A Kaiser Permanente employee died and dozens of workers were infected with the coronavirus after a staffer appeared at one of its Northern California medical centers wearing an inflatable holiday costume on Christmas Day, the hospital and health care company said Monday.

Irene Chavez, senior vice president and area manager at San Jose Medical Center, said Sunday that 44 staff members of the emergency department have tested positive for the virus since Dec. 27.

The Oakland-based hospital chain is investigating the outbreak and says the source could be a blow-up, holiday costume worn by an infected staffer who “briefly” visited the department Dec. 25, according to its statement.

The staffer did not have symptoms at the time, but subsequently tested positive, said Hilary Costa, a Kaiser Permanente spokeswoman.

Inflatable costumes usually rely on battery-operated fans to suck air into the costume to keep its shape. COVID-19 spreads through droplets in the air, which is why investigators are looking into the air circulation function of the fan.

“Any exposure, if it occurred, would have been completely innocent, and quite accidental, as the individual had no COVID symptoms and only sought to lift the spirits of those around them during what is a very stressful time,” Chavez said, adding that the holiday costume was not approved beforehand by Kaiser.

Costa, the spokeswoman, said she did not know what kind of costume the person was wearing. NBC affiliate KNTV-TV in San Jose reported Sunday that the costume was a Christmas tree.

Kaiser declined to identify the employee who died of COVID-19 complications.

The emergency department is open and undergoing a deep cleaning, Chavez said. Nearly 40,000 Kaiser Permanente health care workers in Northern California have already received COVID-19 vaccinations, she said.