‘I feel good about the future,’ says MUSC scientist tracking COVID

May 10, 2021
Logo for COVID immunity. Red virus particle with green checkmark.
Rahel Wachs created this image of the coronavirus with a green check mark, representing immunity. Illustration from Wikimedia Commons*

The COVID case rate basically held steady in the Tri-county area in the MUSC COVID-19 Epidemiology Intelligence Project’s latest update. There were 874 new cases during the past week in Berkeley, Charleston and Dorchester counties combined, compared with 863 the week before.

But project leader Michael Sweat, Ph.D., is optimistic.I feel good about the future, because the fact that we're getting growing numbers of people vaccinated is going to have an effect. There's just no question. It protects more people. It also breaks up the ability of that virus to spread through networks. It's not slowing down enough to make it go away right now, but it's helping a lot.”

Sweat has been through global health crises before, honing his expertise in developing mathematical models to predict what viruses will do – and what factors can change their trajectories. 

During the AIDS epidemic, he provided analysis for the Centers for Disease Control and Prevention on HIV. He later worked in international health at the Johns Hopkins Bloomberg School of Public Health, where he’s still an adjunct professor. Sweat currently directs the Center for Global Health at the Medical University of South Carolina and serves as a professor in the College of Medicine.

Since the start of the coronavirus pandemic, he’s been using data from MUSC Health, the South Carolina Department of Health and Environmental Control and other sources, along with emerging research on COVID-19, to give daily updates on cases per day for the Tri-county area and other parts of South Carolina, and weekly big-picture updates.

Dr. Michael Sweat 
Dr. Michael Sweat

As he watches the number of vaccinated people tick up, Sweat is also looking at the role of natural immunity in getting us closer to possible herd immunity. People who have had COVID may keep antibodies to the coronavirus for at least five months. The degree of immunity is affected by the severity of infection, the person’s age and other factors.

But Sweat said new research does appear to confirm that natural immunity works. “There have been a couple of high-quality, very prominent studies that came out in the past few weeks — one in Denmark and one in the U.K. — using huge numbers of people, tracking breakthrough infections for people who already had an infection and also looking at infection rates among people who've been vaccinated.”

Their conclusion: “I think the message, at least based on these two studies, is natural immunity seems to be as effective as vaccination,” Sweat said.

But that message comes with some important caveats. “Seems to be” isn’t the same thing as “is definitely” Sweat said. He brought up four key points.

  • Vaccines have been proved to reduce the severity of, hospitalization rate related to and mortality from COVID-19.  We do not have evidence that natural immunity has the same benefit.
  • Vaccines are highly effective against the current variants in circulation. We do not know if natural immunity is effective against variants.
  • Vaccines work well for all age groups. Natural immunity has been shown to be much less effective among people 65 years and older.
  • Many people mistakenly think that they have had COVID-19 in the past. If you avoid vaccination because of this, it is very risky.

Sweat’s team at MUSC has been calculating immunity estimates for every county in South Carolina. The current statewide estimate is 66%. That number goes up and down as people who had COVID more than five months ago are no longer considered immune.

Jane Kelly, M.D., an assistant epidemiologist with SCDHEC, called Sweat’s work high quality. “I’m excited by this analysis, but we don't know enough yet about duration of immunity. I am concerned that people will think they will be long-term protected by natural infection, but we don't have definitive proof of this," she said.

Using what has been proved, Sweat did some extra analysis this week on South Carolina data, looking at the impact of both vaccinations and natural immunity.

First, when it comes to vaccinations, the shots clearly make a difference. “We compared the 30 counties that have less than 40% vaccination immunity to the 16 counties that have 40% or more vaccination, and we looked at the growth rate over the past 14 days,” Sweat said.

“Those with lower vaccination saw 17% growth. But the counties with 40% or more vaccination went down about 17%. So the virus is clearly spreading more easily in places with lower vaccinations.”

Not surprising, right? But then he looked at natural immunity in a separate calculation. “In the eight counties that have less than 20% natural immunity, the growth rate in the past 14 days was up an average of 68%,” Sweat said.

“But in the 38 counties where there's 20% or more natural immunity, the growth rate declined by 8%.”

Finally, Sweat looked at the effect of vaccines and natural immunity combined. His conclusion: “Vaccination immunity is really working. Natural immunity is working almost as well. And when you combine the two, you can get an even better effect.”

All of the findings were statistically significant, Sweat said, meaning they were caused by something other than chance. 

He said everyone who can get vaccinated, should. 

Kelly said about 200,000 people across South Carolina are overdue for their second dose of Pfizer or Moderna. “A frequent comment I hear that surprised me was people saying, ‘I haven’t gotten my second dose because I’m waiting to get the booster for the variant.’ But the vaccines we have right now work against the variants. All of our vaccines are highly effective in preventing severe disease and death,” Kelly said.

And Sweat said people who haven’t had a COVID vaccine or been infected within the past five months are especially vulnerable now. The number of cases per 100,000 people is holding steady — even as more people get vaccinated. “All these infections are now getting more and more clustered in a shrinking group of people, suggesting to me that the seriousness of the epidemic is really bad for people without immunity.”

*Lead image license link

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