With COVID-19 spreading, New London County’s health districts notified municipal officials and the public Thursday that the U.S. Centers for Disease Control and Prevention had recommended all county residents — including those fully vaccinated against COVID-19 — wear masks indoors.

The alert went out the day after the CDC listed New London County among U.S. counties identified as areas of “substantial” transmission of the coronavirus disease.

New Haven County joined New London and Hartford counties on the list Thursday.

Late in the afternoon, the Connecticut Department of Public Health echoed the CDC recommendation, calling on all residents in the designated counties over the age of 2 to wear masks regardless of their vaccination status. The department strongly recommended people who live in, work in or are visiting towns in the three counties follow the guidance.

“In addition, residents across our entire state who have underlying medical conditions that put them at high risk for complications of COVID, or residents who live with high-risk or unvaccinated individuals should also consider wearing masks in indoor public spaces,” acting public health Commissioner Deidre Gifford said in a statement. “This includes vaccinated family members who live with young children who are not yet eligible for vaccination.”

Still in effect is Gov. Ned Lamont’s executive order requiring those who have not been fully vaccinated and are unable to adequately distance themselves from others to wear masks indoors. He has yet to extend the mandate to the fully vaccinated.

In emails to town and school officials and heads of businesses and organizations in their respective jurisdictions, Stephen Mansfield, director of health for Ledge Light Health District, and Patrick McCormack, director of health for the Uncas Health District, wrote that the increase in COVID-19 transmission in New London County is cause for concern.

Ledge Light encompasses nine municipalities in the southern part of the county while Uncas includes 11 municipalities surrounding Norwich.

“In areas with substantial transmission, CDC recommends that everyone (including fully vaccinated individuals) wear a mask in public indoor settings to help prevent spread of the Delta variant and to protect others,” Mansfield wrote. “This is important because new data show that fully vaccinated people who are infected with the Delta variant might be infectious and might potentially spread the virus to others.”

Ledge Light’s weekly COVID-19 report, released Thursday, indicated the district had counted 102 new COVID-19 cases in the week that ended last Saturday, up from 44 the previous week and nine the week before that. In the two-week period from July 11 to 24, town-by-town average daily case rates ranged from less than five in Lyme and North Stonington to a high of 11.2 in Ledyard. New London and Groton had the most new cases in the period, 35 and 31, respectively.

In a Facebook post, Ledyard Mayor Fred Allyn III announced the town will reinstitute a requirement Monday that all employees and the public wear masks inside town facilities. The town library was to put the mandate into effect Friday. The Town of Groton also announced that as of Monday, all employees and visitors entering town facilities must wear masks.

"Please do your research and consider getting a vaccine shot if you have not done so yet," the notice says. "We all need to do our part to keep our family and community healthy and to avoid returning to more austere measures."

On Thursday, the latest data from the governor’s office showed 339 new cases of the disease had been detected in the state since the previous day out of 14,401 results — a one-day positivity rate of 2.35%. Hospitalizations had increased by nine to 112, and seven additional deaths linked to the disease since the previous Thursday had pushed the toll since March 2020 to 8,293.

During a virtual news conference, Marna Borgstrom, chief executive officer of Yale New Haven Health, reported that the system’s five hospitals were treating 53 COVID-19 patients, nearly five times as many as the 11 they were treating two weeks ago. Ten of the current patients were in intensive care and five were on ventilators. Two weeks ago, two patients were in intensive care and none was on a ventilator.

Of the current patients, 10 are at Lawrence + Memorial Hospital in New London and three are at Westerly Hospital. Neither hospital had a COVID-19 patient two weeks ago.

Backus Hospital in Norwich, a member of Hartford HealthCare, had three COVID-19 patients Thursday.

Dr. Thomas Balcezak, Yale New Haven Health’s chief clinical officer, attributed the uptick in cases to the delta variant, the “dominant strain in Connecticut.” He said the system “sequenced” 53 cases from July 6 to 13 and found that 72% of them stemmed from the delta variant. The strain’s prevalence is concerning, he said, because it has a much higher “viral load” than previous strains, enabling it to infect more people.

Increasingly, Balcezak said, people who have been vaccinated are contracting the disease. Between December and July 14, the system saw 28 “breakthrough” cases involving people who had been vaccinated. Three of them died. In the last two weeks alone, the system has seen 13 breakthrough cases, none of them fatal.

“Does this mean it’s ineffective?” Balcezak said of the vaccines. “No. ... It’s still our best way out of the pandemic. The greater the percentage of people that gets vaccinated, the less the spread (of the disease).”

Given that a person infected with the delta variant typically infects six other people, he said he believes the number of cases will continue to rise for a time. Previously, a person infected with COVID-19 was believed to infect, on average, two other people.

b.hallenbeck@theday.com