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Spokane, Washington  Est. May 19, 1883

Are there blood tests to measure immunity?

A patient has her blood drawn at a hospital in Philadelphia.  (Jacqueline Larma/Associated Press)
By Lindsay Bever Washington Post

Is there a test to measure immunity to COVID-19? That is, can we tell if the vaccine took or if we’ve developed natural immunity? – Chad in Arizona

Yes, scientists can absolutely measure antibodies produced by the coronavirus vaccine and natural infection from COVID-19. There are even fancy tests that researchers can use to distinguish between those produced by vaccines and the actual virus.

But because scientists are still learning about the virus and the immune system is such a complex machine, an antibody test might not tell the whole story. That is, it might not show whether a person is protected or how protected that person is from the disease, said William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health.

Here’s why:

First, commercial antibody tests might not be searching for the antibodies that protect against infection. Moss said many tests done in commercial laboratories measure a variety of antibodies but not necessarily the ones that stop the virus.

Neutralizing antibody tests measure the neutralizing antibodies that bind to the spike protein and block the virus from entering healthy cells, thereby keeping it from replicating in the body, he said. But, he said, these tests are not widely available.

Second, although scientists have a clearer grasp these days of what level of immunity is protective, there is no magic number.

“There is a strong correlation between the levels of neutralizing antibodies and protection, but we’ve not yet defined a cutoff, and I’m not sure we’ll be able to,” Moss said.

“What we would like to be able to say is that antibody levels above a certain threshold are protective and that antibody levels below a certain threshold are not protective. But biology is not that simple,” he added.

And, in any case, antibodies are only part of it.

The immune system also produces memory cells, white blood cells known as memory B cells and protective T cells. So even when antibody levels wane over time, the immune system can still call on those memory cells to respond – B cells start making more antibodies while T cells work to attack and kill other cells infected with the virus.

So, yes, you can get a blood test to see whether you have antibodies against the coronavirus, but it might not tell you much more than that.

That’s why the Centers for Disease Control and Prevention and many medical experts agree that antibody testing is not recommended to assess immunity.