May 25 (UPI) -- More than 10,000 people in the United States have been infected with COVID-19 after being vaccinated against the virus, according to data released Tuesday by the Centers for Disease Control and Prevention.
They are among the roughly 101 million people nationally who received both doses of the two-shot Pfizer-BioNTech or Moderna vaccines or the single-shot vaccine from Johnson & Johnson as of April 30, the agency said.
Of the more than 10,000 people diagnosed with so-called "breakthrough infections" -- or infections that occur after full vaccination -- 27% experienced no symptoms of the virus, the data showed.
However, 10% of these patients were hospitalized following infection and 2% ultimately died.
"Even though FDA-authorized vaccines are highly effective, breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission," the agency researchers wrote.
"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," they said.
The vaccines from Pfizer-BioNTech and Moderna offer more than 90% protection against serious illness from the coronavirus, while the Johnson & Johnson shot is more than 70% effective, research suggests.
The protection offered by the vaccines is believed to last at least several months.
However, the vaccines do not necessarily prevent infection. Rather, they bolster the immune system so that infected people do not develop severe symptoms, according to the CDC.
A person is considered fully vaccinated 14 days after receiving the second dose of one of the two-dose products or 14 days following receipt of the single-dose shot, the agency said.
Sixty-three percent of the breakthrough infections in fully vaccinated people occurred in women, and most of those diagnosed were older adults age 40 to 74.
Nearly 60% of the breakthrough infection cases involved the B.1.1.7, or "U.K." variant of the virus, while one in four were the B.1.429, or California, strain.
These strains are believed to be more contagious than the one first identified in Wuhan, China, in late 2019, according to the CDC.
Only 5% of the samples collected from the more than 10,000 breakthrough cases, however, underwent analysis for genetic variants.
"The proportion of reported vaccine breakthrough infections attributed to variants of concern has also been similar to the proportion of these variants circulating throughout the United States," the agency researchers wrote.
Still, "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," they said.