New data released by federal health authorities offer a glimpse into how breakthrough infections may affect the health of U.S. adults who are fully vaccinated against COVID-19. 

As of April 30, fully 10,262 SARS-CoV-2 breakthrough infections — illness contracted despite vaccination — were voluntarily reported by health departments in 46 U.S. states and territories, according to the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report, published Monday. The CDC defines breakthrough infections as those occurring 14 days or more after full vaccination.

The data, which the agency called preliminary, show that approximately 27% of patients with breakthrough cases had asymptomatic infections, 10% were known to be hospitalized, and 2% died. Among hospitalized patients and patients who died, approximately one third of each group either were asymptomatic or were hospitalized for a reason other than COVID-19 infection. The 2% of patients who died tended to be older than age 65, with a median age of 82 years, according to the CDC’s COVID-19 Vaccine Breakthrough Case Investigations Team.

Breakthrough cases are to be expected with vaccines in general, the CDC said; however, “there is some evidence that vaccination may make illness less severe for those who are vaccinated and still get sick,” it reported this month.

In fact, some U.S. long-term care operators have reported breakthrough cases among residents and staff. Riverview Retirement Community, a continuing care retirement community operator in Spokane, WA, for example, recently discovered an outbreak that included 21 residents and five staff members in its assisted living community. All had been vaccinated previously, Mike Drew, CEO, told McKnight’s Senior Living.

A number of factors can contribute to these cases. Mutating virus variants, for example, can sometimes bypass the protections of a vaccine, leading to post-vaccination transmission, the CDC stated.

“Current data suggest that COVID-19 vaccines authorized for use in the United States offer protection against most SARS-CoV-2 variants currently circulating in the United States. However, variants will cause some vaccine breakthrough cases,” it reported.

Some scientists contend that breakthrough cases also mean that ongoing testing of immunized individuals will be necessary to help prevent future outbreaks.

More than 123 million people in the United States have been fully vaccinated as of May 17, according to the CDC.