The fast-spreading Delta variant is believed to be contributing to the recent uptick in COVID-19 cases in certain areas, particularly among those who have not been vaccinated, a UMass Medical School infectious disease expert and member of Gov. Baker’s COVID-19 Advisory Group said.

“Most of the cases we see and essentially all of the hospitalized cases we have seen have been in unvaccinated people,” said Robert W. Finberg, MD, distinguished professor of medicine.

Nationwide, the Delta variant is responsible for 83 percent of new COVID-19 cases and 97 percent of people hospitalized with COVID-19 are unvaccinated, according to the U.S. Centers for Disease Control and Prevention.

Seventy-two percent of Massachusetts residents have received at least one dose of the vaccine and 63 percent were fully vaccinated as of July 18, according to the CDC. Nationally, 57 percent of the population has received at least one dose and 49 percent are fully vaccinated.

A cluster of 132 COVID-19 cases following the Independence Day holiday weekend was reported to the Massachusetts Department of Public Health on July 16 by health officials in Provincetown, a vacation destination where the summer population swells to 60,000 from 3,000 year-round inhabitants. One-third of those positive cases were among people from out of state and 93 percent were male, with an average age of 35, the Cape Cod Times reported. The positive case count leapt to 256 on July 20.

Provincetown officials issued a public health advisory encouraging everyone, whether vaccinated or not, to wear a mask indoors in businesses and public places when social distancing can’t be achieved.

Meanwhile, on the West Coast, Los Angeles County recently reinstated an indoor mask mandate for businesses and public places, regardless of vaccination status.

Locally, Worcester Division of Public Health Medical Director Michael P. Hirsh, MD, professor of surgery and pediatrics and assistant vice provost for wellness and health promotion at UMass Medical School, said the city reached a milestone this week with 60 percent of those eligible being fully vaccinated. Among the remainder, one-quarter are children under age 12, who are not yet eligible.

Dr. Hirsh said there has been an increase to 10 to 15 COVID-19 positive cases seen per day, from two to four cases per day a few weeks ago, but health officials believe the trend could be related to holiday group gatherings.

“Certainly, we haven’t pushed the panic button,” Hirsh said. “We haven’t gone to recommending that we have to take a step backward with across-the-board indoor masking or things like that.” But he said vaccinated people with compromised immune systems or underlying respiratory conditions should still consider wearing a mask and avoiding tight quarters indoors.

Hirsh pointed to data released last week on breakthrough infections by the state Department of Public Health, which he called “an incredible testament to how effective the vaccines are.” Fewer than 4,500 people tested positive for COVID-19, among the 4.3 million in the state who have been vaccinated, a breakthrough infection rate of one-tenth of 1 percent.

“And none of those people are going to the hospital,” Hirsh said. “They’re all presenting either with cold symptoms and getting checked and finding out they’re positive, or through contact tracing or because they wanted to travel and got tested.”

“So, breakthrough infections are exceedingly rare, and vaccines are definitely keeping people out of the hospital,” said Hirsh. “But the unvaccinated in a sea of Delta variant: Those are the ones who are getting admitted.”

Hirsh said city health leaders, the Worcester District Medical Society and UMass Medical School students in the Vaccine Corps, Gold Humanism Society and the Student National Medical Association are working to get vaccines to the 30 percent of eligible residents who are unvaccinated. Among the strategies they are using are incentives such as gift cards and reviving a coalition of barber shops to promote vaccination, as trusted leaders in Black and LatinX communities.

Researchers and public health officials are watching the COVID-19 case trend closely.

“We expect that as in the United Kingdom, where the Delta variant was seen earlier, the disease will reach a peak soon and then will decline,” Dr. Finberg said. “We do not expect another surge as we had earlier in the year.”

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