When distributing the COVID-19 vaccine, prioritizing older adults first will save the most lives, according to a new study by the University of Colorado-Boulder.

The study, published in the Journal of Science on Friday, uses mathematical modeling to estimate how different distribution strategies would play out in the U.S. and other countries.

“Common sense would suggest you want to protect the older, most vulnerable people in the population first. But common sense also suggests you want to first protect front-line essential workers,” said senior author Daniel Larremore.

Larremore said the study helps to mathematically determine the best strategy, finding that in most scenarios, across countries, vaccinating adults 60 years old and older first will save the most lives.

“Age is the strongest predictor of vulnerability,” Larremore said, adding that age boosts vulnerability more than preexisting conditions like asthma. “You have an exponentially higher likelihood of dying from COVID-19 as you get older.”

This aligns with Colorado’s strategy, focusing on vaccinating Coloradans 70 years old and older in Phase 1B, before offering vaccinations to the rest of the phase, including essential workers in education and grocery stores.

On Friday, Gov. Jared Polis said Coloradans 70 and up account for more than 75% of the state’s nearly 5,000 COVID-19 deaths. He plans to have 70% of the age group vaccinated by Feb. 28.

“Our focus in distributing the vaccine has been to save the most lives and end the public health crisis as soon as possible,” Polis said.

The study also found that the speed of vaccination distribution is important and the longer it takes, the more essential it is to prioritize the elderly.

The study said that if the vaccine rollout speed was doubled from current rates under current conditions, COVID-19 deaths could be decreased by 65,000 over the next three months.

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The only circumstance that the study said would warrant vaccinating younger adults first is if the virus was under control and the vaccine was known to block transmission; that is not the case in the U.S.

“For essential workers who might be frustrated that they are not first, we hope this study offers some clarity,” said lead author Kate Bubar. “We realize it is a big sacrifice for them to make, but our study shows it will save lives.”

The results of this study have been used to inform policy recommendations by the Centers for Disease Control and Prevention and the World Health Organization.

The full study is available at sciencemag.org.