Written by Dr. Sanjana V.B Bhms,dbrm,cdn

Founder & medical director of siahmsr wellness.in

All rights reserved with siahmsr digital healthcare[siahmsr wellness]

Reviewed by SIAHMSR medical team.

 Covid 19 Antibody testing

   COVID-19 (coronavirus disease 2019) is a disease caused by a virus named SARS-CoV-2.  Although COVID-19 is an infection causing a multisystem inflammation, respiratory symptoms are predominant.  The virus infects your lungs and respiratory system causing mild flue like symptoms to severe illness. It is contagious and spreads to other people through droplets mostly.

  Some people affected with Covid 19, including those with minor or no symptoms will develop Post-COVID Conditions or “Long COVID.”

How does COVID-19 spread?

 COVID-19 is a droplet infection and it spreads when an infected person breathes out droplets [very small particles that contain the virus]. Other people can breathe in these droplets and particles and these particles get settled on their eyes, nose, or mouth which can cause infection. These droplets can contaminate surfaces also. On contact with these surfaces people may get infection. 

Covid Antibody testing

   Antibodies are proteins your immune system makes to help fight infection and protect you from getting sick in the future. A positive antibody test result can help identify someone who has had COVID-19 in the past or has been vaccinated against COVID-19.

Antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, can be detected in the blood of people who have recovered from COVID-19 or people who have been vaccinated against COVID-19. It is important to remember that some people with antibodies to SARS-CoV-2 may become infected after vaccination (vaccine breakthrough infection) or after recovering from a past infection (reinfection). For many diseases, including COVID-19, antibodies are expected to decrease or “wane” over time. As their antibodies wane, a person may become more vulnerable to SARS-CoV-2 infection. Even after a person’s antibodies wane, their immune system may have cells that remember the virus and that can act quickly to protect the person from severe illness if they become infected.

Covid infection and significance of Antibody testing

Antibody tests are blood tests conducted to know the response of your body to SARS-CoV-2. This test also shows the response of body to covid vaccine in people who are vaccinated.

 Antibody tests with very high sensitivity and specificity are preferred since they are more likely to exhibit high positive (probability that the person testing positive actually has antibodies) and negative predictive values (probability that the person testing negative actually does not have antibodies) when administered at least 3 weeks after the onset of illness.

This test is also known as serology testing. A negative serology tests result indicates no antibodies present.

Antibody test cannot diagnose current infection with covid. After infection with the COVID-19 virus or a COVID-19 vaccine, your body may take 2 to 3 weeks to make sufficient antibodies to be found in an antibody test. It may take 3 weeks for the antibodies to appear in blood and last for 2-3 months.

Antibody tests may detect some types of antibodies related to the COVID-19 virus:

Binding antibodies- It shows the antibody response of the body to  covid infection.But it doesn’t give any data about your immunity level.

Neutralizing antibodies

This test finds a subgroup of antibodies that prevent the virus from infecting your cells. It shows how well your antibodies are blocking the virus to help protect you from another COVID-19 infection.

A positive antibody test suggests:

·       You may have been infected with SARS-CoV-2, causing COVID-19.

·       You may have been infected with another  related virus from the same family of coronavirus. This is interpreted as a false positive test for SARS-CoV-2.

·       People who are vaccinated against covid virus may show positive antibody tests to the S (spike) protein. That means a positive antibody test does not always mean SARS-CoV-2 infection.

·       It does not assure that you are immune to the covid causing virus in future.

A negative test doesn’t always rule out infection with covid.

·       It typically takes 1 to 3 weeks after infection for antibodies to show up in your blood. If you are tested in the beginning stage of covid infection before antibodies are formed in the blood, the result will be shown as negative. In short, it means that you could have recently been infected with COVID-19 and still test will be negative.

·       A negative antibody test does not rule out previous infection. A proportion of persons who are infected with SARS-CoV-2 might not develop measurable antibodies, thereby limiting the sensitivity of any antibody test to detect previous infection in these individuals. Also, the extent to which seroreversion occurs varies according to the antibody test used.  In addition, measurable antibodies also can wane over time.

Antibody testing is not a replacement for virology testing [ antigen tests] for Covid infection. It should not be used to confirm the presence or absence of acute SARS-CoV-2 infection.

How much time does infection with Covid need to form antibodies in blood?

Persons suspected of having COVID-19 who test positive by direct viral detection methods for SARS-CoV-2 (e.g., NAAT or antigen detection tests) typically begin to develop measurable antibody 7–14 days after illness onset, and by 3 weeks most persons will test positive for antibody.

 During this interval, the sensitivity of detecting infection using NAAT or antigen detection testing decreases and the sensitivity of serologic testing increases.

What are the other uses of antibody tests?

Antibody testing may be useful to support the diagnosis of COVID-19 illness or complications of COVID-19 in the following conditions of covid:

·       A positive antibody test at least 7 days following acute illness onset in persons who had a previous negative antibody test (e.g., seroconversion) but did not receive a positive viral test might indicate SARS-CoV-2 infection between the dates of the negative and positive antibody tests.

·       A positive antibody test can help support a diagnosis when patients present with complications of COVID-19, such as multisystem inflammatory syndrome or other post-acute sequelae of COVID-19. This may indicate previous covid infection.

·       Antibody testing can be used for clinical and public health purposes to help differentiate antibodies produced due to past infection from those produced by vaccination by using tests that measure antibodies against different protein targets. 

 The anti-SARS-CoV-2 IgG antibody tests results interpreted in the following way:

·       In a people who are not vaccinated against Covid virus:

Testing positive for antibody against N (nucleocapsid protein), S (spike protein), or RBD (receptor-binding domain of S protein) indicates prior infection.

·       In vaccinated people:

Testing positive for antibody against the vaccine antigen target, such as the S protein, while testing negative for other antigens (e.g., N) suggests that

 A] They have produced vaccine-induced antibody.

B ] They may have never been infected with SARS-CoV-2, or they may have had a previous infection, but the N protein antibodies have since waned.

·       Testing positive for antibodies other than the vaccine-induced antibody, such as the N protein, indicates resolving or past SARS-CoV-2 infection that could have occurred before or after vaccination.

 The presence of antibodies to N protein indicates previous infection irrespective of a person’s vaccination status.

 The presence of antibodies to S protein indicates either previous infection or vaccination. Since vaccines induce antibodies to specific viral protein targets, post-vaccination antibody test results will be negative in persons without a history of previous infection if the test used does not detect antibodies induced by the vaccine.

Antibody testing is currently not recommended to assess for immunity to SARS-CoV-2 following COVID-19 vaccination.

References

1.    https://www.cdc.gov/coronavirus/2019-ncov/your-health/about-covid-19.html

2.    https://www.cdc.gov/coronavirus/2019-ncov/hcp/testing/antibody-tests-guidelines.html#Introduction

3.    Centers for Disease Control and Prevention website. COVID-19 Testing: What You Need to Know. www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/testing.html#testing-for-antibodies.