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So far, there have been about 5,800 such cases, the CDC told CNN.
At the time that data was collected, 77 million American adults had been fully vaccinated, meaning 99.992 percent had dodged the disease.
The efficacy rates of the double-dose Pfizer and Moderna vaccines are both around 95 percent.
"To date, no unexpected patterns have been identified in case demographics or vaccine characteristics," the CDC told CNN.
The number of American adults who are now fully vaccinated is now 84 million adults.
Though breakthrough cases are rare, they do serve as a reminder that vaccinated people should wear masks and maintain social distance while COVID is circulating widely, medical experts say.
"These measures have to continue until a larger segment of the population is vaccinated," Dr. Francesca J. Torriani, an infectious disease specialist at UC San Diego Health, told The New York Times.
The United States cleared the 50-percent mark over the weekend, as half of US adults -- nearly 130 million people 18 and older -- now have received at least one COVID-19 vaccine shot.
The country’s vaccination rate of 61.6 doses administered per 100 people is No. 2 worldwide, behind only Israel.
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