The latest info shows that the number remains extremely low.
In those totals, released on Friday, April 30, more than 95 million Americans were fully vaccinated as of Monday, April 26.
To be fully vaccinated, two weeks or more must have passed since they received the second dose of the Pfizer or Moderna vaccine or the single-dose Johnson & Johnson vaccine.
Of those nearly 100 million fully vaccinated people, only 9,245 became infected with so-called vaccine breakthrough cases. That's an efficacy rate of 99.999.
A total of 5,827 (63 percent) of the cases were women, and 4,245 (45 percent) were adults age 60 and over.
In initial data on breakthrough cases reported in mid-April, of the then 77 million Americans fully vaccinated, there were 5,800 such cases for an efficacy rate of 99.992.
The efficacy rates of the double-dose Pfizer and Moderna vaccines were both around 95 percent in clinical trials. The J&J vaccine was 66.3 percent.
"It is important to note that reported vaccine breakthrough cases will represent an undercount," the CDC said in a statement after the newest results were released. "This surveillance system is passive and relies on voluntary reporting from state health departments which may not be complete.
"Also, not all real-world breakthrough cases will be identified because of lack of testing. This is particularly true in instances of asymptomatic or mild illness. These surveillance data are a snapshot and help identify patterns and look for signals among vaccine breakthrough cases."
The CDC said it will now be releasing data on the number of breakthrough cases on a weekly basis.
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