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DENVER (KDVR) — If vaccinations continue at their current rate, Colorado could see herd immunity as soon as summer, but complications make late summer or even fall likelier.

As of March 31, there have been 464,000 COVID-19 cases and 1 million Coloradans who have received either both doses of dual dose vaccines or a single dose of the Johnson & Johnson vaccine.

That makes just under 1.5 million Coloradans with immunity either from prior exposure or through inoculation – about 38% of the number of Coloradans needed to achieve a minimal herd immunity threshold.

Herd immunity occurs when the virus cannot transmit to enough hosts to sustain itself. There is no consensus on how many people need to have immunity to achieve herd immunity, but experts speculate anywhere between 70% and 85%.

Daniel Larremore, an assistant professor at CU Boulder’s Department of Computer Science & BioFrontiers Institute, likens herd immunity to a forest fire.

“With vaccination, we’re fire proofing some of those trees,” he said. “Fundamentally, once you’ve fireproofed enough of the forest, the fire can’t spread very well. It runs out of fuel.”

But herd immunity isn’t quite as simple as having X amount of Americans or Coloradans vaccinated or infected.

Just as a host of factors complicate the way the forest fire works – the susceptibility of each tree, their distance from each other or the intensity of the fire – many issues complicated herd immunity. The virus’ strength makes a difference, as well as the strength of the immunity.

In particular, Larremore singles out variants as a complicating factor. New coronavirus variants such as the B.1.1.7 variant are as much as 50% more transmissible than the original wild COVID virus. That intensity raises the threshold for herd immunity.

“You can think of B.1.1.7 variant as wind,” said Larremore. “That makes it easier for the fire to jump from one tree to another.”

Still, Larremore said he can see herd immunity achieved by late summer or fall if the current vaccination rate continues and no new complications emerge.

On average, each day in March saw a 2-3% rise in the number of fully vaccinated Coloradans. This rate may quicken or slow down as it has in the past.

Depending on the herd immunity threshold and the rate, this puts Colorado herd immunity sometime between May and late August.

Assuming a 3% daily growth in fully vaccinated residents and the lowest 70% herd immunity threshold, Colorado could see herd immunity by May 13. This assumes daily cases will continue growing at their current rate.

On the other hand, assuming a much slower 1% daily growth and a higher threshold of 85% of the population, Colorado won’t see herd immunity until Aug. 29.

These estimates fall in line with Dr. Anthony Fauci’s predictions and earlier projections Data Desk ran based on population. In those, Colorado achieved herd immunity somewhere between May and August at the highest vaccination rates.