COVID tracking team releases estimates on immunity

March 02, 2021
Bar graph showing estimated immunity rates in Charleston, Florence and Lancaster areas.

An estimated 37% of South Carolinians may be immune to COVID-19 through vaccination or from being infected with the coronavirus during the last three months. The numbers come from the Medical University of South Carolina’s COVID-19 Epidemiology Intelligence Project, a group of public health experts that’s been tracking the coronavirus’ impact for almost a year. 

Project leader Michael Sweat, Ph.D., director of the MUSC Center for Global Health and a professor in the College of Medicine, did the analysis. “We are getting there. That's the message now,” he said.

The Tri-county area, MUSC Health’s home base, has an estimated immunity rate of 35.2%. That comes from combining the 20.9% of people vaccinated with the 15.2% of people who had an infection within the past three months in Berkeley, Charleston and Dorchester counties.

Dr. Michael Sweat 
Dr. Michael Sweat

The Florence metro area, which has an MUSC Health hospital, has an estimated immunity rate of 50.3%. That’s from combining the 27.5% of people vaccinated with the 24.7% estimated to have had a recent infection in Florence, Darlington, Marion and Williamsburg counties.

And the Lancaster metro area, another MUSC Health hospital site, comes in at 36.3%. There, 15.4% have been vaccinated, and 21.8% are estimated to have had a recent infection in Lancaster and Chester counties.

Sweat cautioned that his team is still working on refining the analysis, but said he was conservative with the numbers he used. “The percentages are probably even higher.”

An example of that conservative approach: The analysis only included people infected with the virus that causes the illness COVID-19 during the past three months.“There's some uncertainty about how long native natural immunity lasts, but everybody pretty clearly says at least three months. It may be up to a year, maybe longer, but we just don't know,” Sweat said.

He arrived at the percentages through a complex series of computer-assisted calculations that factored in such questions as: How many people have been diagnosed? How many have gone undiagnosed? How effective are the vaccines based on clinical trial data? How do you include people who have been both infected and vaccinated without double counting them? 

The answers were based on scientific standards, leading to the percentages. Sweat, who specializes in the statistical analysis of health care threats and treatments, said his team’s figures are not a sign that the pandemic is over. “We’re nowhere near herd immunity. That’s unique to each pathogen, and when something is highly transmissible like COVID-19, it requires a pretty high rate of immunity, and many think with COVID-19 it will probably require no less than 70% of people to have immunity, perhaps more.”

But Sweat said people should be optimistic. “Everything’s looking good. It’s just not going to happen overnight. We have to watch the signals and wait for the science to address some of the nuanced issues. Until we get to herd immunity levels it’s important to still practice prevention behaviors.”

Those issues include the threat of variants, the possibility of another surge and the ongoing need for vaccines to be updated.

Sweat and his team update their online project daily when it comes to cases per day and give a big picture update once a week, typically on Wednesday or Thursday. The team’s goal is to help officials and the public understand COVID-19 trends, predict any strain on the health care system and give everyone the best possible information so they can make good decisions. Next week, the team plans to start including data about estimated immunity.

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