As millions of Americans consider rolling up their sleeves to get a booster shot of the COVID-19 vaccine, it makes sense many want to know what their level of immunity is before and after getting the vaccine. For that reason, many people are requesting antibody blood tests to measure their levels of immunity.

However, unlike other vaccines, experts say it’s not so simple to measure the level of protection provided by getting a simple blood test to measure antibodies.

According to NPR, these standard antibody tests are not recommended by the Food and Drug Administration (FDA). The FDA says “currently authorized SARS-CoV-2 antibody tests should not be used to evaluate a person’s level of immunity or protection from COVID-19 at any time, and especially after the person received a COVID-19 vaccine.”

The FDA says that the antibody blood tests do detect antibodies from prior COVID-19 infection, but they have not been researched sufficiently for their accuracy in measuring immune response from vaccinations. The antibody tests were part of studies that determined how quickly the efficacy of vaccines waned, but scientists say that the basic antibody blood tests do not give you the whole picture of how your body will respond to the virus.

“We have a mix of antibodies,” explains Ali Ellebedy, an associate professor of pathology and immunology at Washington University School of Medicine. “Some of them are very good and protective. Some are not as protective.”

Studies have shown that all three shots available in the U.S. — the Pfizer, Moderna, and Johnson & Johnson — can effectively neutralize the COVID-19 virus, including the Delta variant. Natural infection also boosts immunity, although a recent study by the Centers for Disease Control and Prevention (CDC) stated that vaccines offer superior protection.

In the CDC study published in Morbidity and Mortality Weekly Report researchers examined more than 7,000 people across nine states hospitalized with COVID-like illness. The study authors found that those who were unvaccinated and had a recent infection were five times more likely to have COVID than those who were fully vaccinated and didn't have a prior infection.

"We now have additional evidence that reaffirms the importance of COVID-19 vaccines, even if you have had prior infection," CDC director Dr. Rochelle Walensky said in a statement.

Experts point out that antibodies are just one part of the entire story of immune response from vaccines. Your immune system kicks into action if your body encounters the virus, says NPR, producing new antibodies to attack the pathogen and generating T cells that help remove infected cells.

Some antibody tests only look for IgG antibodies that are specific for nucleocapsid protein of the virus that causes COVID-19 and appear in response to the infection. These are helpful if your health care professional suspects you’ve had, or have been exposed to, COVID-19.

The spike protein IgG antibody test is designed to detect antibodies specific for the virus spike protein that develops once a person has received a COVID-19 vaccination. These tests confirm that the vaccinated person has developed the antibodies that offer protection against severe illness from the disease.

But having an antibody test could give you some indication of your level of immunity, says Dr. Nicole Bouvier, a specialist in the diagnosis and treatment of infectious diseases at the Icahn School of Medicine at Mount Sinai.

“That can give you a benchmark for the immune response to natural infection, and then you can sort of gauge your vaccine response against that,” she said. And experts suggest that if someone has no detectable antibodies on a test, even if they still have some protection from T-cells, they may not be as well protected as someone who has an antibody level of 1,000.