A volunteer working on a recount of ballots at the Portland Expo this week initially tested positive for COVID-19, but a second test was negative for the disease, according to city officials.

City staff members were working with the Maine Center for Disease Control and Prevention to locate anyone having had contact with the volunteer, who on Thursday and Friday was helping recount some 39,000 ballots in a referendum initiative to place more restrictions on short-term rentals.

After confirming the first test was positive, Jessica Grondin, the city’s spokeswoman, said late Friday that a second test showed the volunteer had tested negative. A report by the Portland Press Herald posted Friday evening and in Saturday’s print edition was based on confirmation of the initial positive test; details of the second test were not confirmed until after deadline.

False positive tests are exceptionally rare, according to a report by Harvard Health Publishing, an online publication of Harvard Medical School.

“Most false-positive results are thought to be due to lab contamination or other problems with how the lab has performed the test, not limitations of the test itself,” Dr. Robert H. Shmerling wrote in an August report, explaining the differences in COVID-19 tests.

The recount confirmed that the ballot measure was defeated by a narrow margin.

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