The breakthrough cases presented with mild or asymptomatic infections, of which more than 90 percent were attributed to the Delta variant.
How effective are the vaccines against the new Delta variant? A newly released study provides data that may answer some of these questions.
The study found the Moderna vaccine produced more than double the antibodies compared to Pfizer, but doctors urge caution.
While Pfizer is currently the only vaccine to be fully approved, it is also the only vaccine authorized for emergency use in adolescents between the ages of 12 and 17, though Moderna filed for such approval earlier this year.