COVID-19 Serology Antibody Test

Dr. ZEE Sze Tsing, Jonpaul

Specialist in Infectious Disease

With the ongoing COVID-19 pandemic, the Government continues to call on citizens to get vaccinated as early as possible.

An immune response usually arises after the completion of 2 doses of vaccination. Antibody testing can ascertain if an immune response occurs after COVID-19 vaccination, with blood samples being collected for serological testing at least 2 to 4 weeks after the second dose.

What is Antibody?

Antibodies, also known as immunoglobulins, are proteins produced by the immune system to fight against invasion of pathogens. There are different types of immunoglobulins, e.g. Immunoglobulin M (IgM) and Immunoglobulin G (IgG). In the context of viral infection, IgM is first produced by the immune system for a rapid, short-term protection, followed by the production of IgG for a more potent, long-term protection.

The presence of a specific IgM thus usually indicates one as being “currently infected” or “recently infected”, while the presence of IgG means one has been “previously infected” or has “attained immunity”.

Neutralising antibodies refers to immunoglobulins that can reduce the infectivity of virus by binding to a specific site to block the virus from entering human cells and cause infection.

Positive? Negative?

According to the current laboratory testing standard in HKSH, the result of COVID-19 antibody test is deemed positive when COVID-19 IgG level reaches 50 AU/mL or above. Neutralising antibody testing is also available to give a better picture of one’s immunity against COVID-19.

A positive antibody result in serological testing may indicate prior natural infection or an immune response to vaccination. It remains unknown as to the minimum level of antibody required to confer protection against future infection. The presence of antibody does not completely preclude future infection, especially by the variant strains.

Antibody Level and Protection

Vaccine response is affected by a myriad of factors, e.g. a lower response is noted in the elderly. Other factors include chronic illnesses, e.g. kidney disease, liver disease, cancers, and use of immunosuppressive medications will also result in a lower response.  Most of the people with normal immunity will mount an adequate immune response after completing two doses of vaccination.

Immune memory is generated after natural infection. It means a single dose of vaccination suffices to induce an adequate antibody level in those who have recovered from COVID-19 infection.

Currently, there is insufficient data to suggest what is the optimal antibody level that can best protect us from infection. There are other components of our immune system that are also important in fighting against COVID-19. Furthermore, antibody level may wane after vaccination, it is important to maintain social distancing and other precaution measure after vaccination.          

The Clinical Pathology Division of Hong Kong Sanatorium & Hospital is one of the local testing institutions recognised by the HKSH SAR Government toconduct COVID-19 Serology Antibody Test. For appointment, please visit https://covid.hksh.com.