What is breakthrough COVID and who's most at risk?

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Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, during a briefing Tuesday defended the agency’s updated approach to monitoring breakthrough coronavirus infections, or COVID-19 occurring after a person is fully vaccinated.

While the recently approved coronavirus vaccines demonstrated over 94% efficacy in clinical trials, they are not 100% effective and health officials expected breakthrough illnesses.

Starting May 1, the CDC shifted its focus in monitoring breakthrough infections; narrowing studies only to cases resulting in hospitalization or death. A reporter questioned Walensky whether the new approach was the right strategy and if there was any increased concern over breakthrough cases as people begin to remove their masks.

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"These vaccines were studied to prevent severe illness, hospitalization and death and as we look at these breakthrough infections those are the ones we’re most concerned about," Walensky said during the briefing, adding that researchers are sequencing the reported cases to see if any variants are behind the infections.

"Before we started only studying breakthrough infections in the hospitalized patients, we were studying all breakthrough infections," she continued. "What we were starting to find is … a large portion of them were fully asymptomatic and in fact when we went to study [them], sequence them, there was inadequate virus to even do so."

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Walensky said the academic world is continuing to learn about breakthrough infections. Dr. John Whyte, the chief medical officer of the health care website WebMD, previously told Fox News that breakthrough cases could occur if someone is infected at the time they receive the vaccine or was exposed to the virus before their body had time to build up immunity following vaccination. 

The agency also discontinued reporting breakthrough case counts on its webpage. As of April 26, the latest available information, the CDC recorded 9,245 breakthrough infections amid a backdrop of over 95 million fully vaccinated Americans. Over half of cases were among women, 45% involved people over 60 and nearly a third were listed as asymptomatic infections. Less than 10% resulted in hospitalization, and there were 132 deaths (1%).

The figures likely represent an underestimate due to voluntary reporting from state health officials and inadequate testing, especially in people not experiencing symptoms, the CDC says.

Fox News' Madeline Farber contributed to this report.