The latest versions of COVID-19 vaccines were 54% effective at preventing symptomatic infection in adults, according to the first U.S. study to assess how well the shots work.

The shots became available last year and were designed to better protect against more recent coronavirus variants.

In Thursday's study, the Centers for Disease Control and Prevention looked at 9,000 people who got tested for COVID-19 at CVS and Walgreens pharmacies, checking who tested positive and whether they had gotten a new shot or not.

The 54% finding is similar to what's been reported in other countries, and it's also about what was reported for an earlier vaccine version, said the Ruth Link-Gelles of the CDC, the study's lead author.

Studies coming out later this year will assess how effective the shot was at preventing symptoms severe enough to send patients to a doctor's office or hospital, she said.

The CDC recommends the new shots for everyone 6 months and older, but most Americans haven't gotten them. The latest CDC data suggests only about 22% of U.S. adults have gotten the shots, and only 11% of children. The slow uptake meant that it took longer for researchers to gather enough data to assess how well the shots work, Link-Gelles said.