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Who is spreading COVID-19? CU Boulder did a study to find out

"As vaccination rates go up, there are more vaccinated targets for the virus to try and infect, and so at some point we see a crossover point," the researcher said.

BOULDER, Colo — Computer models helped University of Colorado Boulder researchers figure out how COVID continues to spread throughout the community.

Daniel Larremore and his team at CU Boulder and the BioFrontiers Institute published a study that found testing only unvaccinated people in highly vaccinated communities had little impact on community spread, because there are so many breakthrough cases.

"As vaccination rates go up, there are more vaccinated targets for the virus to try and infect, and so at some point we see a crossover point where in fact 50% of the people who are infected are vaccinated and 50% are unvaccinated," Larremore said. 

"Testing only the unvaccinated is going to be low impact, but if we know there are plenty of breakthrough infections, it may be better to test everybody, not just the unvaccinated folks," he said.

Demand for testing has surged in Colorado. According to CDPHE, its 150 free community test sites average 11,000 tests per day. During the holidays, that volume peaked at 20,000 tests per day.

RELATED: Colorado sees surge in demand for COVID-19 testing

Larremore told 9NEWS with at-home rapid tests often being the first line of defense, he expects positivity rates to increase.

"More people who get in line for that PCR test are more than likely to be positive. Fewer are likely to be negative. This has the effect of pushing up those test positivity rates in the PCR line," he said.

Positivity rates in Summit County have county officials warning all to be cautious. The seven-day average went from roughly 7.5% to about 24% in just a week. Boulder County's seven-day average also increased by 160% in one week.

Larremore's study was done before the omicron variant surfaced. CU researchers said now that that variant is spreading, the majority of infections in highly-vaccinated regions will soon be breakthrough cases.

RELATED: Denver, neighboring counties extend mask mandate for another month

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