Vermont has the highest vaccination rate in the country. So why are cases surging?

Vermont is one of the most vaccinated states in the country and has served as a model for its COVID-19 response throughout the pandemic. But now, the state is experiencing its worst COVID-19 surge yet, with several factors -- including its own success -- to blame, officials said.

In Vermont, nearly 72% of residents are fully vaccinated against COVID-19 -- more than any other state, according to U.S. Centers for Disease Control and Prevention data. At the same time, it has the 12th-highest rate of new COVID-19 cases over the last week, state data released Tuesday shows.

Vermont has seen a "significant" increase in COVID-19 cases in the past week, Mike Pieciak, commissioner of the state's Department of Financial Regulation, said during a press briefing Tuesday.

The seven-day average for COVID-19 cases rose 42% as of Tuesday, according to state data. Vermont does more testing than nearly any other state, though testing only increased 9% during the same period. The statewide positivity rate also increased 30%, with the seven-day average positivity rate just under 4%. The number of new cases increased by nearly 700 in the past week, state officials said Tuesday.

"We just haven't [previously] seen an increase in terms of that raw number of cases during the pandemic," said Pieciak, noting there were just over 2,100 cases reported for the week in Vermont, one of the least-populated states in the country.

Case rates in Vermont residents who are not fully vaccinated are nearly four times higher than in fully vaccinated residents, according to state data. Essex County, the least-vaccinated county in the state, is reporting the highest case rates of any county in Vermont, with 1,022 cases per 100,000 people reported from Nov. 2 to 8. In Grand Isle County, which has the highest vaccination rate in the state, that number was 160.

Statewide, those driving the surge include people in their 20s, who are the least vaccinated among Vermont adults, as well as children ages 5 to 11, who are just now eligible to get vaccinated, Dr. Mark Levine, Vermont's health commissioner, said Tuesday.

There's no "one simple answer" behind the surge, according to Levine. Though one major factor is the delta variant, experts said.

"Across the United States and in Vermont, we're seeing the impact of the highly contagious delta variant," Dr. Jan Carney, associate dean for public health and health policy at the University of Vermont's Larner College of Medicine, told ABC News. "It really is so contagious, it seeks out pretty much every unvaccinated person."

The delta surge in Vermont mirrors rising cases in the region, as northern parts of the country that were largely spared over the summer are now seeing increases during colder weather. Vermont is one of 22 states, many of them with colder temperatures, that has seen an uptick in daily cases of 10% or more in the last two weeks, according to an ABC News analysis of CDC and Health and Human Services data.

Vermont is also one of 14 states that have seen an increase of about 10% or more in hospital admissions over the last week, the ABC News analysis found. About two-thirds of hospitalized COVID-19 patients in Vermont are unvaccinated, with those in the intensive care unit also largely unvaccinated, state officials said this week. COVID-19 patients make up between 10-15% of ICU patients; if that number increases to around 25%, "then the system could be in jeopardy," Levine said.

Regarding the recent case surge, Vermont may also be a "victim of our success," Levine said Tuesday, pointing to a lack of natural COVID-19 immunity among unvaccinated residents "because we kept the virus at such low levels throughout the entire pandemic." Vermont has one of the lowest levels nationwide of people who have developed natural immunity to the virus, CDC data shows.

By the same token, waning immunity among residents who were "efficiently and effectively" vaccinated early on is also likely contributing to rising cases, Levine said. Breakthrough cases among vaccinated residents are up 31% over the past week, according to state data.

The COVID-19 vaccines have proven to be highly effective at preventing serious illness, hospitalization and death. No vaccine is 100% effective, and the waning immunity among residents, especially those who may not have mounted a robust immune response, may be tested by high community spread, experts say.

"You still have pockets of unvaccinated people, even in a highly vaccinated state," said Dr. John Brownstein, an epidemiologist at Boston Children's Hospital and ABC News contributor. "Unvaccinated individuals are the primary host by which the virus will spread and continue to allow for transmission to take place in the community and ultimately create challenges for those that are vaccinated."

Changes in behavior, including more travel and indoor gatherings, and Halloween festivities have also helped fuel the surge, state officials said. At Saint Michael's College in Colchester, Halloween parties were blamed for causing an outbreak on campus that led school officials to briefly move classes online and suspend in-person social gatherings through Thanksgiving. Post-Halloween, 87 students have tested positive for the virus, compared to just 11 between Aug. 27 and Oct. 22, according to school data.

"We were doing really well as a community up to the point where there were numerous Halloween parties where students were unmasked and in close contact," the college's president, Lorraine Sterritt, said in a letter to students earlier this week.

Statewide, COVID-19 cases are not expected to decrease over the next four weeks, state modeling shows, as hospitalizations are on the rise. Vermont has among the lowest COVID-19 hospitalization rates in the country "thanks to vaccines doing their jobs," Gov. Phil Scott told reporters Tuesday. But ICU capacity is the "biggest concern at this point" as hospitals are currently "under stress from an increase in patient care for health issues that are not related to COVID," he said.

Health officials are stressing vaccination and urging residents to get booster shots and vaccinate newly eligible children. Nearly 50% of Vermonters aged 65 and older have gotten a booster dose, while over 30% of children between the ages of 5 and 11 have made an appointment to get vaccinated or already started the process, state officials said Tuesday.

Reaching the remaining unvaccinated adults will also be key, Carney said.

"If there are people who have not yet decided to get vaccinated, I strongly urge them to talk to whoever they seek for their health care and have a conversation," she said. "Vaccinating as many people as we can who are eligible for the vaccine will help us -- in the short-term and in the long-term."

Maintaining high levels of testing will also help, Brownstein said. "Testing is such an important way for us to identify those who have been exposed and infected and to limit transmission," he said.

Scott said he isn't reissuing a mask mandate amid the increase in cases, saying he feels it would be an "abuse of power," but encouraged residents to "take a few extra precautions," including wearing masks indoors while in public and getting tested before gatherings.

"If we make smart decisions in the coming weeks, and make an extra effort to protect the vulnerable, we can help reduce hospitalizations," Scott said. "But it takes all of us committing to these smart, practical choices, starting with getting vaccinated."

"None of us wants to step backwards," he added.

ABC News' Arielle Mitropoulos contributed to this report.