The two main types of antibodies relevant to Covid-19 are immunoglobulin M, which the body generates during an active infection but that gradually dissipate, and immunoglobulin G, which are long-term antibodies. But Dr. Wilson emphasized: “The mere presence of antibodies does not mean that those antibodies are protective.”
Of the majority of people who do make antibodies, are they so-called neutralizing antibodies that can stop the virus from multiplying? In a laboratory study of plasma from 111 recovered Covid-19 patients treated at Rockefeller University Hospital in New York, 33 percent had too few neutralizing antibodies to prevent a reinfection. “The results here were not so encouraging,” Dr. Wilson observed.
To help both medical professionals and lay individuals make the best possible use of Covid tests, the Center for Infectious Disease Research and Policy at the University of Minnesota compiled a document on “smart testing to help ensure that the right test is given to the right person at the right time, with test results provided in a timely manner to allow for actions that minimize illness, deaths, and transmission.”
Dr. Osterholm, who directs the center, explained that accurate testing requires the use of tests that are least likely to result in either false-negative or false-positive results. He characterized some of the drive-in tests that people now seek as “terrible,” generating lots of false-negative and false-positive results, in part because of how samples are collected.
There are two critical hallmarks of a reliable test that can detect true-positive and true-negative results: sensitivity and specificity. Sensitivity refers to “how well it correctly identifies those who have the virus infection or antibodies,” the center’s document explains. “A test with low sensitivity will produce more false-negative results compared with a test that has higher sensitivity.”
Specificity of a test refers to “how well it correctly identifies those who do not have” a Covid-19 infection or antibodies to it. Early in the pandemic in this country, the test endorsed by the White House was insufficiently sensitive and specific, potentially missing a significant number of infections, the Food and Drug Administration reported mid-May.
You might think any test with 95 percent sensitivity and 95 percent specificity would be highly accurate. But while these would be great grades on an organic chemistry final, the ability of such a test to render a reliable result is extremely poor: 50 percent of the positive results would not be true positives, Dr. Osterholm said. (You’ll have to take my word for this — explaining the statistics would require half a column!)