Citing slack in demand for COVID-19 vaccinations, Yale New Haven Health officials said Friday they’ll open up the health system’s eight mass vaccination sites to walk-up patients, starting next week.

The sites include Mohegan Sun’s Earth Expo & Convention Center, where people have been getting vaccinated by appointment since late February. Walk-up patients will be accepted Sunday and the following Saturday and Sunday, May 1 and May 2, according to Vin Perini, a Yale New Haven Health spokesman.

“We’re disappointed when we hear about growing hesitancy and open appointments,” Marna Borgstrom, Yale New Haven Health’s president and chief executive officer, said during a virtual news conference. “We’re a long way from herd immunity.”

Dr. Thomas Balcezak, Yale New Haven Health’s chief clinical officer, said the system, which includes Lawrence + Memorial and Westerly hospitals, has administered 100% of the vaccine doses it’s received weekly since mid-December but some vaccination appointments are going unfilled.

“Yes, I’m worried,” he said. “Around 50% of the state of Connecticut has been vaccinated, but we should still be seeing robust demand for vaccinations. ... Slack in demand tells us the second half of the state isn’t pursuing the vaccine the same way the first half did.”

Balcezak said the state needs to vaccinate 75% to 80% of its adult population to achieve herd immunity, the point at which the coronavirus disease theoretically can no longer spread. He said the vaccination rate among Yale New Haven Health employees “essentially” has reached 80%.

Despite the slowdown in vaccinations, their effectiveness in reducing hospitalizations and severe outcomes among older people is clear, Balcezak said.

Before November, COVID-19 patients 75 years of age and older represented 30% of the system’s patient population. Now, they’re 10%. Similarly, those 65 to 74 years old have gone from 20% to 10% of patients. Those now hospitalized with the coronavirus disease are mostly 55 and below, Balcezak said.

Overall, the number of COVID-19 hospitalizations across the system has plateaued at about 200, a number Balcezak said could begin to fall as more people in the 35-to-54 cohort get vaccinated. Borgstrom reported that there were 194 COVID-19 patients in the system’s five hospitals Friday: 96 at Yale New Haven Hospital, 64 at Bridgeport Hospital, 18 at Greenwich Hospital, 14 at Lawrence +Memorial and two at Westerly Hospital.

In response to a question, Balcezak said the system has seen “about a dozen” cases in which fully vaccinated people have contracted COVID-19. In one such “breakthrough” case, he said, the patient died.

The patient was hospitalized with an underlying respiratory illness but COVID-19 was a contributing factor in the death, he said.

Yale New Haven Health announced Friday it had reached a significant milestone earlier in the week when it conducted its 1 millionth COVID-19 test. It said more than 95% of the tests were both collected by YNHH personnel and tested by YNHH laboratories. Earlier in the week, YNHH announced it had administered 300,000 doses of COVID-19 vaccine.

On Friday, Gov. Ned Lamont’s office reported 1,034 new COVID-19 cases had been detected among 39,469 test results, a one-day positivity rate of 2.62%. Hospitalizations decreased by 37 to 578, while eight additional deaths pushed the statewide toll since the pandemic began in March 2020 to 8,047.

b.hallenbeck@theday.com