Why does Connecticut have the lowest rate of COVID-19 in the country? It starts with vaccination.

Connecticut has recorded fewer COVID-19 cases per capita than any other U.S. state over recent weeks, and experts say there’s one simple explanation above all others: vaccination.

“If you look at the relationship between vaccination coverage and new cases, they seem to be strongly related,” said Dr. David Banach, an epidemiologist at UConn Health. “I think that offers a good part of the explanation.”

As of Friday, 69.2% of Connecticut residents were fully vaccinated, behind only Vermont. That total included 79.5% of residents 12 and older, most of any state. Although geography, weather and human behavior may also help explain why Connecticut’s recent delta variant spike was not as intense as surges elsewhere, experts say vaccination is clearly playing a key role.

Throughout the recent nationwide wave, states with high vaccination rates have typically recorded fewer cases, hospitalizations and deaths. Though there are exceptions — Maine, for example, is currently facing a COVID-19 spike despite high levels of vaccination — the safest states have tended to be the ones, like Connecticut, where most people are inoculated.

Even within Connecticut, the inverse correlation between vaccination and new cases is on full display. According to state data, many of the Connecticut municipalities with the lowest rates of vaccination are in the eastern half of the state. Sure enough, so are a majority of the 37 towns and cities that currently qualify for the state’s “red alert” status. The state’s two eastern counties, New London and Windham, are the two that still have “high” COVID-19 transmission, as defined by the U.S. Centers for Disease Control and Prevention.

Dr. Jim Cardon, chief clinical integration officer at Hartford HealthCare, said he thinks Connecticut has done a fairly good job of distributing COVID-19 vaccines even across the state, preventing severe outbreaks from festering in certain communities. Though disparities endure, nearly every municipality in the state, including Hartford and other major cities, now has at least half of residents vaccinated.

“We knew that vaccinating a certain number of the population would help with this, but we were always worried there would be pockets that were really under-vaccinated substantially that could allow a disproportionate continued bubbling of this event,” Cardon said. “And I do like to think that the focus the state made on trying to get into the communities that are typically underserved … may have had a little extra added benefit.”

According to data released Thursday, 78.7% of those hospitalized with COVID-19 in Connecticut are not fully vaccinated. Most vaccinated people hospitalized with the disease were admitted for other reasons and do not have severe symptoms, according to hospital officials.

Vaccination alone likely doesn’t explain why Connecticut has survived the delta variant better than other states have, experts say. It’s possible the state benefited from summer weather that allowed residents to socialize outdoors while those in southern states raced for the AC. It’s also possible, as Gov. Ned Lamont has suggested, that Connecticut residents simply take COVID-19 more seriously than those in other places and therefore behave more cautiously.

Though Lamont declined to impose a statewide mask mandate this summer as the delta variant surged, local officials in most of Connecticut’s largest towns and cities chose to do so. According to a recent survey from the nonprofit DataHaven, 51% of adults in the state report wearing a mask “very often” when leaving their homes, and 84% say they do so at least sometimes.

To Dr. Ajay Kumar, chief clinical officer at Hartford HealthCare, this is key.

“One thing I like to believe is that Connecticut has generally been a bit more compliant with masks and social distancing than most of the states in our country,” Kumar said. “I’m really proud of our state in how we have managed the pandemic at this time.”

Of course, Connecticut’s status as the model of COVID-19 containment could change quickly. States such as Maine and New Hampshire, which had recorded relatively low coronavirus levels for much of the pandemic, are currently among the states experiencing the sharpest upticks.

If COVID-19 cases can increase in those places, public health commissioner Dr. Manisha Juthani noted last week, they can also increase in Connecticut.

“We don’t live on an island, and it’s very easy to get from any of those states here,” said Juthani, an infectious disease specialist. “We’re coming up on a colder season and times that people want to gather. I don’t know what that’s going to mean when it comes to our state.”

All the more reason, Juthani said, for people to get vaccinated.

Alex Putterman can be reached at aputterman@courant.com.