Early results from antibody testing have prompted renewed speculation about the nature and severity of the coronavirus, but epidemiologists warn the results should be interpreted with extreme caution.
“These point of care instruments need to be considered thoughtfully before just going and using them out in the population,” said Dr. Michael Mina, professor of epidemiology at Harvard T.H. Chan School of Public Health.
Antibody tests look at blood samples to find whether a person has been infected with the coronavirus and has built up antibodies in the immune system to fight off future infection. They differ from the majority of tests conducted since the coronavirus pandemic began, known as PCR tests, which detect if the virus is active in a person’s system and can be spread to other people.
Public health officials have said that antibody tests will be a key part of the government response to the pandemic because they allow epidemiologists to track the spread and prevalence of the virus beyond the number of people ailing or dying. In particular, they promise to give researchers a sense of how the disease spreads asymptomatically.
“I’ve been talking about the level of asymptomatic spread and my concern about asymptomatic spread,” coronavirus task force response coordinator Dr. Deborah Birx said at the White House earlier this month. “Because with flu and other diseases, when people are sick, it’s easy to contact trace. When people are not sick and shedding virus, you have to have a very different approach.”
In recent weeks, the first batches of antibody testing results have come in, most notably in New York, where Gov. Andrew Cuomo has discussed the findings in daily briefings.
The findings have led some commentators to assert sweeping conclusions about the virus, including the suggestion that it is much less dangerous than previously thought, but epidemiologists have cautioned that those assertions are premature.
For example, New York Gov. Andrew Cuomo announced last week that antibody testing showed 1 in 7 New York residents, roughly 2.7 million people, had been infected with the virus at some point without showing symptoms. The figures have convinced many that the death rate is vastly lower than what was believed.
Epidemiologists caution, though, that the results might not reflect the entire state’s population. The test results from New York in question were gathered from testing people at big-box retailers and grocery stores, but the sample was relatively small, with about 3,000 people.
The sample excluded large factions of the New York population, said Dr. Danielle Ompad, an epidemiologist at New York University School of Public Health, said. For instance, the sample population did not include affluent New Yorkers who have groceries delivered or people who live in food deserts who are less likely to go to a grocery store. It also did not reflect the unemployed struggling to make ends meet and would not be at the grocery store, or healthcare workers who go food shopping at night after the end of their shift.
“So you can imagine that people who were in grocery stores during that period may be different from the general population,” Dr. Ompad said. “We have to be a little thoughtful about extrapolating to the larger population.”
The accuracy of antibody tests also remains in question. Manufacturers have pushed over 150 more antibody tests onto the market with little vetting from the Food and Drug Administration. In fact, the FDA said April 17 that the agency “does not review the validation, or accuracy, data for these tests unless an [Emergency Use Authorization] is submitted.”
The tests are “not ready for prime time,” former Centers for Disease Control and Prevention Director Dr. Richard Besser told Stat reporter Helen Branswell.
“As we learn whether that means you are protected in the future, there could be value in that,” Besser said, but “the science isn’t there yet to be able to say what those tests mean.”
Commercial antibody tests are known to generate false positives, resulting in unreliable data. George Q. Daley, Dean of Harvard Medical School, cautioned against treating two widely disseminated analyses from two areas of California, Santa Clara and Los Angeles Counties, as anything more than preliminary. He and other Harvard professors noted in the Washington Post that the tests used in California have been known to provide inaccurate results.
They cautioned against conclusions, reached by some commentators, that because the number of asymptomatic carriers in the studies was so high, the true mortality rate was about as low as that of the seasonal flu. “That would be great news, but until proved otherwise, these findings should be treated as preliminary at best,” the professors said.
Building a proper sample representative of the general population is possible, Dr. Ompad said, but would be difficult to carry out. For instance, researchers could create a random stratified sample based on a list of all households in New York.
“And then, you would go knock on people’s doors, but imagine trying to do that in New York right now,” she said. “Even if you did that in New York, you may have some selection bias, because some more affluent people left the city. And some people may not open the door to you because you’re a stranger, and they’ve been told to socially distance themselves.”
Creating more reliable antibody tests, she said, will take time. Despite having used antibody tests for past coronavirus epidemics, “now, we’re in a better position to begin to consider what the actual death rate is from the infection … And we know that 1% is probably too high,” she added.
The United States surpassed 1 million cases Tuesday, a new milestone, according to the Johns Hopkins University tracker. Estimates put the death toll at over 60,000, but the true fatality rate is likely much higher, given the lack of data on those who died before being diagnosed with the coronavirus.
