COVID-19 vaccine antibody test

COVID-19 vaccine antibody test

Updated on 22 July 2021

Recent update on quarantine policy

The Hong Kong government has recently announced that quarantine from other countries can be shortened to 7 days post-arrival who could present a positive antibody test (test completed in Hong Kong within 3 months), also with valid Covid-19 vaccination record, in addition to a negative PCR test. 

What is a vaccine antibody test (also known as “immunoglobulin test)?

A vaccine antibody test, also known as a vaccine immunoglobulin test, is conducted to check whether individuals have developed antibodies after being vaccinated.

 

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What is the difference between an antibody versus antigen test (also known as a “viral test”)?
What is the difference between an antibody versus antigen test (also known as a “viral test”)?

Difference between an antigen and antibody test

Antigen:  The proteins that exist on the outer shell of the virus itself.

Antibody: Produced by the human body itself against those specific antigens.

So in short, the COVID-19 Antigen test is to test whether the person has a presence of the antigens (protein fragments) that are specific to COVID-19.  If tested positive, it means that the person most likely does have an active infection by COVID-19 virus (SARS-CoV-2) and can infect others. However, there is always a chance to have a “false positive” which means the person does not carry the virus.

Antibodies test, as mentioned earlier is to detect the presence of the antibody within the human body. Showing a positive antibody result means the person either HAD an infection before or have been vaccinated.  (We can always refer to the table in another article in detail.)

How many different types of antibody tests are available?
How many different types of antibody tests are available?

Different types of antibody tests available

The body makes different antibodies, or immunoglobulins, to fight the different viruses. For example, the antibody for Covid-19 isn't the same as the antibody for influenza (seasonal flu).

There are 5 types of antibodies inside our bodies:

 

Immunoglobulin A (IgA)

Immunoglobulin G (IgG)

Immunoglobulin M (IgM)

Immunoglobulin E (IgE)

Immunoglobulin D (IgD)

Found in

 

Linings of the respiratory tract and digestive system, as well as in saliva (spit), tears, and breast milk

 

 

Blood and other body fluids

 

Mainly in blood and lymph fluid

 

Normally found in small amounts in the blood 

Small amounts in the blood 

 

Remark

 

N.A.

Most common antibody. These antibodies protect you against infection by "remembering" which germs you've been exposed to before. If those germs come back, your immune system knows to attack them

The first antibody which body makes when fighting against a new infection. When your body has an infection, your IgM level will rise for a short time. It will then begin to drop as your IgG level kicks in and increases to protect you long-term 

The amount may increase when the body overreacts to allergens or is fighting an infection from a parasite

 

Least understood antibody

 

 

Among all, IgG and IgM antibody tests are often used for Covid-19 testing.     

What is the difference between a Covid-19 IgG antibody versus an IgM antibody test?
What is the difference between a Covid-19 IgG antibody versus an IgM antibody test?

Difference between a Covid-19 IgG antibody versus an IgM antibody test

Below table is basic summary on the IgM and IgG test:

 

Immunoglobulin M (IgM)

 Immunoglobulin G (IgG)

Test type

Blood test 

Test object

IgM antibodies 

IgG antibodies

Test result

·         Positive or negative 

·         Level of antibodies  

Appear since

Initial stage when exposed to a virus 

Later stage when exposed to a virus

General result indication  (if positive)

Recent infection or vaccinated

Past infection or vaccinated

Implication for Covid-19 (regarding quarantine policy at Hong Kong) 

 

To verify whether a patient has been vaccinated for Covid-19

 

Individual with positive result, along with other conditions, may shorten quarantine period after arrival at Hong Kong. Click here for details

 

Indeed, both IgM and IgG antibody tests are blood test.

However, an IgM test is to detect the IgM antibodies. IgM usually is the first antibody produce by the immune system when a virus attacks (or after being vaccinated). A positive result indicated that you may still be infected or that you have recently been vaccinated and your system just started to respond towards the vaccination.

On the other hand, an IgG test is designed to detect IgG antibodies that are specifically for the virus. IgG is usually used to determine that a person has developed the antibodies that fight the virus. 

Commonly asked questions
Commonly asked questions

Commonly asked questions regarding Covid-19 antibody test

  1. Who should get a Covid-19 antibody test?
  • If an individual would like to know his/her antibody level
  • Individuals who would like to be subjected to a shortened compulsory quarantine period after arrival in Hong Kong

For more information, please go to https://www.coronavirus.gov.hk/eng/inbound-travel.html#faqs

  1. When should a person get the antibody test after being vaccinated?

For a vaccinated individual, it is recommended to have the test done only after two weeks of vaccination.

  1. Does a positive IgG antibody test mean a person is immune to the COVID-19?

No, although the presence of antibodies in a person body provide some immunity to the COVID-19 virus, currently there isn’t enough evidence to know how long the antibodies last.   

  1. Does a low IgG antibody test result mean the Covid-19 vaccination isn’t working?

No, the result does not necessarily reflect the protection level because you may still have protection even the antibody level is low.

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