Three Connecticut counties in two days have been classified as having a substantial level of community transmission of COVID-19, where the Centers for Disease Control and Prevention recommends everyone should again wear masks indoors regardless of vaccination status.

New Haven County became the latest, elevated Thursday to substantial community transmission as the Delta variant spreads through the state. A day earlier, Hartford and New London counties qualified based on the CDC’s standard of averaging 50 to 99 cases per 100,000 people a week or a 8 percent or 9 percent positivity rate.

New London County reported 64 cases, Hartford County 57 and New Haven 52 cases per 100,000 people, according to recent figures. Fairfield County, with 49 cases per 100,000 people, appears ready to cross into the substantial levels from moderate. The CDC categorizes transmission as low, moderate, substantial and high. (For more information click here.)

The state’s positivity rate was 2.35 percent, according to the state’s daily report, down from 2.96 percent Wednesday.

The sudden jump in cases reflects the power of the Delta variant’s transmissibility. Each person infected with the original SARS-CoV-2 virus, which causes COVID-19, would likely infect 2.5 to three other people, according to research by the Emory University School of Medicine in Atlanta. Someone infected with Delta is more likely to infect eight or nine others. Because of Delta, the CDC reversed its no-mask guidance when it announced Tuesday that fully vaccinated people should again wear masks indoors in areas of the country with elevated COVID-19 infection rates.

Despite Delta, state health officials remain confident in the COVID-19 vaccines. Connecticut is the nation’s fourth-most vaccinated state, with 69 percent of the eligible population (12 years old and up) having received at least one dose of the vaccine. But virtually every state, including Connecticut, is experiencing a surge of cases associated with the Delta variant.

Still, do not expect a revival of a statewide mask mandate anytime soon.

“Everybody says if (the positivity rate) gets above 6.72 percent,” Gov. Ned Lamont said this week, “it’s this rule vs. that rule. There will be guidelines, but not rigid.”

But the CDC’s updated guidance could create mask confusion where people would be advised to wear a mask in a Hartford County store but not if they drove to a Middletown store, part of moderate-risk Middlesex County.

“If you step over the border from some of these signs, you’re not as high risk” says Dr. Virginia Bieluch, Chief of Infectious Diseases at The Hospital of Central Connecticut in New Britain, “and that doesn’t make a lot of sense.”

Connecticut residents have followed the science, both vaccines and masks, which is unlikely to change during the Delta wave.

“My recommendation would be that people should seriously consider wearing a mask,” says Dr. Ajay Kumar, Hartford HealthCare’s Chief Clinical Officer, “in those settings where there’s a high risk of exposing yourself (to the virus) or if you have some underlying comorbidities.”