"Through contact tracing, the disease investigation team discovered more than five laboratory-confirmed COVID-19 cases and many more ill individuals," officials said.
The party was attended by a "large number" of extended family members after officials issued a safe-at-home order to prevent the spread of the virus.
Health officials said the first patient in the outbreak identified with COVID-19 was "coughing and not wearing a face covering at the party," where guests were also not wearing face coverings or practicing social distancing.
"As a result, COVID-19 spread among party attendees," city officials said in a statement.
Officials did not disclose on what date the party took place.
"This is an example of how good contact tracing can identify disease clusters and tell us more about the spread of disease in our community," Public Health Department epidemiologist Dr. Matthew Feaster said in a statement. "We're grateful to our large team of public health nurses, case investigators, and contact tracers who help track the virus and prevent the disease from spreading to other members of our community."
Dr. Ying-Ying Goh, who heads the health department, said that while there have been small modifications to the stay-at-home order, gatherings of people who do not live in the same household are still prohibited.
"The virus remains highly contagious," Goh said. "Social distancing, frequent hand washing and wearing face coverings remain our best defenses against the further spread of COVID-19 in our community.”
The city is offering free testing for Pasadena residents experiencing symptoms at the Rose Bowl and ChapCare’s Kathryn Barger Health Center.