The Centers for Disease Control and Prevention has identified about 5,800 fully vaccinated people who have contracted COVID-19 so far, a fraction of the 66 million Americans who have been vaccinated.

Why it matters: The infections, called "breakthrough cases," are rare. The findings are consistent with previous studies that indicate positive coronavirus cases are extremely unlikely among vaccinated people, and that the vaccines prevent severe disease.

• "Like is seen with other vaccines, we expect thousands of vaccine breakthrough cases will occur even though the vaccine is working as expected," the agency said in a statement on Thursday.

By the numbers: About 40% of the infections were in people older than 60, but breakthrough cases occurred among all age groups. 

• 65% were female, and 29% of the breakthrough infections were reported as asymptomatic. 
• 7% of people with breakthrough infections were known to be hospitalized, and 1%, or 74 people, died. 

The state of play: Cases among vaccinated people happen when the body either doesn't amount an adequate immune response or when immunity fades over time, NIAID director Anthony Fauci said on Monday.

• "We see this with all vaccines in clinical trials," Fauci said. "And in the real world, no vaccine is 100% efficacious or effective, which means that you will always see breakthrough infections regardless of the efficacy of your vaccine."
• Two studies published in the New England Journal of Medicine last month showed similarly small percentages of breakthrough cases among fully vaccinated health care workers.

What to watch: The CDC said it's collecting genomic sequencing on respiratory samples from breakthrough patients to further understand if any of the variants affect a vaccine's effectiveness.

• To date, it has identified no unexpected patterns in case demographics or vaccine characteristics.