What is a vaccine booster shot? Moderna is already starting volunteer trials

Moderna announced Wednesday that the first trial volunteers have received boosters of their vaccine, some of them modified to target a variant.

Seattle resident, Neal Browning, is one of the Moderna trial volunteers. ABC7 has been following him since last year.

"So March 16th 2020, I was the second human in the world to be given the, experimental at the time, Moderna vaccine. Which now, everyone's getting in their arms," said Browning.

VIDEO: First COVID-19 vaccine volunteers in US describe experience as Bay Area launches vaccine trials

With a trial for the AstraZeneca COVID-19 vaccine set to begin in the Bay Area this month, we talked to some of the first people in the U.S. who've tried a vaccine. One volunteer in Seattle said she's felt "fantastic" since being injected in March.

A year after his first two shots, Browning enrolled in another Moderna trial and is now recovering from a third Moderna vaccine injection that he got Tuesday.

Kate Larsen: "How are you feeling today?"

Neal Browning: "Not great earlier this morning, but I'm actually doing pretty good right now.... This morning I couldn't lift my arm, but now I have full range of motion, it doesn't hurt anymore. I had slight fever and chills earlier today, took a nap and popped some ibuprofen and I'm great now."

Moderna is enrolling dozens of previously vaccinated people in trials to test:

• A third booster of the original vaccine, like the shot Neal received
• A modified vaccine designed to better target the variant first identified in South Africa
• A booster containing both the modified vaccine and the original formula

"I think this is really good news," said Dr. Dean Winslow, an infectious disease specialist at Stanford. He points out that Moderna, Pfizer, and Johson&Johson are all working on second generation COVID-19 vaccines.

RELATED: COVID-19 vaccine trial: Doctor calls promising results a 'wonderful triumph'

"It means that as soon as later this summer or early this fall, that Americans and others around the world will be offered a booster dose of this modified vaccine."

"All of this is going to move faster. That's why I hope the public is feeling really secure about the future with COVID. The way out is vaccines and all of our technology now is highly adaptable and fast," said UCSF infectious disease specialist Dr. Monica Gandhi.

Dr. Gandhi also addressed why Browning may have had a strong immune response after his third vaccine dose.

"I think that's actually a good sign, if you get more sick after a booster. It's why actually there's some increasing data from Lancet and JAMA now that you only need one dose if you've had COVID-19. The FDA hasn't made that primetime, but we're seeing very good data that after one dose, if you've had COVID-19, you get ten times the antibody responses than if you did two doses and never had COVID."

Moderna has said that its original vaccine appears to still offer some protection against the COVID strain identified in South Africa.

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