CDC identifies only 10,000 COVID-19 infections in fully vaccinated, but likely an undercount

More than 10,000 fully vaccinated people in the U.S. have experienced a "breakthrough" COVID-19 infection, according to a new Centers for Disease Control and Prevention (CDC) study.

According to the CDC, a total of 10,262 vaccine breakthrough infections had been reported from 46 U.S. states and territories as of April 30, out of the approximately 101 million people who were fully vaccinated against COVID-19 at the time.

Among these cases, 63 percent occurred in women, and the median patient age was 58 years old.

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The coronavirus vaccines are highly effective, but they will not completely prevent all infections, so some breakthroughs are expected. 

"The number of COVID-19 cases, hospitalizations, and deaths that will be prevented among vaccinated persons will far exceed the number of vaccine breakthrough cases," the CDC said.

The agency noted, however, that the breakthrough number in the report is likely a substantial undercount.

State health departments voluntarily report vaccine breakthrough infections to the CDC, and people voluntarily report infections to states, so the data might not be complete or representative. 

Many persons with vaccine breakthrough infections, especially those who are asymptomatic or who experience mild illness, might not seek testing, CDC said.

Going forward, the CDC is limiting the monitoring of people who have been infected while vaccinated. The agency is only sequencing a limited number of the cases, which has drawn concern from scientists. 

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CDC said sequence data were collected from only 5 percent of the breakthrough infections, and 64 percent of the sequenced viruses were identified as "variants of concern," including the  B.1.1.7 variant (first found in the U.K.) and B.1.429 (first found in California).

In addition, beginning May 1 the CDC shifted from monitoring all reported breakthroughs to only those that result in hospitalization or death. The agency said the narrowed focus means only the cases of "highest clinical and public health significance" get reported.

Tracking and sequencing help identify who might be more at risk, and whether certain variants can escape the vaccine's neutralizing effects. Scientists have questioned the benefit of limiting surveillance, when casting a wider net would likely be more beneficial.

CDC said it will continue to lead studies in multiple U.S. sites to evaluate vaccine effectiveness and collect information on all COVID-19 vaccine breakthrough infections, regardless of clinical status.