At least 63 people tested positive for COVID-19 after attending a dance party in the western German city of Muenster, local media reported on Tuesday.
Authorities were surprised at the high number of infections as only vaccinated or recovered people-both widely believed to have immunity against the disease-were allowed to attend.
Nearly 380 people were at the club during the party, and so far 63 of them tested positive for COVID-19, public broadcaster WDR reported.
Many had only minor symptoms, however, according to the report.
A number of German states and cities relaxed coronavirus restrictions last month and allowed restaurants, cafes, and dance clubs to reopen their indoor areas, but only for people who can present proof of vaccination or recovery from the virus.
The new entry rule is often referred to as "2G rule," from the German words Geimpfte (vaccinated) and Genesene (recovered).
But critics say the rule is creating a false sense of security among vaccinated people, as recent studies show that vaccinated individuals can still get infected and transmit the virus to others.