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Some Tucson areas reach “theoretical” COVID herd immunity threshold
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Some Tucson areas reach “theoretical” COVID herd immunity threshold

A growing number of neighborhoods in Pima County have COVID-19 vaccination rates high enough to cross a “theoretical” threshold into herd immunity.

The threshold is not a fixed number, but it's currently how the county’s chief medical officer, Dr. Francisco Garcia, is thinking about herd immunity, he said. It’s when around 75% of the adult population is fully vaccinated, or maybe as low as 70%.

More than 70% of adults have been fully vaccinated in 41 census tracts in Pima County, according to data, through June 1, from the county’s online vaccination dashboard.

Census tracts are roughly equivalent to neighborhoods. Rates from tracts on the Tohono O'odham Nation are not included. 

These 41 census tracts are about 17% of all tracts in the county. They are mainly located in the Catalina Foothills, Green Valley, Oro Valley and scattered around areas near downtown Tucson and the University of Arizona.

They are generally areas with little social vulnerability, as measured by an index of U.S. Census data on income, age, race, language, housing type, occupation, education attainment, access to transportation and more.

The herd immunity threshold that these census tracts have crossed is theoretical, Garcia explained, because health experts still don’t have a precise estimate for the number of immune people needed to reach herd immunity, but they know it will take some combination of vaccine-induced immunity and natural immunity to get there.

Multiple factors move herd immunity’s goalpost and make it difficult to calculate the number of vaccinated people we need. For one, the real-world efficacy of COVID-19 vaccines will be different from the efficacy seen in clinical trials. And on the other side of the equation, new, more contagious coronavirus variants change the number of immune people we need to stop the virus from spreading.

The vaccination rates in census tracts where 70% of adults have been fully vaccinated are higher, by a fairly large margin, than the county’s vaccination rate as a whole.

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