Antibody test type, timing key in gauging immunity

Now that tens of millions of Americans are vaccinated against the coronavirus, many are wondering: Do I have enough antibodies to keep me safe?

For a vast majority of people, the answer is yes. That has not stopped hordes from stampeding to the local doc-in-a-box for antibody testing. 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.

Actually, scientists would prefer that the average vaccinated person not get antibody testing at all, on the grounds that it is unnecessary.

In clinical trials, the vaccines authorized in the United States provoked a strong antibody response in virtually all of the participants.

“Most people shouldn’t even be worrying about this,” said Akiko Iwasaki, an immunologist at Yale University.

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 take steroids or other drugs that suppress the immune system.

There is mounting evidence that suggests a significant proportion of these people do not produce a sufficient antibody response after vaccination.

If you must get tested or just want to, it is essential to get the right kind of test.

“I feel a little bit hesitant to recommend everybody getting tested, because unless they really understood what the test is doing, people might get this wrong sense of not having developed any antibodies,” Iwasaki said.

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.

But these antibodies are not as powerful as those required to prevent virus infection, nor do they last as long.

More importantly, 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 the spike, they may be in for a rude shock.