By: The New York Times

Declining antibody levels do not mean less immunity, experts say. Besides, two widely used tests may detect the wrong antibodies.

The most powerful antibodies recognize a piece of the coronavirus’s spike protein, the receptor binding domain, or R.B.D. That is the part of the virus that docks onto human cells. Only antibodies that recognize the R.B.D. can neutralize the virus and prevent infection.

But the Roche and Abbott tests that are now widely available — and several others authorized by the Food and Drug Administration — instead look for antibodies to a protein called the nucleocapsid, or N, that is bound up with the virus’s genetic material.

Some scientists were stunned to hear of this choice.

“God, I did not realize that — that’s crazy,” said Angela Rasmussen, a virologist at Columbia University in New York. “It’s kind of puzzling to design a test that’s not looking for what’s thought to be the major antigen.”

The N protein is plentiful in the blood, and testing for antibodies to it produces a swifter, brighter signal than testing for antibodies to the spike protein. Because antibody tests are used to detect past infection, however, manufacturers are not required to prove that the antibodies their tests seek are those that actually confer protection against the virus.

There’s another wrinkle to the story. Some reports now suggest that antibodies to the viral nucleocapsid may decline faster than those to R.B.D. or to the entire spike — the really effective ones.

“The majority of people are getting tested for anti-N antibody, which does tend to wane more rapidly — and so, you know, it may be not the most suitable test for looking at neutralizing capacity,” Dr. Iwasaki said.

Read more: https://www.nytimes.com/2020/07/26/health/coronvirus-antibody-tests.html