The agency's disease investigation team used contact tracing to find more than five laboratory-confirmed COVID-19 cases and "many more ill individuals," according to a news release.
Health officials said the party was attended by "a large number" of extended family members and friends after the coronavirus stay-at-home order was issued.
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The index case, or first patient in the outbreak identified with the disease, was coughing and not wearing a face covering at the party, the Health Department said.
Guests did not wear face coverings or practice social distancing, according to the statement, resulting in the spread of the coronavirus among the celebration's attendees.
Dr. Matthew Feaster, a Health Department epidemiologist, described the findings as "an example of how good contact tracing can identify disease clusters and tell us more about the spread of disease in our community."
COVID-19 remains "highly contagious," said Dr. Ying-Ying Goh, the agency's director and health officer, adding that social distancing, frequent hand washing as well as wearing face coverings continue to be the best defense against the virus.
Free COVID-19 testing is available at the Rose Bowl and ChapCare's Kathryn Barger Health Center. Appointments are required. Register on the city's officials website.
A cluster of #COVID19 cases in Pasadena has been traced to a birthday party. Remember, the best way to celebrate #MothersDay and any other occasion is by STAYING HOME and staying healthy: https://t.co/YyAqMebKhF #StayHomeSaveLives pic.twitter.com/WI9uh6zTKs— City of Pasadena (@PasadenaGov) May 9, 2020