The patient, who has not been identified, had been in close contact with someone who had been inoculated, health officials said. However, Dr. Jonathan Fielding, Los Angeles County's director of public health, said it remained unclear exactly how the patient became infected.

The vaccine is made with a live virus that can be spread by touching a vaccination site before it has healed or by touching bandages, clothing or other material contaminated with the live virus.

"We really don't know how it happened — it could have happened in a variety of ways," Fielding said. "What's important is they had direct contact with the person, rather than this being something that was just in the air."

Public health officials have declined to release the patient's gender or age, but have said the patient's condition was improving.

It was the first known case in California linked to the national drive to vaccinate emergency health workers and military personnel.

In Florida, officials are investigating medical problems among three health care workers who became ill after receiving the vaccine. Only one appears to have had a reaction to the inoculation, with the other two experiencing symptoms not normally related to the inoculations.

As of last week, 7,354 people had been vaccinated in 38 states and Los Angeles and New York, which are running separate programs.

In the military, where more than 100,000 people have been vaccinated, there have been five serious reactions. All five men are in good condition, officials said. In most cases, the symptoms were not severe.