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CHICAGO — We talk a great deal about antibodies when it comes to post Covid infection and vaccinations. But there is another critical immune response that plays a role in Covid protection and it lasts long after antibodies wane, potentially extending vaccine power.

Since the early days of the pandemic, anthropologist Dr. Thomas McDade and his co-researchers have used Chicago as a field study seeking out antibodies in samples sent to their lab at Northwestern University.

“We found about 20 percent of people through the Fall of 2020 were exposed to SARS,” McDade said.

That was the earliest finding from the blots of blood. The cases of Covid ranged from mild to severe. And now most study participants have been vaccinated, providing a key opportunity for the researchers to measure the shots’ efficacy.

“We have samples from before they got vaccinated. We got samples after they got that first dose of vaccine and samples after the second dose,” he said.

Across the board for all groups, when the team measured again two months after their second dose, antibodies had decreased by 20%, a normal process seen with other types of vaccinations.

“It’s normal to generate a lot of antibodies following a vaccine or natural infection,” McDade said. “It’s also normal for those antibodies to decline over time.”

But, thankfully, there’s more to human immunity than antibodies. We have a built-in back up system that remembers the virus long after our first line of defense diminishes. It’s called cellular immunity, and it relies on what are called T-Cells and B-Cells – the same cells that help us fight other viruses and even cancer.

“When they see it again or something that looks similar, they will supercharge production of new antibodies and new cells that will fight the virus very quickly and very effectively,” McDade said.

In those who had COVID-19, did the immune system remember the virus after vaccination? Those who had previous mild or asymptomatic infections did not respond strongly to the first dose of vaccine. That’s likely because they experienced low viral loads during their natural infections — in other words, not enough of the virus to imprint a memory on the immune system. 

“They generated a neutralizing response that was about 50% of where we want it to be. Same with people who had never had any prior exposure to the virus. It took two doses of the vaccine to get everyone to full protection,” McDade said. ”A lot of people who have had Covid previously assume they are now immune and that they don’t need to get vaccinated. …  Our studies show if you were previously infected with the virus that causes COVID-19, you can’t assume you have a high level of natural immunity.”

In contrast, people who had moderate to severe cases of COVID-19, and likely higher levels of virus in their bodies during their illness, “responded very strongly to the first dose of the vaccine,” McDade said.

So, do we need boosters? For the immunocompromised, McDade says yes. For otherwise healthy individuals McDade said more people with the first dose is better.

“I think for the general population, for most people, we’ll actually all be better off is is more people get the first doses of the vaccine rather than everybody get a third who has already had the first two,” he said.

Even with the Delta variant in circulation, McDade’s study shows the vaccine is holding up when it comes to protecting against severe infections and hospitalizations.