Nantucket has experienced a surge in coronavirus case numbers since Labor Day, with nearly 50 cases reported this month and 19 in the past four days — many of them contact-traced back to large gatherings or specific industry sectors.

Meanwhile, Martha’s Vineyard has kept its coronavirus case numbers low and avoided community spread throughout the pandemic, according to public health experts, even as similar communities have struggled to handle the virus and seen spikes in the waning months of the summer.

“It’s very surprising,” Tisbury health agent Maura Valley said. “Coming into the summer, we were very nervous just because of the large numbers of different people who come here.”

Coronavirus cases on the Island have hovered between zero and five cases reported per week since the first week in August. Most weeks have seen three cases or fewer, indicating low numbers of active cases — an important metric to keep low considering the Island’s small hospital capacity, according to health officials.

In newly released data Friday from the Island boards of health compiled by Chilmark health agent Marina Lent, officials reported that the Island has maintained its goal of a two-week average test positive rate under one per cent since July, hovering between 0.4 and 0.9 per cent. The one per cent positive test rate threshold is a state metric used to indicate the prevalence of the virus in a community.

In contrast, Nantucket has seen a sharp spike in cases since Labor Day weekend, reporting nearly 50 since September. Five new cases were reported on Nantucket Sunday and 20 in the past seven days. The island’s seven-day positive test rate is 4.3 per cent. There is currently one person under investigation for Covid-19 hospitalized at Nantucket Cottage Hospital, and one more who was transferred in recent weeks.

Nantucket Cottage Hospital released a statement on Monday saying that one of the individuals who tested positive over the weekend was a hospital employee. Another statement warned Nantucket residents to remain vigilant, and that a recent cluster of cases had been traced to a church gathering on the Island.

“This incident should serve as a reminder to the community to avoid large gatherings, and always wear a mask when out in public when physical distancing is not possible,” the statement said.

Nantucket is currently classified by the state as a high risk, or red, community for Covid-19 transmission. “To bring this current surge in new cases of COVID-19 under control, we will need the entire island to work together to keep cases down,” the Nantucket statement said.

At an emergency board of health meeting held earlier in the month, Nantucket health agent Roberto Santamaria said many of the cases originated from workers in trades, including landscaping and construction, and that the island was experiencing community spread of the disease.

On the Vineyard, no patients are currently hospitalized with the virus.

Ms. Valley, in an interview with the Gazette Monday, said the Vineyard, unlike Nantucket, has avoided community spread.

“I, from the cases we’ve seen through this whole pandemic, don’t see any indication of community spread. If there was community spread, we’d have more cases,” she said. “That’s kind of what happened over in Nantucket.”

Assuming the Vineyard has a 60,000-person population, the Vineyard report said the Island is well-within its population-specific case percentages, averaging far fewer than 17 cases per week. In the past two weeks, the Island has reported two new cases, both of which came from the same family, according to Ms. Valley. Case numbers have even declined through September, according to the health report.

While Ms. Valley noted that luck was a large factor in the Island keeping case numbers low, she also pointed to preventative steps public health officials have taken on the Vineyard in the past few months, such as coordinating with building inspectors. She also credited the early impacts of the construction moratorium, which stopped almost all construction for two weeks in April and then slowly phased building back in with five and 10-person crews over the month.

A similar construction moratorium was instituted on Nantucket.

“I think early on in the pandemic, we got the buy-in from the builders association on the Island. They went along with us on the moratorium and followed those rules. And I think for the most part, that still is the case. The people in the trades here understand,” Ms. Valley said. “We would be in a completely different place right now had they not complied with the moratorium and bought into that.”

The Vineyard has a comprehensive, free asymptomatic coronavirus testing program through TestMV, meaning that many people who otherwise wouldn’t receive laboratory (PCR) exams are getting tested regularly, including weekly tests for VTA drivers and dental hygienists.

About 700 to 800 people are still being tested per week on average at the site, according to Ms. Valley — down only by a small amount from the approximately 1,000 people being tested weekly during the summer. Symptomatic patients are being tested at the Martha’s Vineyard Hospital.

The Nantucket hospital is the only coronavirus testing site on Nantucket and is offering asymptomatic testing to Nantucket residents at a cost of $150 per test though a program coordinated with the town. The hospital has tested just over 9,000 patients, reporting 125 positive PCR tests.

The Vineyard has reported 74 confirmed positive PCR cases at TestMV and the Martha’s Vineyard Hospital combined since the pandemic began. The facilities have tested about 20,000 patients total — twice that of Nantucket.

Ms. Valley also said that unlike Nantucket or Cuttyhunk, where an indoor party led to eight cases earlier this summer, the Island has avoided case-spreading events traced back to large parties, weddings or other gatherings.

“The cases we’re seeing definitely seem to be more isolated individual cases,” Ms. Valley said. “We’ve had a few cases that were travel related, but we haven’t had anything that was linked back to a party or a large gathering like that. It is really surprising.”

She added that even though contact-tracers continue to work vigorously to prevent further outbreaks, the lesson from Nantucket was that it is important to stay vigilant.

“The people on the Island, and public health, and the police and the building inspectors and everything have been out there reinforcing public health guidelines, even though it may seem like we’re not,” Ms. Valley said. “It really has made a difference. I think it’s that, and a lot of luck.”