covid 19 antibody tests

COVID 19 antibody tests have received a lot of attention recently as the pandemic causes havoc across the world and leaves behind a trail of death and destruction. Masking, maintaining social distance, hand sanitisation and vaccines constitute the pillars on which the anti COVID 19 fight is based. With the passage of time many fortunate people would have recovered from COVID 19, or would have been vaccinated.

Chances of reinfection.

What are the chances of unfortunately contracting COVID again if you belong to one of the above groups? A lot of it depends on the immune response your body is able to mount to keep you protected from future infections. The important parts of the immune response consists largely of protective circulating antibodies and T cells (T lymphocytes) that have been primed to identify the pathogen.

covid 19 antibody testsAntibody levels can be measured in the blood using COVID 19 antibody tests and can give some indication of the level of protection that one enjoys. The laboratory measurement of T cell function is still limited to research labs and is not available on a large scale to quantify the degree of protection it confers. Here all  you need to know about COVID 19 antibody tests, and how you should interpret the test result if you’ve had a test.

Role of COVID 19 antibody tests.covid 19 antibody tests

There definitely is a time and place for COVID 19 antibody tests. They can provide critical information that could help scientists predict future transmission of the virus by determining what proportion of the population is still at risk or to identify localities where transmission is particularly high. These bits of information will help to target vaccination efforts, which will be particularly relevant as the supply of COVID-19 vaccines are likely to be constrained in the immediate future.

COVID 19 antibody tests could also be useful for identifying people who could donate antibody containing plasma to be used as a treatment for people with a life-threatening COVID 19 disease. But for any individual receiving an antibody test, placing a lot of emphasis on their result is, at best being optimistic, and, at worst, being dangerous.

Sensitivity of COVID 19 antibody tests.

Any good diagnostic test needs to be highly sensitive, which means that most people who have had the disease or the vaccination will have a positive result. If a test is deemed to be 98% sensitive it means that if the test is done with 100 people who have had COVID 19 or the vaccination, the test will be positive in 98 of them. The test will only fail to elicit a positivity in 2% of the people who have had COVID 19 disease or its vaccination. The 2% group. constitutes the false negative group. A highly sensitive test is good for mass screening programs.

Specificity of COVID 19 antibody tests.

A good diagnostic test should also be highly specific. This means that only people who have had the disease or vaccination should have a positive result. A positive result must not occur from any other cause / disease. If a test is deemed to be 98% specific, it means that test positivity indicates that there is a 98% chance that you’ve had COVID or had the vaccine. There is only a 2% chance that your test positivity is from other (non COVID / non vaccination) causes. The 2% constitutes the false positive group. A highly specific test is good for accuracy of diagnosis.

COVID antibody tests are about 90% sensitive and 98% specific.

Timing of COVID 19 antibody tests.

covid 19 antibody testsThe degree of reliability of COVID 19 antibody tests will also depend on how recently a person has contracted COVID 19 or had a vaccination The immune system needs time to produce antibodies at a high enough concentration to be detectable with the test. According to the recently published Cochrane review, only about 30% of people will test positive if they undergo an antibody test a week after developing symptoms of COVID 19 or having the vaccination. This progressively rises over time – to about 70% in the second week, and 90% in the third week.

It decreases again as antibody levels start to wane with the passage of time. It is also important to remember that if a person develops a mild or asymptomatic COVID-19 disease he may not produce high enough antibody levels to be detected by the test.

Antibody levels and protection.

covid 19 antibody testsAfter taking all of the above factors into account, we do not know for sure if looking for antibodies is a good way to establish whether a person is immune to COVID-19 or how long this immunity might last. Antibodies are just one part of the immune response that is triggered when a person becomes infected with the COVID 19 disease or has been vaccinated.

Lack of antibodies does not necessarily mean that the immune system is not primed to respond to possible reinfection,and, the mere presence of antibodies does not necessarily mean that antibodies are present at a high enough concentration to provide protection from re-infection.

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