The four-month study from January 1 to April 30 found that out of 101 million fully vaccinated people, only 10,000 – .01% – contracted COVID-19.
University of South Florida public health professor Dr. Thomas Unnasch said a 99% effective rate is better than the initial clinical trials.
"It’s really fantastic. It’s amazing how successful and effective the vaccine is," said Unnasch.
Dr. Unnasch said the vaccine does its job well, but it is not perfect. The CDC study found that over the 10,000 so-called breakthrough cases, 10% of people went to the hospital and 2% of them died.
"That doesn’t surprise me too much either because there’s always going to be a small percentage of people who for whatever the reason, their physiology, their immune system and their genetics just aren’t going to develop a decent immune response to the vaccine," said Unnasch.
Some people are waiting to see how the vaccine does in people, and public health scientists said this report shows further proof the shot helps you avoid COVID. Experts said they hope for more details on how the breakthrough cases happen.
"One thing that I would really love to know more about is where people are getting exposed when they become breakthrough cases," said Cindy Prins, an associate professor of epidemiology at the University of Florida. "So, the idea is, are we having breakthrough in people because they had long exposures in the household? Is it a workplace exposure? Are these folks that are working in healthcare?"
More than a quarter of people didn’t show any symptoms, and the average age of those infected was 58 years old. Researchers said a complete age breakdown would give more perspective.
"There have been some studies out there that show that the older you get, the less robust your immune response to the vaccine is going to be," said Unnasch. "And in people over the age of 85, it’s like 30 percent of the people don’t really respond very well to the vaccine at all."
Scientists said the shot gives you a chance, and the CDC’s report reinforces that message.
"I think what makes the findings important is they can have confidence in the vaccine as something that can protect them, especially protect them from getting very, very sick or from dying," said Prins.
Editor's note: An earlier version of this story misstated the percentage of people who contracted COVID-19 after vaccination due to a math error. This version has been updated.