Vermont health officials investigating a possible coronavirus outbreak in Manchester say more than a dozen presumptive positive cases have tested negative using a different kind of test.
“So far we have not found connections that would cause us to call this an outbreak,” Dr. Mark Levine, commissioner of the Vermont Department of Health, said at a press conference on Friday.
Levine said the 59 presumptive positive cases around Manchester came from using new antigen tests that were recently approved by the US Food and Drug Administration.
As of late Thursday evening, 17 of the 59 people had taken traditional polymerase chain reaction (PCR) follow-up tests to confirm the results, he said. Most have come back negative.
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“Of the 17 people, 15 tested negative, two were positive,” Levine said at a press conference on Friday. “What does this mean? Although our investigation is not complete, it appears that many of the positive antigen results reported by Manchester Medical Center might have been false positives.”
The antigen tests, which were approved by the FDA in May, were designed for rapid detection of the virus that causes COVID-19 and can provide results in minutes. But they are not as sensitive as PCR tests, which yield highly accurate results but take longer to complete.
Vermont health officials said PCR tests were used to confirm the results of those 17 antigen tests.
“PCR tests are the most common type of tests used to diagnose or confirm COVID-19 infection, and it is the test used by our public health laboratory,” said Levine. “Antigen tests are a newer type of test, only recently approved by the FDA, that provides results much more quickly than PCR tests.”
Levine said antigen tests are intended to be used as a screening tool for people who are showing symptoms.
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“We don’t have evidence about the accuracy of the antigen test on people without symptoms,” he said. “It might turn out to be a great test for everyone, but we just don’t know.”
So what caused so many presumptive positive cases to test negative?
“Many of the 59 people who tested positive with the antigen test did not have symptoms,” said Levine. Systemic factors related to the performance of the test or the time elapsed between the two tests may have also played a role, he said.
Bennett Truman, a spokesman for the Vermont Department of Health, said the people who received positive results from the antigen tests are believed to be “a mix of Vermont residents and non-Vermont residents from the Manchester area.”
As of Friday morning, state officials had reached all but 11 of the 59 individuals who tested positive. All are being asked to take the PCR test, which is not mandatory, Truman said.
“Remember, we know of two positive cases, only two,” said Levine. “We continue to investigate the situation and are treating all positive antigen tests the same as any positive case, reaching out to each person, giving them guidance to isolate, tracing their contacts, and above all, recommending that they get a confirmatory PCR test.”
Levine urged anyone who is concerned that they may have been exposed to someone who tested positive to contact their health provider to see if they should be tested.
“We are learning more about antigen tests, they are a useful tool for screening patients who do have symptoms,” said Levine. “But our recent PCR results are showing us why they need to be confirmed, so we have a more accurate picture of current infection patients.”
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Emily Sweeney can be reached at emily.sweeney@globe.com. Follow her on Twitter @emilysweeney.
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