The immune system produces specialized proteins called antibodies to combat foreign invaders like viruses. When the SARS-CoV-2 virus, responsible for COVID-19, enters the body, it triggers an immune response. This article explains SARS-CoV-2 spike antibody tests and how to interpret their results.
Understanding SARS-CoV-2 Spike Antibodies
SARS-CoV-2 spike antibodies are proteins generated by your immune system in response to the virus’s spike protein. This distinctive structure on the virus surface allows it to attach to and enter human cells. After exposure, either through natural infection or vaccination, specialized immune cells produce these antibodies. Vaccines, for example, prompt your body to create the spike protein, leading to antibody production. These antibodies then neutralize the virus by blocking its ability to infect cells, helping to protect you from illness.
Purposes of Spike Antibody Testing
SARS-CoV-2 spike antibody tests serve several purposes. They can determine if someone had a prior SARS-CoV-2 infection, especially if symptoms were mild or absent. These tests reveal evidence of a past immune response, even without prior molecular testing.
Testing also assesses the immune response after COVID-19 vaccination. Most vaccines elicit antibodies against the spike protein, confirming an immune reaction. Antibody tests differ from diagnostic tests like PCR, which detect an active viral presence for current infection diagnosis.
Interpreting Your Spike Antibody Test Results
SARS-CoV-2 spike antibody test results are commonly reported as positive/reactive, negative/non-reactive, or as quantitative values. A positive or reactive result indicates spike antibodies were detected, suggesting a past infection or vaccination. Conversely, a negative or non-reactive result means no detectable spike antibodies were found, which might occur if you haven’t been infected or vaccinated, or if it’s too early for antibodies to have developed.
Quantitative values appear in units like AU/mL (Arbitrary Units per milliliter) or BAU/mL (Binding Antibody Units per milliliter). These numerical values indicate the amount or concentration of antibodies present. For instance, < 0.8 U/mL might be considered non-reactive, while higher values like > 200 IU/mL (equivalent to ≥40 AU/mL) could indicate a strong positive response.
Direct comparison of quantitative results between different laboratories is challenging due to variations in testing methods and reporting units. A specific numerical value reflects the immune response but does not currently indicate immunity to reinfection or protection against the virus.
What Spike Antibody Levels Indicate About Protection
The relationship between detected spike antibody levels and protection against SARS-CoV-2 is an ongoing area of study. While higher antibody levels generally correlate with better protection against symptomatic infection, it’s not an absolute guarantee against future infection or severe disease, especially with new viral variants. Research suggests the risk of symptomatic or severe SARS-CoV-2 infection within 12 months decreases as observed antibody levels increase.
A “correlate of protection” refers to a measurable immune response that reliably predicts protection from infection or disease. While antibody levels are significant, a precise threshold for SARS-CoV-2 that guarantees immunity is not yet fully established. The immune system also relies on cellular immunity, involving T-cells, which identify and eliminate infected cells, providing another layer of defense.
Considerations and Limitations of Spike Antibody Tests
Several factors can influence SARS-CoV-2 spike antibody test results. The timing of the test relative to infection or vaccination is important, as it takes time for antibodies to develop and become detectable, typically at least 14 days after exposure or vaccination. Testing too early may yield a negative result even if an immune response is developing.
Different types of antibody tests exist; some detect only spike antibodies (anti-S), while others detect antibodies to the nucleocapsid protein (anti-N). Anti-N antibodies usually suggest a past natural infection, whereas anti-S antibodies can result from either infection or vaccination.
Antibody levels can also wane over time, meaning a lower level might not reflect a complete lack of past immune activity. The effectiveness of antibodies can also be impacted by different SARS-CoV-2 variants, as some variants may be less susceptible to existing antibodies. Therefore, antibody test results should be considered alongside your overall health, vaccination history, and any known exposures, ideally discussed with a healthcare professional.