Parts of Colorado where more people are vaccinated have fewer cases of COVID-19 overall, but some counties where few are inoculated appear to be lucky in avoiding a spike this spring.

The question is whether that luck will hold if their residents continue to go unvaccinated — especially with more-contagious variants lurking in the state. Mesa County came close to running out of hospital beds earlier this month as the delta variant, which spreads more easily and may be more lethal, moved through a population where two-thirds of people remained unprotected.

Those counties shouldn’t count on it, according to Beth Carlton, an associate professor of environmental and occupational health at the Colorado School of Public Health.

“If counties have low vaccination rates, now is the time to address that, especially with the delta variant,” she said.

It’s particularly important to get vaccinated now, she said, because one dose of the vaccine provides significantly less protection against delta than against other forms of the virus, and it takes five to six weeks to reach the highest level of immunity.

In Colorado counties with more than 100,000 people, the link between more vaccination and lower cases is easy to see, despite exceptions in about a dozen small counties. Large counties where more than half of the population is fully vaccinated, such as Jefferson County, recorded about half as many cases, compared to population, as the overall state in the first two weeks of June.

On the other end, El Paso County, where just over one-third of residents are vaccinated, had nearly twice the rate of cases as the state as a whole.

The disparity is even more stark in Mesa County, where the rate of new cases was four times the state’s. That may be at least partially due to spread of the delta variant among unvaccinated people, Mesa County Public Health spokeswoman Amanda Mayle said.

Most cases of the delta variant are in Mesa and El Paso counties, though it accounts for about 40% of variant cases found statewide last week — suggesting it’s starting to crowd out previous versions of the virus. It matches global evidence, where the delta variant caused a spike in highly vaccinated countries, like the United Kingdom.

So far, the delta variant hasn’t been found on the Eastern Plains — which is among the least-vaccinated areas of Colorado. They’ll still need to be watchful, said Trish McClain, public health director at Northeast Colorado Health Department.

The first Colorado cases of the alpha variant, previously known as B.1.1.7, were found in a nursing home in Simla. Alpha was more contagious than the original version of the virus, but has been eclipsed by delta.

“I haven’t been able to figure out a rhyme or reason in how the variants appear,” she said. “It’s a guessing game.”

It’s also not clear whether variants have anything to do with two exceptions to the correlation between vaccination and lower cases — San Miguel and Gunnison counties in western Colorado.

San Miguel County is seeing higher levels of transmission despite 64% of its population being fully vaccinated. That’s because its immunization rates vary in different parts of the county, said Lindsey Mills, a public information consultant for San Miguel County Public Health. The county’s case rate was about three times the state average in the first half of June.

More than two-thirds of cases are coming from the less-populated western side of the county, where fewer people are vaccinated, Mills said. The remaining cases are mostly milder, breakthrough infections in the county’s east side, which is more populated and attracts tourists.

“We have a really diverse community with staggering different views on vaccines and uptake in general,” she said, adding, “For the smaller population to be having these outbreaks, it’s been very much from social gatherings and household transmission.”

Gunnison County also had slightly more COVID-19 cases than the state as a whole, despite an above-average vaccination rate — 59% fully protected.

Joni Reynolds, program director for Gunnison County Public Health, said she’s not sure the rate reflects the true situation, though. In smaller counties, the virus spreading in even one family can throw off the numbers, she said. Gunnison County has about 17,000 residents.

“In a bigger county or a bigger population, that gets diluted,” she said.

Six of the 10 counties with the lowest case rates also had below-average vaccination rates, and all but one of them, Morgan County, have fewer than 5,000 residents.

Statistics fluctuate more in small populations, and the case numbers may be less reliable in rural areas with younger populations, who are less likely to develop symptoms or know they should get tested, Carlton said.

And it’s possible that the number of cases may be underestimated in some Eastern Plains counties because it’s not easy to get tested, said Trish McClain, public health director at Northeast Colorado Health Department. For example, in Washington County, the state sends a mobile testing unit once a week, she said.

The region’s agriculture-centered economy also could be helping, because the virus is less likely to spread in outdoor workplaces, McClain said.