RALEIGH, N.C. (WNCN) — Many North Carolina counties with the most new COVID-19 cases lately have some of the lowest rates of immunity to the virus — either through vaccination or recovery from a previous infection — a CBS17.com data analysis found.

“If we look across North Carolina, we’re going to see a lot of variation with regard to where we are towards herd immunity,” RTI epidemiologist Dr. Pia MacDonald said.

There are two ways to develop immunity to the virus that causes COVID-19 — getting the vaccine, or catching it and recovering — so adding those rates together can give a rough estimate of the immunity in those counties.

For example, in Wake County, state Department of Health and Human Services data shows 57 percent of people are at least partially vaccinated while another 8 percent have been infected — for a total of 65 percent, though of course there may be some overlap between those groups. 

(This table shows the combined first-dose vaccination rate and infection rate for every county in North Carolina. Counties where that combined rate exceeds 50 percent are shaded blue while those where it falls short of 50 percent are shaded red.)

Not surprisingly, Wake County ranks in the bottom third in the state in terms of new cases per capita.

Of the 15 counties that had at least 100 new cases for every 100,000 residents over the past two weeks, the combined vaccination and infection percentage was below 50 percent in 12 of them.

Conversely, there were 16 counties with 15 or fewer new cases per capita during that span — and that combined percentage exceeded 50 percent in 11 of them.

“We need to be monitoring the vaccination rates, as well as the previous infection rate, to understand where to look most closely for outbreaks,” MacDonald said.

With herd immunity still a long way off and vaccination rates slowing down despite incentives including cash drawings for $1 million, health experts are concerned about outbreaks in localized pockets of unvaccinated people that have the potential to turn into a big problem.

A study by Bloomberg News found hundreds of counties across the nation where fewer than a quarter of residents are fully vaccinated. North Carolina state health data shows three such counties in North Carolina — Hoke, Robeson and Onslow.

“Globally, we have not got anywhere near a sense of herd immunity for this virus,” said Dr. Cameron Wolfe, an infectious disease specialist at the Duke University School of Medicine.

From the start of the vaccination rollout, public health experts said we wouldn’t reach herd immunity — when enough people acquire protection from the virus, either through inoculation or infection, to severely slow its spread — until 70 percent or 80 percent of the population had some immunity.

Wolfe says those estimates have held up, although it’s more of a sliding scale than a firm rule. Sparsely populated places might be able to get by with lower rates of protection while it might need to be higher in dense areas.

“It’s not as if you suddenly hit 75 percent,” Wolfe said, “and COVID vanishes.”

CBS 17’s Joedy McCreary has been tracking COVID-19 figures since March 2020, compiling data from federal, state, and local sources to deliver a clear snapshot of what the coronavirus situation looks like now and what it could look like in the future.