A “large number” of extended family members and friends were at the party, the Pasadena Public Health Department said in a news release, adding the event took place after the city issued a stay-at-home order in March.
One patient at the party was coughing and not wearing a face covering, health officials said, and other party guests were also not covering their face or social distancing.
“She was joking with people at the birthday party,” Lisa Derderian, a spokeswoman for Pasadena, said of the woman who was coughing. “She said I may have Covid-19, and lo and behold, she did.”
“It was selfish behavior that could have been avoided,” Derderian told CNN.
Through contact tracing, investigators were able to confirm five Covid-19 cases traced to the party, all Pasadena residents.
“But we think five or six more party guests who live outside Pasadena May have been infected, because they showed symptoms of the virus,” Derderian said. “They need to quarantine and eventually be tested.”
“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,” said Dr. Matthew Feaster, an epidemiologist with the department.
More than 66,550 people have been infected with the virus in California and at least 2,687 have died, according to Johns Hopkins University.
California was the first state to implement a stay-at-home order for its nearly 40 million residents on March 19, closing nonessential services and asking residents to stay home and maintain social distancing.
Pasadena officials issued a similar order. Essential businesses that remained open needed to practice social distancing, keep people 6 feet apart and provide access to hand washing, among other requirements.
California is now in the midst of a phased reopening and allowed some businesses like retailers, manufacturers and warehouses to reopen beginning Friday. Offices, gyms, restaurant dine-in service, malls, museums, salons and bars remain closed.
“Although we are moving forward with small modifications to the Safer at Home Order, gatherings of people who do not live in the same household are still prohibited,” Dr. Ying-Ying Goh, director and health officer of the department, said. “The virus remains highly contagious. Social distancing, frequent hand washing, and wearing face coverings remain our best defenses against the further spread of Covid-19 in our community.”