An upward trend of COVID cases and hospitalizations has federal and state health officials recommending people in particularly high transmission areas - including most of Connecticut - to again wear masks indoors. But the advisory stops short of reinstating a statewide mandate, leaving many local municipal and business leaders to establish and enforce their own rules. Some Connecticut cities - Norwalk, Norwich, New London and Hartford - have already announced the return of mask mandates in municipal buildings. They joined New Haven, which never dropped the requirement. And Danbury Mayor Joe Cavo said he hasn't "ruled out" a mask mandate for municipal buildings as he examines the city's infection data. Some businesses and organizations are starting to take action as well. The Connecticut Science Center in Hartford announced Friday it will require everyone age 2 and older to wear a mask indoors, regardless of vaccination status. Walmart stores in the affected areas will also require everyone to wear masks indoors, the retail giant announced Friday. While Stew Leonard's is not requiring customers to wear masks at its Norwalk, Danbury and Newington grocery stores, all employees are now required to wear face coverings again. The SoNo Collection will distribute masks to shoppers who request one, but there will be no requirement to wear one inside the South Norwalk mall. And some businesses are not planning to change their rules. Will Maxwell, of Outdoor Sports Center in Wilton, said requiring a mask indoors is not "a concern of ours." "At this point, there will be no policy shift," Maxwell said Friday. It will be up to Gov. Ned Lamont whether an indoor mask mandate is reinstated. While some municipal leaders have considered local mandates for businesses, they said only the governor through his executive powers can issue the requirement. Lamont has been reluctant to reinstate a statewide indoor mask mandate. A Lamont spokesperson on Friday said the governor's stance hasn't changed and referred to the state Department of Public Health advisory that recommends everyone wear masks indoors in Connecticut counties where COVID transmission is considered "substantial." With the exception of Litchfield County, all of Connecticut has reached the Centers for Disease Control and Prevention's "substantial" COVID spread level in recent days. On Saturday, Middlesex County was the latest to exceed the threshold of more than 50 cases per 100,000 people over a seven-day period. COVID cases have been rising in Connecticut in recent weeks with the daily positivity rate now regularly climbing over 2 percent. State health experts have attributed the surge to the highly transmissible delta variant, which now comprises 73 percent of Connecticut cases sequenced in the past three weeks, according to the latest data from the Yale School of Public Health. The CDC has released a report on a recent outbreak in Cape Cod, where officials say most of the infections involved people who were fully vaccinated. The data also showed the infected vaccinated people carried a viral load that was similar to unvaccinated people who were infected, the report said. "High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus," CDC Director Rochelle Walensky said Friday in a statement. Walensky said the finding was pivotal in the agency's decision to update its indoor mask guidance for everyone - regardless of vaccination status - in areas with "substantial" and "high" transmission. "The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones," Walensky said. Connecticut has had a total of 1,133 breakthrough COVID cases - infections involving people who have been vaccinated. The figure represents a small fraction - less than 0.06 percent of the people who have been fully vaccinated in the state, the data shows. Of those, 171 have been hospitalized and 27 have died, according to the state's data. Of those who died, all but three were over the age of 75, and everyone was over 55, the data shows. While Connecticut has among the nation's highest vaccination rates, some areas of the state continue to lag behind. Nearly 70 percent of Connecticut's population has received at least one dose of the vaccine and about 63 percent are fully vaccinated, according to the latest CDC data. But as of last week, four Connecticut towns were marked red on the state's color-coded map of COVID hotspots with 15 or more cases per 100,000 residents during a two-week period, the DPH data shows. Of the four towns in red - Hartland, Sprague, Salem and Bozrah - only Salem reported more than 60 percent of residents with at least one shot of the vaccine, the data shows. "It's really important for people to take this seriously, get on top of this quickly," New Haven Mayor Justin Elicker said. "The longer this virus is in our community, the more likely it is to have yet another variant." In response to Elicker's recommendations, Eva Geertz, operations manager with The Institute Library, an independent institution on New Haven's Chapel Street, said she has reinstated the indoor mask policy for all patrons. Geertz said the decision was not particularly difficult. She said the library is "very oriented toward the common good," visited by people from a "million different backgrounds." "This isn't anyone's idea of a good time," Geertz said, "but it feels like the right thing to do." Staff writers J.D. Freda and Ben Lambert contributed to this story.