Got Vaccinated = Got Antibody = Got Protection? - Dr. Jonpaul ZEE Sze Tsing

Q1. What is antibody? What do IgG, IgM and neutralising antibody mean?
Antibodies, also known as immunoglobulins, are proteins produced by the immune system in response to an infection. There are Immunoglobulin M (IgM) and Immunoglobulin G (IgG). IgM is usually the first antibody produced by the immune system when a virus attacks and plays a short-term role. IgG is then produced for long term protection. Therefore, positive IgM level usually indicates “currently infected” or “ recently infected”, while positive IgG level usually means “previously infected” or “attained immunity”.

Neutralising antibodies refer to immunoglobulins that have the ability to reduce the infectivity of the virus by binding to a specific site to block t the virus from entering and infecting human cells.

Q2. What is the purpose of antibody testing after COVID-19 vaccination? When should I have the test?
The purpose of antibody testing is to ascertain if there is an immune response following COVID-19 vaccination. Blood samples should be collected for serological testing 2 to 4 weeks after the second dose of vaccination.

What is the cutoff for positive SARS-CoV-2 antibody test? What does positive antibody test mean in terms of immunity?

According to the laboratory testing standard currently applied in Hong Kong Sanatorium & Hospital, COVID-19 antibody test is regarded as positive when COVID-19 IgG level reaches 50 AU/mL or above. However, different laboratories have different equipment and measurement standards for positive and negative antibody levels.

Neutralising antibody testing can help us better understand the immunity level of people who have received vaccinations. According to the latest research, positive antibody level can only reflect the immunity level, it is yet unknown how much protection antibodies might provide against future infection.

Q4. Which are the factors affecting the antibody level?
Many factors affect the vaccine response. For example, elderly usually have a lower response to the vaccination. People with chronic illnesses such as liver disease, kidney disease, cancers, or taking immunosuppressive medications will have impaired responses to the vaccination.

Since immune memory is generated after natural infection, one-dose vaccination will induce adequate antibody for people who have recovered from COVID-19 infection. For people with normal immunity, a favourable antibody level can usually be achieved after two-dose vaccination.

Q5. How long does the vaccine-induced immunity last?
There are many factors that may affect the duration of protection offered by vaccination, such as general health, immune status of an individual as well as circulation of variant virus in the community. Also, antibody level may wane after vaccination, it is important to maintain social distancing and other precautions after vaccination.

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