Niki Marienburg told KGW-TV, a Portland-based television news station, that family members from across the country attended the reunion at Sunriver Resort, a luxury resort in Central Oregon, for 10 days in June.
She told the station her family "had a great time" after not being able to see each other due to the COVID-19 pandemic.
"It sort of snowballed and we got text messages from other people in the family who actually tested positive," Marienburg told the news station.
Marienburg told the station that people need to be taking the virus seriously.
There will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from COVID-19," the CDC's website reads.
Symptomatic breakthrough infections appear to be happening among .0098 percent of fully vaccinated people, according to an ABC News story of cases reported by the CDC.