SC coronavirus immunity estimate shocks expert

April 08, 2021
MUSC nurse Rebecca Hale puts on PPE prior to going into a patient’s room.
Nurse Rebecca Hale gets ready to see a COVID-19 patient. The number of such patients is down from earlier surges as more people become immune to the coronavirus. Photo by Sarah Pack

Public health experts at the Medical University of South Carolina estimate statewide immunity to the coronavirus that causes COVID-19 has hit about 60%. 

“I'm a little shocked,” said Michael Sweat, Ph.D., leader of the MUSC COVID-19 Epidemiology Intelligence Project. “But as we went back through the numbers, it was kind of hard to argue.”

His team has put not only the statewide estimate but also estimates for every county in South Carolina online, updating the website as more information comes in. It’s part of a public service MUSC has been offering since the early days of the pandemic, tracking everything from COVID case numbers to vaccinations to give the public, health care leaders and politicians solid data for decision making.

Pickens, McCormick and Greenville counties have the highest estimated immunity, all over 70%.

Saluda, Jasper and Berkeley counties have the lowest, coming in below 40% each.

The Charleston Tri-county area, which includes Berkeley, Charleston and Dorchester counties, has a combined estimated 52% immunity.

“I've always been pretty optimistic about the more recent period we're in, because vaccinations are going pretty well,” Sweat said. 

Vaccinations are one part of the complicated formula that helps his team estimate immunity. “We take into account whether you've had one vaccination or two. You just get partial credit for immunity if you've only had one dose. Once you've had two, we give you full credit.”

Infections, both documented and estimated, are another key factor. Sweat said for every known case of COVID-19, there may be at least five other people who either didn’t get sick or didn’t get sick enough to get tested.

And the length of time people keep their immunity to the coronavirus after recovering from an infection is part of the formula as well. It’s an evolving area. As more time passes, scientists know more about it.

Just this week, Sweat’s team extended the length of time it considers people who have had COVID-19 are likely to retain their immunity from three months to five months, based on high quality research published online in the respected journal Science. Some doctors think immunity extends well beyond that, but that hasn’t been documented yet.

You can get a lot more detail about how the epidemiology team estimates immunity on its website.

The epidemiology team’s latest update covers a lot of other COVID information as well. COVID cases in the Tri-county area dropped 7% over the past week. It’s easy to get a test and hospitals aren’t under any strain from the virus.

However, Sweat cautions people not to let the good news obscure the fact that about 40% of South Carolinians may still be vulnerable to the coronavirus because they haven’t been vaccinated or infected. People who aren’t eligible to get a vaccine yet, such as children, are included in his estimates since they can get sick and transmit the virus to other people.

“There’s still a lot of virus around. I mean, it doesn't take much for this to blow up,” Sweat said. “This virus seems to be whack-a-mole ever since the beginning. Michigan, in particular, is having this massive outbreak. It could happen again here, too.”

He also worries about variants that help the virus spread more easily. But those concerns come against the backdrop of more and more people getting vaccinated every day - and that encouraging 60% immunity estimate for South Carolina.

“There is a point where you get to, traditionally called herd immunity, where the virus just can't propagate anymore. It can’t get through the networks and goes away. I personally don't think we'll ever completely do away with this, because there are reservoirs of the virus in some places," he said.

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