With more than 30 million people in the United States at least partially vaccinated against covid-19, you may wonder whether the shot has done its duty, arming your immune system to fight off infection. Or whether the vaccine is needed at all, particularly if you have had the coronavirus.

But health experts say antibody tests — the tests designed to detect proteins created by the immune system that protect against the virus — are not necessary and can be unreliable.

“Don’t try to second-guess the vaccine. Just get vaccinated,” said Sarah Fortune, chair of the department of immunology and infectious diseases at Harvard’s T.H. Chan School of Public Health.

Should I get an antibody test to see whether I need the vaccine?

Infectious-disease expert Rob Murphy explained that immunity provided by the antibodies is a “mixed bag” — although some people who have had covid have kept their immunity, others have lost it.

Also, preliminary data shows that such immunity may not protect against the new variant first identified in South Africa at all.

Because no one knows how long natural immunity lasts, vaccination is crucial, Murphy said.

Aside from that, the vaccine may be more a potent immunogen than immunity from the actual virus, offering better protection against it, said Murphy, executive director of the Institute for Global Health and a professor of medicine and biomedical engineering at Northwestern University.

The bottom line is that antibody testing “for the purposes of vaccine decision-making” is not advisable, according to the Centers for Disease Control and Prevention.

How about an antibody test to determine if the vaccine was effective?

The CDC discourages antibody testing for assessing immunity after getting the vaccine. In clinical trials, the Pfizer-BioNTech and Moderna vaccines were shown to protect people against the virus 95 percent of the time and 94 percent of the time, respectively. But that does not mean everyone who gets the vaccine would test positive for antibodies, Murphy said.

Commercial antibody tests may not be searching for the same antibodies that the vaccine triggers. Someone who has had the vaccine and is immune to the virus may still test negative for certain antibodies, Murphy explained.

In any case, Murphy added, antibodies are only part of a person’s immune system response. Some studies have shown that protective T cells, a white blood cell that helps protect against disease, have been elevated in some patients with covid, even though the patients had no detectable antibodies.

“After the vaccines, a lot people are going to get antibody testing — ‘Oh, I want to see if it’s working.’ It actually has very little correlation,” he said. “Many people will test negative on the antibody test, and that does not mean the vaccine didn’t work.”

Then when should I get an antibody test?

Health experts say antibody testing is useful in assessing the prevalence of the disease in the community to inform public health officials.

For individuals, Murphy said the tests “can tell you about past exposure to the virus.”

“But it doesn’t change anything else. It doesn’t change the fact that you can get infected again from the regular virus, and it may not offer you enough protection from these variants that are out there,” he said.

So, when it comes to the vaccine, he said, testing is useless. “Even if you’re positive [for covid antibodies], you would still get the vaccine. If you’re negative, you would get the vaccine. And then after the vaccine, it really doesn’t matter whether that antibody test is positive or negative. It doesn’t help you at all.”

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