Reaching an accurate number for Covid breakthrough cases?

New Jersey has recently reported a Covid breakthrough fraction of 21.6%. In New Jersey from Aug. 23 through Aug. 29, fully vaccinated individuals accounted for: 2,602 positive tests out of a statewide total of 12,051

link: https://patch.com/new-jersey/mendham-chester/covid-breakthrough-cases-rise-nj-data-says-vaccines-work