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If you have completed the COVID-19 vaccines and are considering an antibody test, you should understand how to interpret the test result first.

What are antibody tests?

When a virus, such as the COVID-19 virus, invades your body, the B lymphocytes in the blood will produce antibodies to destroy the virus. After the virus is cleared, these antibodies will stay in the body. If the virus ever comes back, the antibodies can immediately defend against the virus. Vaccines allow your body to mimic this reaction and produce antibodies without actually getting infected with the virus. This is why you develop antibodies after natural infection with a virus or after getting a vaccine. However, not all antibodies are the same, and not all tests look for the same types of antibodies. The type of the test can affect the results you receive.

Binding antibody tests

When you were infected with COVID-19 virus, your body will produce antibodies against a variety of proteins of the virus, including nucleocapsid and spike proteins.

The antibody test detects the presence of antibodies that bind to the purified proteins of COVID-19 virus. Most of these tests detect antibodies against one of the two types of proteins from the virus:(i)Nucleocapsid protein
(ii)Spike protein

There are two types of COVID-19 vaccines in Hong Kong – Cormirnaty and SinoVac. Comirnaty is an mRNA vaccine that triggers antibodies against spike protein only, so the antibody test performed after this vaccine should detect antibodies to the spike protein, not to the nucleocapsid protein. SinoVac is an inactivated virus vaccine and stimulates your body to make antibodies against multiple viral proteins including spike protein and nucleocapsid protein. Most antibody tests that are available commercially aim at detecting antibodies against spike protein only. Some tests look for antibodies to both spike protein and the nucleocapsid protein, and these tests are usually reserved for research purposes.

What conclusion can you draw from the antibody test result?

For vaccines that have been in use for a long period of time, we have a better idea of what antibody levels correlate with protection. For example, after hepatitis B vaccination, we can check hepatitis B antibody levels to show if we are immune to the virus. If we do not have high enough antibody levels, we can receive booster vaccines, or even the entire vaccine series.

However, even experts do not know what level of antibodies confer protection against COVID-19 virus infection. If the antibody level is thought to be too low, doctors do not know what to do. In the future, doctors may recommend additional booster shots for people who do not have an adequate antibody response. But it is not possible to make such a recommendation now. Therefore, the Centers for Disease Control and Prevention (CDC) in the US does not recommend testing antibody levels after COVID-19 vaccination.

Although rare, it is also possible to receive vaccines and not develop antibodies. This can be a failure of the immune system of an individual to respond to the vaccine and to generate antibodies. It is also possible that the individual indeed has some antibodies, but the level is too few for a laboratory test to detect. Or the test is performed too early, and the antibody level has not risen to a detectable level yet.

Protection Against COVID-19 Is More Than Just Antibodies

The COVID-19 vaccine may confer protection that goes beyond antibodies, including making the body generate T cell immunity. The T cells might not be able to prevent COVID-19 virus infection, but they can help to clear the virus from the body once infection has occurred, and limit the severity and duration of infection. But the presence of T cells cannot be reflected in the antibody test.

The Bottom Line

There are currently too many unanswered questions about whether an antibody test should be performed after a COVID-19 vaccine. The best thing you can do right now is to get the COVID-19 vaccine and continue to follow standard mitigation measures to avoid exposure to the virus.