COVID tracking team debuts important new feature in online updates: Estimated immunity

March 19, 2021
Graph showing the estimated immunity to COVID-19 in Charleston is about 40%.
We're a good way from herd immunity, but Charleston is making progress.

The Medical University of South Carolina’s COVID-19 Epidemiology Intelligence Project has added the category of estimated immunity to its weekly updates. The new metric gives an overview of where things stand in the push to end the pandemic. Right now, the team of public health experts behind the project estimates immunity to COVID-19 in Charleston County is almost 40%.

Michael Sweat, Ph.D., a professor in the College of Medicine and director of the MUSC Center for Global Health, leads the tracking team. “It’s a great metric,” he said. “I think it has the potential to help target where you put your vaccination efforts.”

Dr. Michael Sweat 
Dr. Michael Sweat

It also gives people an idea of where we stand in the quest for herd immunity. That’s the time when enough people are protected from COVID-19, because they’ve gotten sick from it or been vaccinated, to make it hard for the virus to spread. The South Carolina Department of Health and Environmental Control reports that 70% to 80% of the population needs to be immunized to reach herd immunity.

Sweat described how his team calculates estimated immunity. “We know how many people have been vaccinated, and there's some additional information that we feed in around that. For example, how effective the vaccines are. The two main vaccines being used are about 95% effective two weeks after getting the second dose.”

They also look at how many people were infected in the last three months. “We assume anybody in the last three months certainly has some sort of immunity. We have an estimate for the efficacy of natural immunity as well. That's not perfect, but we have a conservative estimate we use,” Sweat said.

“There are other little assumptions we have to grapple with, like how many people who got vaccinated also got infected, so you don't double count people. So we do those things and then we plug it into this kind of complicated model to look at uncertainty and all that stuff. In a nutshell, that’s what we do.”

Sweat’s team continues to track other categories as well. This week’s update shows there were almost 1,000 cases in the past week in Berkeley, Charleston and Dorchester counties combined.

The weekly change of reported COVID-19 infections was up 5%. “We were having a really significant decline there for a couple of weeks, but it’s beginning to plateau at a high number. There’s a lot of virus out there.”

Sweat is also keeping a close eye on COVID-19 variants.The CDC now is reporting 36% of all cases in the U.S. are the British variant. Here in South Carolina, it's much more likely to be the South African variant. But both of those transmit much more easily,” he said. “In Europe, they found when it got to about 50% of cases involved variants, they just had explosive growth, which they're dealing with right now.”

Does that mean the same thing will happen here? “If we get to this point where it really takes off, which will probably happen in a few weeks where we get to that 50% mark, for the proportion of cases that are variant, we'll have this battle going on between that force and vaccines,” Sweat said.

“The faster we can vaccinate, the faster we can suppress the variants. And I tend to be on the optimistic side. I think by summer, when we get a larger and larger proportion of the people vaccinated, the variant will not win that battle. But we're in this interim period, right?”

He urged people to keep up their pandemic precautions for now. “Only about what 20% of the population in the U.S. has gotten a vaccine, which is great, but it's got to get higher, you know? Don't run the race and give up just before you get to the finish line.”