The state's top health official is recommending everyone wear masks indoors in three Connecticut counties that have been classified by the Centers for Disease Control and Prevention as having "substantial" COVID transmission. Cases over the past seven days have increased in New Haven, New London and Hartford counties, elevating the risk of community spread from moderate to substantial, according to the CDC. At nearly 49 cases per 100,000 people as of Thursday, Fairfield County was approaching the CDC's substantial spread threshold of 50 cases per 100,000 over a seven-day period. Deidre Gifford, the acting commissioner of the state Department of Public Health, said she "strongly recommends" that people who work, visit or live in New Haven, New London and Hartford counties wear a mask in indoor public spaces. Her statement stopped short of an outright mandate. City leaders in New Haven and Hartford said Thursday they do not have the authority to require people to wear masks indoors in private establishments. New Haven Mayor Justin Elicker encouraged everyone to wear masks indoors regardless of their vaccination status. "I think people need to consider their role in keeping themselves and others safe, even if they're vaccinated," he said. Gifford said people across the state who have underlying medical conditions that put them at greater risk of COVID-19 complications should wear a mask indoors. That includes parents of children too young to be vaccinated, she said. Gifford's recommendation follows the CDC releasing new guidance this week, calling on everyone to wear masks indoors in areas where there are substantial and high virus transmission. Gifford's statement reiterated that people who are not fully vaccinated are still required to wear a mask indoors. Connecticut's outdoor mask mandate ended in May. "Emerging science indicates that with the delta variant in circulation, in some cases, vaccinated individuals may become infected with and subsequently transmit COVID infection to others," the statement reads. "The risk of contracting and transmitting COVID infection is far greater for the unvaccinated, and vaccination remains the safest and most effective way to protect yourself and your community from COVID." Gov. Ned Lamont, whose emergency powers are set to expire at the end of September, has so far issued no new statewide mask mandates. A spokesperson for the governor's office said Thursday Lamont has not changed his position. On Thursday, 339 new cases of COVID-19 were found statewide out of 14,401 tests, for a one-day positivity rate of 2.35 percent. Nine more patients brought the total hospitalized statewide to 112 - the most since late May. Seven COVID-related deaths were reported in the past week, bringing Connecticut's death toll to 8,293. The state released data Thursday that showed there have been 195 more breakthrough infections - COVID-19 cases in people who are fully vaccinated - in the past week. Nearly 70 percent of all Connecticut residents have started vaccination and just over 63 percent are fully vaccinated, according to the CDC data as of Thursday. Connecticut vaccinations have slowly risen over the past two weeks, but still hover below 40,000 doses per week - far below the state's peak of more than 315,000 weekly doses in early April. In Fairfield County, more than 72 percent of residents have received at least one dose of vaccine and more than 64 percent are fully vaccinated, the data shows. In New Haven County, about 68 percent of residents have received at least one dose, while more than 61 percent are fully vaccinated, the data shows. In New London County, about 69 percent have received at least one dose and about 64 percent are fully vaccinated. In Hartford County, nearly 67 percent are partly vaccinated and about 61 percent are fully vaccinated. 'This was avoidable' Dr. Thomas Balcezak, chief medical officer of Yale New Haven Health, said infections will likely continue to rise in Hartford and New London counties. Each infected person is now estimated to pass the disease to six other people, he said during a news conference Thursday afternoon. He attributed the increased spread to the delta variant. Marna Borgstrom, the Yale health system's president and CEO, said 53 patients are hospitalized for COVID-19 across its health care system. Ten are in intensive care units, with five on ventilators. In areas with surging infections, doctors are now "angry," Borgstrom said. Staff in areas with low vaccination rates feel they are being put at risk by unvaccinated people, she said. "There is a sense that this was avoidable," Borgstrom said during the news conference. In Hartford, where a "substantial" amount of transmission in the county means people should be masking indoors under the CDC guidance, municipal buildings still require face coverings in areas where staff and the public interact, a spokesperson for Mayor Luke Bronin's office said. West Hartford Town Manager Matt Hart said his team plans to meet next week to discuss whether to require everyone - employees and visitors - to wear masks in municipal buildings. The town of about 63,000 only requires people to wear a mask if they are unvaccinated, which is run on an honor system. "I think one of the things we learned over the last two years is we need to be prepared to move quickly if case counts start to go in the other direction," Hart said. In New London, Mayor Michael Passero said Thursday the city will begin requiring everyone to wear masks in municipal buildings. "Who knows who's vaccinated and who isn't?" Passero said. "We're much more comfortable just putting a sign on our public buildings saying, 'Everyone please wear a mask.'" The new CDC guidance hinges on a color-coded, county-level system of transmission levels. Counties marked in blue are "low" transmission areas, counties marked in yellow are "moderate," orange is "substantial" and red is "high." The CDC uses two data points to determine the community transmission level: The total number of new cases per 100,000 people, and the test positivity rate, both over seven days. Both conditions do not need to be met and the CDC takes whichever is higher. Low transmission areas are those with less than 10 cases per 100,000 or a positivity rate below 5 percent. Moderate areas have 10 to 49 cases per 100,000 or a positivity rate of 5 to 7.9 percent. Substantial areas have 50 to 99 cases per 100,000 or a positivity rate of 8 to 9.9 percent. Areas with high transmission have 100 or more cases per 100,000 or a positivity rate over 10 percent. Hartford, New Haven and New London counties are marked as areas of "substantial" transmission, while the state's five other counties are marked in yellow, for moderate transmission. Staff writer Ed Stannard contributed to this story.