing and what that might mean.

“In Australia we are only using spike vaccines, therefore when only spike antibodies are present in a patient, we can speculate that it is because of the vaccine. If we had chosen whole virus vaccines, such as Sinopharm, then we would see a response to both spike and nuclear proteins which would make it a lot more complicated to distinguish vaccination from infection. At the moment our tests are able to make this distinction, but this may become more difficult once the borders open and more vaccines are made available,” said Dr Hueston.



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