The entire state of Connecticut qualifies as an area of substantial COVID-19 transmission after Litchfield County was added to the list Tuesday afternoon by the Centers for Disease Control and Prevention.

Litchfield Country averaged 55.45 cases per 100,000 people in the past seven days, an increase of almost 89 percent, according to CDC data. The agency defines substantial transmission as 50 to 99 cases per 100,000 people or an 8 percent to 9.9 percent positivity rate.

Health officials have renewed calls for masks and other safety precautions for everyone, regardless of vaccination status.

Why does the CDC now recommend that everyone, even the fully vaccinated, wear a mask in public indoor spaces in areas of substantial and high transmission?

The Delta variant, and its rapid spread, upended the CDC’s May advice that freed the fully vaccinated from masking up when leaving home. But the agency’s about-face July 27 was attributed to an investigation into a COVID-19 outbreak during the Fourth of July weekend in Provincetown, Mass., that found three-quarters of the 469 cases were fully vaccinated. Even more alarming, the breakthrough infections of fully vaccinated people showed a similar viral load detected in cases among the unvaccinated.

“The fear is that the vaccinated will be able to transmit at the same rate as the unvaccinated,” says Dr. Ulysses Wu, Hartford HealthCare’s System Director of Infection Disease and Chief Epidemiologist. “That’s what prompted the CDC to really pivot in terms of masking indoors.”

What does this mean for Connecticut?

The state remains the nation’s fourth-most vaccinated state, with 63.4 percent of the eligible population fully vaccinated. If you’re vaccinated, you have little risk of serious illness. If you’re unvaccinated, you have a much greater chance of serious illness, hospitalization and death.

The state Department of Public Health reported, as of July 28, 1,133 confirmed COVID-19 cases among the fully vaccinated — less than 0.06 percent of the more than 2 million people vaccinated in the state. The unvaccinated account for almost all COVID-19-related hospitalizations. People 75 years old and up have accounted for close to 54 percent of hospitalizations and and 80 percent of the deaths among the state’s breakthrough cases.

The  CDC, through July 19, received reports of 5,914 COVID-related hospitalizations among the nation’s 161 million-plus fully vaccinated people. That means only 0.004 percent of the fully vaccinated later became seriously ill or died from COVID-19.

“Our first line of defense is vaccinations,” says Dr. Wu. “That is still our pathway out. When vaccinations either stop happening or we think that cases are going to be rising, the next two steps are social distancing and masking.”

Where should I wear a mask?

Like the CDC, the DPH “strongly recommends” wearing a mask in public spaces in any area with a substantial rate of transmission. Absent a statewide mask mandate, observe all masking requirements inside local businesses and state and local offices. Elsewhere, use you judgement: If you are fully vaccinated and infected, you might not get sick but you can spread the virus to either the unvaccinated are children under 12 ineligible for vaccination.

“We’ve always known that indoor transmission is much greater than outdoor transmission,” says Dr. Wu. “So outdoor transmission guidelines are obviously not that strict, if occurring at all. . . . I fully agree with with indoor masking for the most part in an area of high transmission, which unfortunately is becoming everywhere at this point.”

A Delta-powered surge spiked the state’s COVID-19 positivity rate above 3 percent this week and the seven-day positivity rate to the highest levels since mid-April, according to the state Department of Public Health.

“Being vaccinated doesn’t mean you cannot get infected,” says Keith Grant, APRN, Hartford HealthCare’s Senior Director of Infection Prevention. “It does mean, based on the data, that you have a significantly lower risk of being and being critically ill and of dying from this disease.”