When it comes to herd immunity, how small can the herd realistically be?

North Carolina

RALEIGH, N.C. (WNCN) — Experts have broken down the pursuit of community protection from COVID-19 on many levels — from the global and national scale, all the way down to census tracts and ZIP codes.

Which raises a question: When it comes to herd immunity, how small can those herds realistically be?

“It is made for a population. It’s not really intended to be something that, you know, narrows down to your household, necessarily,” said Dr. David Wohl, an infectious disease specialist at the University of North Carolina School of Medicine.

While the question might seem frivolous, it does have a practical purpose: Cases have risen 70 percent over the past week in North Carolina, and experts agree the primary drivers of the surge are small pockets of unvaccinated people.

“Where it’s going to spread is those unvaccinated pockets of people, whether they be geographic pockets or socially connected pockets,” said Dr. Pia MacDonald, an epidemiologist at RTI International.

The state Department of Health and Human Services says 48 percent of North Carolina’s 10.5 million people have gotten at least one dose of vaccine.

But that number masks wide-ranging differences between counties and other corners of the state, with county-wide partial vaccination rates ranging from 65 percent in Orange County to 23 percent in Hoke County.

And looking even closer at those counties reveals differing vaccination rates — and, subsequently, different levels of community protection — among ZIP codes and census tracts.

So, it’s important to understand the demographics of those pockets.

“The concept of herd immunity assumes that everyone mixes equally, and we know that that’s not true in the social setting,” MacDonald said.

Some are determined by location. Others by social connections — those who gather together at churches, grocery stores and neighborhoods.

“Various ways that people mix and match,” MacDonald said. “And so from an epidemiological perspective, we look at how people mix and match based on communities and the geographic community is just one factor.

“But similarly, it’s important to understand the differences among groups that are socially connected rather than just locationally connected,” she added.

The state averaged 575 new cases over the past seven days. While far below the peaks six months ago, that does represent a 70 percent increase from the seven-day average of 338 a week ago.

While the most populous counties had the highest total cases, three counties stand out on a per capita basis: DHHS says three averaged more than 100 new cases each day last week for every 100,000 residents — Graham, Robeson and Richmond counties.

None of those three counties has a partial vaccination rate higher than 36 percent, a CBS17.com analysis found.

“You talk about counties and census tracts … where there’s only 30 percent of people who are vaccinated,” Wohl said, “that’s where you’re going to see the spread of Delta variant.”

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