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Companies are talking COVID-19 booster shots, what would signal it's time?

Vaccine-makers are already testing extra shots as well as updated formulas to target variants.

CHARLOTTE, N.C. — There is growing chatter signaling the likelihood that fully vaccinated people will need some form of a COVID-19 booster shot, somewhere down the line.

This week, it surfaced that Dr. Albert Bourla, the chief executive of Pfizer, said people might need a third shot within 12 months of being fully vaccinated. Earlier this year, Johnson and Johnson's CEO Alex Gorsky said a COVID-19 shot might be needed yearly.

Dr. David Priest, an infectious disease specialist with Novant Health, states the need for a booster or additional shot comes down to the length of immunity from the first round of shots as well as mutation of the virus.

"If we see immunity is lasting six months, a year, two years and numbers remain low, it's likely that a booster won't come every year," Priest said.

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The latest research shows mRNA vaccines by Pfizer and Moderna offer at least six months of protection and high levels at that. However, immunity duration beyond that is still unknown, and only time will tell.

"If we're seeing increases in numbers and individuals that are vaccinated that are still being infected with other variants, then, I think it's going to change the equation," Priest said.

In other words, tracking "breakthrough" cases will be important, along with tracking mutations of the virus.

The Centers for Disease Control and Prevention recently released data showing how rare breakthrough cases have been so far. Out of more than 76 million vaccinated people in the U.S., roughly 5,800 developed a COVID-19 infection anyways, an incidence rate of less than 0.01%.

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In South Carolina, there have been about 150 breakthrough cases, and that is with more than one million people fully vaccinated in the Palmetto State.

Dr. Jane Kelly, Assistant State Epidemiologist with South Carolina's Department of Health and Environmental Control, said most of the cases came with only mild or even no symptoms.

Kelly said SCDHEC is tracking the role of variants in these infections and their presence in the state, but noted that the good news is, for now, it appears the existing vaccines are still very effective with B.1.1.7, also known as the U.K. variant, and are only slightly less so with B.1.351, also known as the South African variant.

"They're still plenty effective. They will still keep you out of the hospital and keep you alive," Kelly said.

If the time comes for a pivot in the vaccine formula, Kelly said, it should be relatively easy.

"We are in a really new era of vaccinations," Kelly said. "The beauty of it, is they don't have to grow things out over months of cultures. They can do this is in a very short period of time. I'm talking about days to weeks."

Have a relative or friend in another state and want to know when they can get vaccinated? Visit NBC News' Plan Your Vaccine site to find out about each state's vaccine rollout plan.

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