As the number of confirmed cases of COVID-19 in Maine and other states appears to plateau, scientists and public health experts are looking for ways to track people who may have had it and recovered. One way is with antibody tests, which are used to determine infections and, potentially, who may have immunity to the disease. But the antibody tests now being offered in Maine and elsewhere are far from conclusive.
In a nutshell, antibody tests, like the ones being offered by Maine health care providers, attempt to measure blood proteins that are produced by the body when it is fighting an antigen, a foreign substance such as a flu strain or the novel coronavirus.
Maine Center for Disease Control Director Dr. Nirav Shah has been fielding a lot of questions about the COVID-19 antibody tests because they were once touted as a tool that public health experts could use to track the prevalence of infections and also establish whether someone has built up immunity to the disease.
"The hope is that by detecting those antibodies we might be able to say with some degree of certainty, 'Hey, you had the coronavirus infection and we hope that you don't get it again,’" Shah said.
The promise of such tests is appealing for a multitude of reasons. For public health officials, it can help track the prevalence of the disease in a particular area and potentially the degree of immunity. And that can also inform important decisions: an older person might be able to visit their grandchildren again; a worker who closely interacts with customers might be cleared to return to the office; or an immune person may be able to donate antibody-rich blood that could help a COVID-19 patient fight off the disease.
But Elitza Theel, the Mayo Clinic's director of infectious disease, said in a recently posted video that there's a basic problem with measuring antibodies to the coronavirus.
"Because we've only really been dealing with this virus for four or five months, we don't have a good sense of the duration of that protective immunity that you're talking about," Theel said.
Shah, with the Maine CDC, also says it is unclear from the early research whether the antibody tests are actually finding antibodies to COVID-19 or something else.
"Although antibodies can be detected, we don't know yet whether the antibodies that are being detected are specific to the coronavirus that's causing COVID-19 or any of the cousins of the coronavirus," Shah says.
Cousins such as the Middle East Respiratory Syndrome (MERS) or the severe acute respiratory syndrome (SARS), which became its own pandemic in 2003.
And there are some problems with the tests themselves.
The World Health Organization (WHO) reported in a scientific brief posted last week that some trials have shown too many false positives or false negatives — bad results that can be problematic both for the person tested and the people attempting to monitor and control the virus.
That, Shah says, means the value of the antibody tests isn't yet known.
"The interpretation of those results and the clinical utility, right now there's just not enough science to know what it means," he says.
Nevertheless, some providers in Maine are beginning to offer the antibody tests, including AFC Urgent Care in South Portland.
AFC Medical Director Dr. Andre Couture says he is cautioning patients not to read too much into the test results. He says roughly three of the 100 tests AFC has conducted have come back positive. But Couture has told those positive-testing patients that they are not necessarily clear of COVID-19, much less immune to it.
"We just don't know if that (a positive result) confers you a short period of immunity, or a long period of immunity, or any immunity," Couture says. "So, it's really important to be very explicit with patients about what the tests can show us and what it can't."
Couture says AFC and other providers conducting antibody tests are required to send their positive results to the Maine CDC. And Shah says that right now, those results are being recorded in a new category – “probable” cases of COVID-19, but not “confirmed.”
Originally published 4:14 p.m. May 1, 2020