But to get a reliable answer from testing, vaccinated people have to get a specific kind of test, and at the right time.

Take the test too soon, or rely on one that looks for the wrong antibodies — all too easy to do, given the befuddling array of tests now available — and you may believe yourself to still be vulnerable when you are not.

But antibody tests can be crucial for people with weak immune systems or those who take certain medications — a broad category encompassing millions of people who are recipients of organ donations, have certain blood cancers, or who take steroids or other drugs that suppress the immune system.

Early in the pandemic, many commercial tests were designed to look for antibodies to a coronavirus protein called the nucleocapsid, or just N, because after infection, those antibodies were plentiful in the blood.

More important, antibodies to the N protein are not produced by the vaccines authorized in the United States; instead, those vaccines provoke antibodies to another protein sitting on the surface of the virus, called the spike.

If people who were never infected are vaccinated and then are tested for antibodies to the N protein instead of to the spike, they may be in for a rude shock.

He realized it was a test for antibodies to the N protein, not to the spike.

In May, the Food and Drug Administration recommended against the use of antibody tests for assessing immunity — a decision that has drawn criticism from some scientists — and provided only bare-bones information about testing to health care providers.

A certain type of lab test, called an Elisa test, may offer a semi-quantitative estimate of antibodies to the spike protein.

It’s also important to wait to be tested at least two weeks after the second shot of the Pfizer-BioNTech or Moderna vaccines, when antibody levels will have risen enough to be detectable.

In November, the World Health Organization set standards for antibody testing, allowing for comparison of different tests.

For example, a transplant patient with poor antibody levels might be able to use test results to convince an employer that he or she should continue to work remotely