SARS-CoV-2 Antibodies: Test Guide

What are SARS-CoV-2 antibodies

SARS-CoV-2 antibodies are immune system proteins that appear after COVID-19 infection or vaccination. The test shows whether the body has mounted an immune response against the SARS-CoV-2 virus. The most commonly measured are IgG-class antibodies against the viral S (spike) or N (nucleocapsid) protein.

The test is performed from venous blood. The result is typically expressed in BAU/mL (Binding Antibody Units) — a standardized WHO unit that allows comparison of results across different laboratories.

Reference ranges

Specific cutoffs depend on the laboratory's test kit. A positive/negative threshold is usually indicated (e.g., above 0.8 BAU/mL is considered positive). A quantitative result shows the relative antibody level, but there is no universally agreed protective threshold that would guarantee immunity from infection.

When might the result be positive

• After a past COVID-19 infection — antibodies appear within 1–3 weeks.
• After vaccination — especially anti-S antibodies, which form in response to the vaccine.
• After both natural infection and vaccination — hybrid immunity typically produces higher titers.

A higher numerical result generally indicates a stronger immune response, but it does not directly reflect the degree of protection against infection.

When might the result be negative

• The person was not infected and was not vaccinated.
• The test was performed too early after infection or vaccination — antibodies have not yet had time to form.
• Antibodies have already declined after a long period — this is a natural process and does not mean complete loss of immunity, since T-cell memory persists.
• Immunosuppression — some patients do not produce sufficient antibodies.
• The wrong antibody class was tested (e.g., anti-N antibodies were tested after vaccination, which does not stimulate their production).

What to do with the results

The SARS-CoV-2 antibody test is used not for diagnosis but for assessing the immune response. It cannot confirm or rule out an active infection — a PCR test is used for that purpose.

If the result is positive after vaccination, it indicates that the immune system recognized the vaccine antigen. A negative result after vaccination may be a reason to consult your doctor, especially if you belong to a risk group.

Monitoring over time

Antibody levels naturally decline over several months after infection or vaccination. This is normal and does not mean complete loss of immunity. Repeat testing can be useful for immunosuppressed patients to assess the response to vaccination.

At present, routine antibody monitoring is not recommended for most people.

Questions for your doctor

1. Does my antibody level indicate a sufficient immune response? 2. Should I have a repeat test after vaccination? 3. In my case, are there indications for an additional booster dose? 4. What is the difference between anti-S and anti-N antibody tests, and which is more relevant for me?