So-called "breakthrough" infections with the novel coronavirus among fully vaccinated individuals are rare and often mild, according to a CDC report released on Tuesday—news that comes as the agency announced it will no longer track breakthrough cases that don't lead to hospitalization or death.

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Details on the new report

In the report, CDC found that, as of April 30, just 10,262 Americans developed Covid-19 after they had been fully vaccinated, accounting for about 0.01% of the more than 101 million people who were fully vaccinated at the time—although CDC warns the figure is likely "a substantial undercount" due to the difficulty tracking breakthrough infections.

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Among individuals with reported breakthrough cases, 27% were asymptomatic, 10% were hospitalized, and 2% died. Of those who were hospitalized, 29% were either asymptomatic or hospitalized for a reason not related to Covid-19.

The median age of those who experienced a breakthrough infection was 58, and the median age of those who died was 82, according to the report.

Nearly two-thirds of those who experienced a breakthrough infection were women. In addition, for the 5% or so of breakthrough cases for which CDC had genomic sequencing data, more than half involved a coronavirus variant of concern.

"Even though FDA-authorized vaccines are highly effective, breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission," the report said. "However, vaccine breakthrough infections occur in only a small fraction of all vaccinated persons and account for a small percentage of all Covid-19 cases."

John Brownstein, a CIO at Boston Children's Hospital and a professor of pediatrics at Harvard Medical School, said the report "helps confirm, in a real-world setting, that breakthrough infections are rare and when they do happen, they mostly have no clinical significance,"

"We always tell vaccinated people that you can still get Covid," Simone Wildes, an infectious disease specialist at South Shore Health and a member of the Massachusetts Covid-19 Advisory Group, said. "We know in general that vaccines may not work as well in immunocompromised individuals, but, overall, we haven't seen a lot of breakthrough clusters."

CDC announces it will no longer track mild breakthrough infections

The report comes as CDC announced this month that it will no longer track breakthrough infections unless they lead to a hospitalization or death.

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CDC said it will still conduct studies on vaccine effectiveness that will include data on breakthrough infections, but only in specific populations such as health care workers, older adults, and residents at long-term care facilities, a CDC spokesperson said.

According to CDC's website, the change "will help maximize the quality of the data collected on cases of greatest clinical and public health importance."

A spokesperson for CDC said the agency made the change because, while no vaccine is 100% effective, the number of breakthrough infections is small and no significant demographic trends among these infections have been identified to date.

"We have to prioritize what we're doing, and the priority is to understand the cases associated with severe disease," Kathryn Edwards, a professor of pediatrics at Vanderbilt University School of Medicine, said of the new policy.

Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, said CDC's change makes sense, as a breakthrough infection is "such a rare phenomenon, and it doesn't change the trajectory of the pandemic."

However, some experts criticized the change, saying CDC is missing an opportunity to learn more about the real-world effectiveness of Covid-19 vaccines that could help identify certain trends in the pandemic.

"We are driving blind, and we will miss a lot of signals," Ali Mokdad, an epidemiologist at the University of Washington and a former senior scientist at CDC, said.

Michael Kinch, an immunologist and associate vice chancellor of the Centers for Research Innovation in Biotechnology at Washington University in St. Louis, said collecting more data is always better than not collecting enough. "The virus is constantly changing, and we need to stay three steps ahead of it," he said.

"What if a variant arises that is less response or, Lord forbid, unresponsive to the vaccine?" Kinch added. "The way you stop it is good old-fashioned epidemiology, which the CDC has historically done very well. But if you don't see it coming, you can't stop it" (Reed, "Vitals," Axios, 5/26; Smith-Schoenwalder, U.S. News & World Report, 5/25; Ramanathan, ABC News, 5/25; Rabin, New York Times, 5/25).