Antibodies are specialized proteins produced by the immune system to identify and neutralize foreign invaders like viruses and bacteria. When the body encounters a virus such as SARS-CoV-2, which causes COVID-19, immune cells recognize parts of the virus as antigens. In response, the body generates specific antibodies designed to bind to these antigens, helping to either slow down or stop the infection.
How COVID Antibody Levels Are Measured
Measuring COVID antibody levels involves specific tests that look for different types of antibodies, primarily those targeting the spike (S) protein or the nucleocapsid (N) protein of the SARS-CoV-2 virus. Spike protein antibodies are typically generated in response to COVID-19 vaccination or natural infection, as vaccines commonly use the spike protein to elicit an immune response. Nucleocapsid protein antibodies, however, are generally produced only after a natural infection with the virus, making them a marker for past infection rather than vaccination.
Different laboratories and testing platforms use various units of measurement to report antibody levels, which can make direct comparisons challenging. Common units include Binding Antibody Units per milliliter (BAU/mL) or an index value, with conversion factors sometimes applied to align with the World Health Organization (WHO) international standard for anti-SARS-CoV-2 immunoglobulin.
Interpreting Antibody Test Results
Understanding COVID antibody test results requires recognizing that no single numerical “high level” universally guarantees protection from the virus. A “positive” result from an antibody test indicates that a person has developed an immune response, which could be due to a past COVID-19 infection or a response to vaccination.
Conversely, a “negative” result suggests that detectable levels of specific SARS-CoV-2 antibodies were not present at the time of testing. Some labs provide specific cutoff values for positivity, such as greater than or equal to 0.8 U/mL or an index value of 1.00, but these are thresholds for detection, not indicators of a protective level of immunity. It is important to remember that antibody tests are not used to diagnose a current infection, and results should always be reviewed by a healthcare professional for proper context.
The Link Between Antibody Levels and Immunity
Having detectable antibodies generally indicates that the immune system has encountered the SARS-CoV-2 virus or a vaccine. While studies show a correlation between higher antibody levels and a reduced risk of infection, this does not provide an absolute guarantee of protection against future illness. Antibody levels naturally decrease over time, a process known as “waning immunity,” which is a normal occurrence for many vaccines and infections.
Antibodies are only one component of the body’s complex immune defense against COVID-19. Other elements, such as T-cells and memory B-cells, also play a significant role in providing longer-term protection. T-cells can recognize and eliminate infected cells, while memory B-cells can quickly produce new antibodies upon re-exposure to the virus. Standard antibody tests typically do not measure these cellular components of immunity, meaning a lower antibody level does not necessarily indicate a complete loss of protection.
Antibodies from Vaccination Versus Natural Infection
The source of immunity, whether from vaccination or natural infection, generally influences the specific types of antibodies produced. COVID-19 vaccines are designed to generate an immune response primarily against the SARS-CoV-2 spike protein. Therefore, individuals who have been vaccinated typically develop antibodies that target this spike protein.
In contrast, a natural infection with the SARS-CoV-2 virus usually leads to the production of antibodies against multiple viral proteins, including both the spike protein and the nucleocapsid protein. This distinction means that if a person tests positive for nucleocapsid antibodies, it is generally an indication of a past natural infection, regardless of vaccination status. Understanding these different antibody profiles can help differentiate the origin of a person’s immune response to COVID-19.