FAQ: Should You Get an Antibody Test After COVID Vaccination?

Should you get an antibody test done after you take your vaccine jab? What will this test tell you? Does it help?

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FIT answers some common questions on antibody testing, how reliable they are and if you should get them done to measure vaccination.
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An essential tool to prevent the spread of Coronavirus is to 'test, test, test.' New variants of SARS-CoV-2 and COVID-19 vaccines have increased the curiosity around testing for antibodies.

This curiosity around whether we already have COVID-19 antibodies, or to know if the the vaccine is effective, has led to an increase in antibody testing. In some cases, elderly patients are being advised to test for antibodies by doctors before taking the second dose.

So, should you get an antibody test done after you take your vaccine jab? What will this test tell you? How will it help? FIT speaks with Dr Shahid Jameel, virologist and director, Trivedi School of Biosciences, Ashoka University.

What does an antibody test tell you?

Dr Jameel says, “Whether we have got the disease or not, if our antibody test is positive, then we get to know whether we have come in contact with the virus or not. Our body produces antibodies in response to exposure to the virus.”

Antibody tests or serological (serum related) tests are done by taking a blood sample.

When infected, our immune system produces antibodies to neutralize the virus in the body. These antibodies remain present in the body for some time even after the virus infection is completely eradicated. This helps doctors identify whether the patient was infected earlier.

A similar reaction occurs in the body after taking the vaccine.

Is it necessary to have antibodies after taking the vaccine?

Antibodies are usually prepared in large quantities after taking the vaccine, and after taking complete dose, you get protection from the disease. It is to be noted that these antibodies will protect against disease and not from infection.

There is a misconception among people that after taking the vaccine, if specific amount of antibodies are not produced, then they’ll not be protected from the disease. Dr Jameel says it’s rare for antibodies to not be formed after vaccination, but it’s known to happen and can be attributed to some genetic abnormalities.

In an earlier story on FIT, we’d explained that there is a threshold of an antibody test.

“Antibodies in the blood are measured in the International Unit (IU). Its range is from 0 to 1000. Usually 10 IU per ml is considered a cut off. Levels between 10 and 1000 are considered protective. However, it may not always be correlated. People with count below 10 can also avoid the disease through cell immunity which is not measured by antibody test.”

The average count comes from 300 to 1000 after the second shot of the vaccine.

Does a higher count indicate more protection?

Dr Jameel says that whether the count is 100 or 1000, the vaccine provides equal protection. But according to the science of vaccineology, for the best response, we should take the booster dose or the second dose of vaccine when the antibody level comes close to the baseline.

“For the best immune response, it is advised to allow the antibody to drop after reaching the threshold level 10. The second booster dose is then most effective. Although this applies equally to all people, it is not necessary. Antibody tests are mostly performed commercially in large pathology labs. But in the private domain, its cost is almost double the cost of the vaccine,” he adds.

But, Dr Jameel also says that it is not feasible for common people to have their antibody level checked repeatedly after infection, or after taking the first dose of the vaccine and wait for the antibody levels to drop, before deciding to take the second dose. This cannot be recommended at population level.

Can an antibody test indicate the efficacy of a vaccine?

No, vaccine efficacy can only be tested in a proper placebo-controlled clinical trial. No antibody test can measure vaccine efficacy.

What’s the difference between a sero survey and an antibody test?

Dr Jameel explains, “For example, we conduct a sero survey in some area and we find that antibodies were found in 57% of the population there. This does not mean that 57% of the people have had the disease, some may have only been exposed to the virus."

Antibody test on the other hand consists of qualitative and quantitative test. The qualitative result indicates whether there has been a virus exposure or not. A quantitative test, which is called an ELISA test, tells us if we have developed antibodies and to what extent.

Confused with all this information? Don’t be.

Dr Jameel explains, “Viruses have 2 different types of proteins - spike protein (S Protein) and nucleocapsid protein (N Protein). The S protein is on the surface of the virus and the N protein is found inside the virus.

Most vaccines against coronaviruses are based on the S protein. Covishield, the vaccine used in India, is based on the S protein and this vaccine will produce S antibodies. Covaxin, on the other hand, is a whole virus vaccine, so it will produce both S and N antibodies.”

But this does not mean that a vaccine that produces both S and N antibodies will offer more protection.

Antibodies neutralize the virus by binding to the surface of the virus. Since the N protein is not present on the surface of the virus, it will not be able to neutralize it.

In addition, there are neutralizing antibodies that prevent the virus from binding to the cell. These are a part of the S antibody. Suppose you have 100 molecules of S antibody in the body, then there will be 10 neutralizing antibodies in them.

Therefore, it makes little sense to obsess over antibody count and let it impact your decision to get vaccinated. Especially, amidst increasing COVID cases, it is important that the elderly, who are more vulnerable, take their complete dose and don’t avoid it. It’s important that we get protection as soon as possible, and that will happen only after you’ve got your two doses.

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