How are antibodies’ strength tested?

Each week, Chronicle health reporters field questions about the latest on COVID-19, vaccines and pandemic living. In this week’s COVID Help Desk, we explain how coronavirus antibodies work in the body, the waiting period after a person is infected before they can take the vaccine and the accuracy of COVID tests.

Is there a way to test COVID antibody strength?

The latest vaccine data shows antibodies drop off fairly steeply in the first few months after the second dose. Then they plateau and other white blood cells become more stable at fending off infection, said Ben Neuman, a virologist with Texas A&M University.

Antibody numbers and strength can differ from person to person, Neuman said.

“In a group, there will be someone’s immune system who got it and made a great response and someone’s who didn’t. They’re the same age, and you can’t tell why without doing some kind of test,” Neuman said. “Each individual antibody either sticks, or it doesn’t.”

If the antibody sticks to part of the virus, it will prevent infection. While

there are ways to laboratory-test how many antibodies a person has, it’s difficult to count how many still work at preventing infection — especially from contagious variants like delta, he added.

In tests that count the number of vaccine antibodies that bind to a coronavirus spike protein, new research shows the number is “going down a little” with delta, Neuman said.

“Any test that counts antibodies is looking at affinity,” Neuman said. “But it’s built around the original version of the spike (protein). Potentially, we do need a panel of tests that are looking at how antibodies bind to different spikes since spikes differ from variant to variant.”

Protein spike changes are the part of the virus that mutates the most, he said.

My friend has COVID right now. How soon can he get the shot?

If you have been sick from COVID, you will have natural immunity for about three months after the initial infection, said Dr. Annamaria Macaluso Davidson, associate vice president of medical operations at Memorial Hermann Medical Group. But doctors still recommend taking the COVID vaccine as soon as it’s safe for you to do so.

Macaluso Davidson said the vaccine creates a more robust immune response.

“With different variants, there’s a wider range of antibodies to offer protection,” she said. “Different studies show vaccines offer longer (immune) memory, antibodies lasting longer and more robust antibody protection.”

People who have active COVID symptoms should wait until they’ve fully recovered and are out of isolation before considering making a vaccine appointment at a pharmacy, clinic or their doctor’s office, said Dr. Galant Chan, infectious disease physician at Baylor St. Luke’s Hospital.

A minimum of 10 days after your symptoms started and 24 hours after any fever goes away is the appropriate time to wait before taking the first shot, Chan said.

If you were treated with monoclonal antibodies or convalescent plasma therapy, you should wait at least 90 days to take the vaccine, she added.

Why did I test positive one day and negative the next for COVID?

With school starting, regular COVID testing will likely become more common for adults and children. But there is a test hierarchy.

If you have been exposed to a person with COVID, the right time to be tested is within three to five days. You can test positive for up to two weeks after the exposure.

A molecular PCR test looks for genetic material and is considered to be the most accurate test to diagnose an active COVID infection, Chan said. Because the test is specific, a positive reading usually means the person has COVID.

Rapid antigen tests look for the virus protein. These can sometimes give inaccurate negative readings, Chan said. If you take a rapid test at a pharmacy or at home and it’s negative, make an appointment for a confirmatory PCR test — especially if you have symptoms or have been in close, unmasked contact with someone who has the virus, she added.

If the rapid antigen tests shows a positive reading, it is highly likely the person has an active infection. Again, a followup PCR test will confirm the diagnosis.

While PCR tests are more accurate, antigen test results come back faster — 15 minutes or so. Newer PCR tests have a turnaround time of 45 minutes to a few hours, Chan said.

Most at-home kits approved by the U.S. Food and Drug Administration are antigen tests. The quality of the nasal swab sample is important, so follow the directions on the at-home kit as strictly as possible.