A group from Massachusetts spends time at Old Orchard Beach in late June. Ben McCanna/Staff Photographer

As Maine moves into the high summer tourist season, the number of out-of-staters testing positive for COVID-19 here has remained stable at a low level, according to data from both hospital laboratories and the Maine Centers for Disease Control and Prevention.

In the nine days ending July 18, the Maine CDC referred nine nonresident cases to other states, or one per day, down an average of 1.4 cases a day in the 21 days ending July 9, the last time the agency disclosed this metric. Between May 11 and June 19, the figure had been 0.6 cases per day. States generally refer infectious disease cases involving nonresidents to authorities in their home state with the expectation that they will take over contact tracing and other aspects of the case.

In a separate metric – the number of positive nonresident tests performed – the agency’s data show a flat trend line this month. A total of 133 such cases had been reported as of July 18, up from 120 on July 9 and 107 on June 30, a trend of 0.7 cases per day in both periods. The primary distinction between the two metrics is that one measures the number of tests performed, and multiple tests can be performed on the same individual.

By comparison, the Maine CDC reported 381 Mainers had tested positive between July 1 and July 18, meaning nonresidents accounted for 6.4 percent of the COVID-19 positive tests here over that period.

Hospital systems contacted by the Press Herald reported broadly similar trends, an encouraging sign that Mainers and out-of-state residents may be keeping the spread of the disease in check by complying with mask-wearing and social distancing measures, even as the pandemic rages in the southern and southwestern United States, overwhelming intensive care units and triggering the deployment of refrigerated morgue trucks.

Northern Light Health, which operates 39 primary care offices and 10 hospitals including Mercy in Portland and Eastern Maine Medical Center in Bangor, had 23 positive results from nonresidents as of July 16, up from 18 on June 30, an increase of five from 348 nonresident tests performed, or 1.48 percent positivity. The positivity rate for the 634 nonresident tests that had been performed as of June 30 was 2.83 percent.

Mount Desert Island Hospital in Bar Harbor has been at the center of a community-wide effort to regularly test hundreds of front-line tourism workers like cashiers, hotel clerks and servers, as well as tourists. They have not had a positive test result of either residents or nonresidents since May 16, spokesperson Oka Hutchins said via email Monday, and only five positive results of any kind since the pandemic began, out of more than 1,000 tests performed.

MaineHealth, the state’s largest hospital network, which includes NorDx Labs and Maine Medical Center as members, declined to share its testing numbers, citing patient privacy. But Dr. Dora Anne Mills, the network’s chief health improvement officer, said Friday they had seen an uptick over the previous two weeks, with roughly five to 10 positive nonresident results, but no indications of an outbreak linked to out-of-state visitors.

“It is certainly worth watching and monitoring to see what happens since the virus is on fire in the rest of the country outside New England and the Northeast,” said Mills, a former Maine CDC director and a sister of Gov. Janet Mills. “The virus does not see borders, and with the increase of travel from outside of Maine, these are like sparks. Are they enough to start a surge? We don’t know.”

Maine has remained an oasis amid the deteriorating pandemic situation in the United States and on Monday was one of only two states with a declining seven-day trend in cases on a widely followed tracker at The New York Times. But the state is especially vulnerable to out-of-state infection vectors, as tourism is its biggest industry and the state has the highest proportion of vacation homes in the country at 19 percent of the housing stock.

The geographic origins of nonresident COVID-19 infections appears to have changed over the past month. As of June 19, when Maine had transferred a total of 81 COVID-19 investigations to health authorities in other states, the majority were from New Hampshire (with 23 such cases) and Massachusetts (with 19). But in the July 18 update provided to the Press Herald, Massachusetts accounted for just five of the 39 new cases, while New Hampshire’s tally had actually gone down by one, because of a data correction. New York, which had not previously been broken out, had accumulated a total of 17 cases since the beginning of the infection, according to Maine CDC spokesman Robert Long, who did not have comparable figures available for the two earlier reporting periods.

Long did not respond to a request for comment from Maine CDC Director Dr. Nirav Shah on the nonresident trends, but he did say via email that the agency would “strive” to release nonresident data on a weekly basis, though this will not follow a fixed schedule. “The day of the week may vary, based on staff availability,” he added.

Two other states with large numbers of seasonal residents – Hawaii and Florida – have posted information on nonresident cases on a daily basis since the early weeks of the pandemic. Maine, by contrast, hasn’t disclosed this information on any regular schedule and only began doing so at all on April 9, the morning after the Press Herald had asked for the information.