Colorado has averaged more COVID-19 cases in recent days than at any point over the past six weeks, a slow incline that’s mirrored by the positivity rate and is driven by the dominant presence of the delta variant.

Over the past week, the state has averaged 455 new cases each day. That number is still far below nearly any other point since September. But it’s also been steadily ticking up over the past 10 days, and six western counties are rated by the state as having “very high” COVID-19 incident rates. The delta variant first emerged there, in Mesa County, and it has become the dominant disease strain statewide. 

The state’s positivity rate has also climbed up from record lows: On average over the past week, 3.7% of COVID-19 tests have returned positive. That’s up from a low of 2.29% in late June and 2.65% two weeks ago. But, like the new cases, that rate is still significantly below spring and fall peaks.

“These trends emphasize the fact that we have not yet stopped the pandemic here in Colorado and across the U.S.,” said Glen Mays of the Colorado School of Public Health. “We still have a large population of unvaccinated residents, so the virus will continue to cause problems for these groups – and occasionally the virus will break through into vaccinated groups as well.”

It’s those pockets of the state that have lower vaccination rates that are playing a significant role in the recent spike. According to a weekly report by the White House, five Colorado counties had a more than 100% increase in their incident rate between July 8 and July 15. Three of those counties – Delta, Montrose and Fremont – have vaccination rates below 50%. Several counties have had a 26% or more increase in mortality. Most of those have sub-60% vaccination rates. 

There are still outliers: Eagle and Summit counties, both with high vaccination rates, were the other two counties to have a more than 100% increase in their incidence rate, according to the White House report, which was released last week. Breakthrough cases – infections among vaccinated residents – are inevitable, health officials have said.

According to data provided by the state Department of Public Health and Environment, there have been 4,970 breakthrough cases since Jan. 15, accounting for 2.7% of infections since then. The agency did not provide breakthrough cases identified since the beginning of July.

The department did provide that data for hospitalizations: Of 207 Coloradans hospitalized with COVID-19 in the second week of July, 88.4% of them were unvaccinated. Since January, the gulf is far wider: 95.3% of hospitalizations have been among unvaccinated people overall.

“The delta variant is more transmissible and more likely to result in hospitalization than the alpha variant,” Jessica Bralish, spokeswoman for the health department, said in an email. The alpha variant was previously dominant in Colorado. “We have seen increased disease transmission as a result, especially in areas with lower vaccination rates.”

Bralish stressed that the vaccine remains effective against the delta variant, “particularly two weeks after both doses are received.” 

Though test positivity and the average number of new cases has risen in recent days, the number of hospitalizations in Colorado have remained relatively flat, floating somewhere between 300 and 350 patients since late June. But if the uptick in cases continues, hospitalizations may follow, Mays said, as has often been the trend.

“Fortunately, in many urban areas of Colorado, we have reached levels of vaccination that should prevent hospitals from becoming overwhelmed with COVID patients,” Mays said. “But we could still see local surges in healthcare utilization that threaten the health systems of smaller communities.”

Mays said that “one of my biggest concerns” is students returning to in-person learning in the fall because many of them – all below the age of 12 – are still not eligible to be vaccinated. If the virus continues to circulate, “we could be in for a very disruptive school year,” he said.