(DENVER) -- While some areas of the country are cautiously celebrating falling COVID-19 cases, hoping the declines might signal the return to a long-awaited sense of normalcy, some states continue to struggle as Americans prepare for winter.
Health officials in Colorado are growing increasingly concerned as the rate of COVID-19 infections grows to levels not seen in more than 10 months.
There is "a clear increase in cases statewide," state epidemiologist Dr. Rachel Herlihy said in a COVID-19 briefing on Thursday.
In the last month alone, the state's daily case average has nearly doubled -- increasing by 91.5% since late September, according to federal data, and state data shows that Colorado's average positivity rate has risen from just under 7% last week, to nearly 8.5% this week.
"Colorado moving in the wrong direction is a clear signal that we are not yet out of this pandemic, especially in under-vaccinated states. Colorado has yet to reach 70% with a first dose and if you layer in colder temperatures and relaxed masking, history is likely to repeat itself," said John Brownstein, an epidemiologist at Boston Children's Hospital and an ABC News contributor.
While southern states in particular are seeing significant declines in their rates of infection, several states with colder weather, like Colorado, are beginning to experience an uptick in cases, as people begin to increasingly head indoors.
"Coronaviruses tend to thrive in winter months and colder weather," Centers for Disease Control and Prevention Director Rochelle Walensky said in a White House COVID-19 briefing last week. "Right now is not the time, as cases are coming down, to become complacent because we do know colder weather is ahead of us."
Five states -- Alaska, Colorado, Maine, New Hampshire and New Mexico -- have all seen a percent increase in hospital admissions of about 15% or more in the last two weeks.
"We are continuing to move very much in the wrong direction," Scott Bookman, Colorado's COVID-19 chief, said at a briefing on Wednesday.
According to state officials, the highest coronavirus case rate is among the 5- to 17-year-old age group.
Coronavirus-related hospitalizations have also been increasing in the state, a trend that is particularly worrying health officials.
Approximately 90% of the state's surgical and intensive care unit beds are currently in use, according to state officials. There are currently nearly 1,300 patients hospitalized with COVID-19 -- the highest number of patients receiving care since December, and on average, federal data shows that nearly 200 residents are being admitted to the hospital with COVID-19 each day.
"With the increase in percent of positivity and the concern of increase in cases in the coming weeks, we are all very concerned at this point about what we are seeing in our hospitals," said Bookman.
And as more patients stream into emergency rooms in need of care, the average number of available beds is rapidly declining.
Thirty percent of hospitals anticipate an ICU bed shortage in the next week. State health officials have told ABC News that hospitals in El Paso County have had days when they've had to turn away transfer requests.
The majority of those COVID-19 positive patients -- 77% -- are unvaccinated individuals.
To date, 61% of the total population in Colorado has been fully vaccinated, leaving a significant number of residents still unvaccinated. People who have not been fully vaccinated are 6.1 times more likely to test positive with the virus and 11.3 times more likely to die from it, compared with people who are vaccinated, according to the CDC.
The notable divide between vaccinated and unvaccinated is evident in counties across the state.
In Crowley County, home to just over 6,000 residents, less than 49% of the eligible population has been vaccinated with at least one shot, according to state data. High transmission across the county remains rampant. In El Paso County, which currently has one of the state's highest number of hospital admissions rate, approximately 65% of the county's population over 12 have been fully vaccinated.
In total, 15 Colorado counties are significantly lagging, partially vaccinating 50% or less of their eligible population.
Comparatively, 11 counties have vaccinated at least 80% of their total population with at least one shot. San Miguel County, with a population of over 8,100, has 74% of its population fully vaccinated, and its infection rate has remained steadily low, despite increasing figures across the rest of the state.
If the situation in Colorado does not improve in the coming days, Colorado Gov. Jared Polis said Thursday he is prepared to take certain steps to address the uptick, including bringing in federal medical surge teams to help local hospitals in need of extra support, halting elective surgeries, expanding the use of monoclonal antibody treatment and possibly reactivating crisis standards of care, which determine how to most efficiently use medical resources, such as ventilators or ICU beds.
"A new surge once again places a challenging burden on our already tired health care professionals while also deferring important hospital procedures. This should really send a message to those still on the fence to do their part," said Brownstein. "Remaining unvaccinated populations still represent opportunities for this virus to spread. This surge in Colorado should serve as an important warning to other states as we head into the winter months."
ABC News' Jeff Cook contributed to this report.