The proportion of COVID-19 tests from out-of-staters that are coming back positive in Maine increased in July, even as the volume of tests remained steady.

Nonresidents are now testing positive in Maine at a rate more than four times that of residents, though the numbers remain reassuringly small, according to the Press Herald’s analysis of data from the Maine Center for Disease Control and Prevention.

Between July 1 and July 28, the agency reported nonresident positive test results four times and this week provided the negative test numbers as well, allowing the positivity rate to be calculated. This rate steadily increased from 2.3 percent of nonresident tests performed July 1-9 to 4.9 percent of tests performed July 24-28.

By comparison, the seven-day average positivity rate for Mainers, which stood at 2.1 percent at the start of the month, has remained around 1 percent since July 10. Nonresidents still account for a small share of the positive samples collected in Maine, but their share has grown from 6.5 percent to 11.4 percent over the month’s four reporting periods.

The trend, while worrisome, involves a small number of people, and a month into the peak summer tourist season there is no indication yet that nonresidents have spread the disease to Mainers. Maine’s seven-day positivity rate remains lower than that of New Hampshire (2.3 percent) and Massachusetts (2.6) and a tiny fraction of that in hotspots such as Alabama (18.5), Florida (19.3) and Arizona (20.2), according to trackers at the Johns Hopkins University School of Medicine.

From the start of the public health crisis here in mid-March to July 28, 159 nonresidents and 3,861 Maine residents have tested positive for the virus, and Maine has remained a relative oasis, with the second-lowest per capita prevalence of the disease in the country after Vermont as of Thursday, according to a widely followed tracker at The New York Times.

Asked about the Maine trends, two public health experts were cautiously optimistic because the absolute numbers remain so low and the bulk of Maine’s tourists and summer visitors come from states and regions where compliance with mask wearing and other public health recommendations remains high, as it is in Maine.

“I’d feel far better knowing that the visitors are coming from the Northeast, since states across the South have had high positivity rates that, although falling, are still close to 20 percent,” Dr. Jennifer Horney, founding director of the epidemiology program at the University of Delaware’s College of Health Sciences, said via email. “Requirements for face coverings and other social distancing measures are stronger in the Northeast and compliance is higher as well.”

Dr. Peter Millard, an epidemiologist and former U.S. Centers for Disease Control and Prevention staffer who is now medical director at Seaport Community Health Care in Belfast, agreed.

“Borders are very porous, and even if you make rules, if there’s an epidemic in one state it’s going to spread to other states,” he said. “But people have been careful, and I think people from other states in the Northeast are, if anything, being even more careful than Mainers are. If they are following the appropriate guidelines, they really shouldn’t be infecting other people.”

Dr. Nirav Shah, director of the Maine CDC, said his agency will continue to watch the trend, but noted that Maine’s overall seven-day average positivity rate, including residents and nonresidents alike, stands at just 0.95 percent. “That provides a fuller look at potential spread of the virus in Maine,” he said via email.

“One hypothesis that we’re exploring is that visitors to Maine are more likely to seek testing when they become symptomatic – seeking testing here when they start feeling ill – which would drive up the positivity rate in a relatively small cohort,” Shah added. “Case investigations and contact tracing are other tools we use to determine if nonresidents are posing a greater risk of virus transmission, and the information we have from those tools does not indicate that nonresidents have caused any kind of statewide or localized spike.”

Several testing providers contacted by the Press Herald reported broadly similar results in recent days.

Northern Light Health, which operates 39 primary care offices and 10 hospitals, including Mercy in Portland and Eastern Maine Medical Center, performed 178 COVID-19 tests for nonresidents July 22-29, with just three positives, a rate of 1.7 percent. Most of the tests have been performed on symptomatic individuals who present themselves for treatment, spokesperson Patti Patterson King said.

Mount Desert Island Hospital in Bar Harbor, hub of a community-wide sentinel testing program to protect front-line tourism workers, saw three positive COVID tests since July 23 after not having one since May. All three were from people who were not residents of Hancock County, said spokesperson Oka Hutchins, though she could not say whether they were from out of state. The hospital has now seen a total of eight positive tests since the start of the pandemic, of which five were from outside the county.

Rep. Brian Hubbell, a Bar Harbor Democrat who represents the island in the legislature and helped create the sentinel program, said the sense in the community was that “things still seem relatively under control, and mask usage seems increasingly well-observed.”

The situation remained encouraging at the other end of the state in York, whose beaches are within an easy day trip from Massachusetts and New Hampshire. At the drive-thru rapid testing site it established at its Route 1 urgent care center, York Hospital hasn’t seen any sharp changes. From July 1 to Thursday morning, the lab had run over 2,000 tests and had only 15 positives, up by just four since July 23, according to testing director Erich Fogg. Sixty-nine percent of those who use the service – which gets results in 20 minutes but now has a wait list three weeks long – are from out of state.

“We’re in a vulnerable area because York and Ogunquit and Wells are very popular, but there’s no uptick of the disease in our out-of-state folks,” Fogg said. “That’s encouraging.”