Public health officials in Pasadena have traced a cluster of COVID-19 cases back to a single birthday party that occurred in early April, officials announced Saturday, May 9.

The disease investigation team discovered “more than five laboratory-confirmed COVID-19 cases and many more ill individuals” among those who attended the party, according to a city statement.

The first person from the party identified with the disease, which is caused by the coronavirus, was coughing and not wearing a face covering at the event, according to the Pasadena Public Health Department. Other guests also did not wear face coverings or practice social distancing, investigators said.

The party, which included the host’s extended family and friends, took place both indoors and outdoors in a private home. People of all ages were present, according to city officials.

City spokeswoman Lisa Derderian said five people “that we know of” — the five whose cases have been confirmed — were tested for COVID-19. As of Saturday morning, there were not any pending test results from the party.

As for the “many more” individuals who are ill, Derderian could not provide a specific number but said, “several people have reported symptoms but weren’t tested.”

Derderian said the city referred those people to the Rose Bowl testing site and recommended they quarantine, but Pasadena does not have the regulatory authority to require them to do so.

As for whether the number of cases linked to the party may continue to climb, Derderian said it’s possible.

On Saturday, Pasadena health officials reported 11 new cases of the virus in the city, for a total of 542 since the outbreak began, but added no new deaths to its total of 65.

Health officials said the cluster should serve as a reminder about the city’s health order, which bans gatherings of all sizes.

“Pasadena residents who stay home keep themselves and their loved ones protected from COVID-19,” said Dr. Ying-Ying Goh, the Pasadena Public Health Department’s director and health officer. “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.

“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.”

The news came as health departments throughout the state double down on contact tracing efforts to identify as many people as possible who may have been exposed to the virus.

Earlier this week, Gov. Gavin Newsom announced a partnership with UC San Francisco and UCLA to mobilize 20,000 so-called “disease detectives” over the next two months as the state slowly begins to ease stay-at-home orders.

As part of the program, the universities will work with health departments, including Pasadena’s, to train workers to track and help isolate those possibly infected with the deadly virus.

In Pasadena, it seems contact tracing — which Derderian said is a “very staff- and time-intensive process” — has already begun to pay off.