LOS ANGELES- Two more students at a Sacramento-area school have been confirmed to have swine flu, health officials said on Monday, bringing to 13 the number of known cases in California.

More infections are expected to be found in the coming days and authorities said it was too late to contain the spread.

The word 'containment' is really inappropriate or ... unneeded at this time. It's too late. The swine flu is here, said Dr. Bonnie Sorensen, chief deputy director of the California Department of Public Health.

The two latest cases both involved students at St. Mel Catholic School in Fair Oaks, which earlier in the day was ordered closed until at least Thursday after a teenage boy there tested positive.

They were confirmed positive for swine flu tests at the Sacramento County Health Department.

A fourth student at St. Mel, which has about 270 students in grades 1 through 8, tested negative for the disease that has killed up to 149 people in Mexico.

A parochial school in the Los Angeles suburb of Claremont also canceled classes through Thursday, saying that a girl and her mother developed flu-like symptoms after a trip to Mexico.

No Californians have died from the swine flu, although two people have been hospitalized. One of them, a 35-year-old woman, was treated in intensive care before recovering.

Ten of California's 13 cases were reported in San Diego and Imperial counties, on the border with Mexico, and officials were puzzled by the lack of any clear link between them.

Except for a San Diego County father and daughter, none of those infected appeared to have anything in common.

They live distant distances from each other, they don't go to any of the same schools, churches, workplaces. They're really very independent cases from each other, said Dr. Gil Chavez, deputy director of the state's Center for Infectious Disease.

He said it was not known why the flu found in California was mild compared to that in Mexico. But he said he doubted that immunity played a role because the virus strain that appears to be involved in both countries is new in each.

Sorensen said the state was not considering quarantines because the incubation period is as short as too days.

So by the time someone is sick, they've already shed virus before anyone even knew they were ill, and they can spread it so quickly, she said. So it's really not logically realistic to use quarantine during influenza.

State health officials have not ordered any border restrictions but Gov. Arnold Schwarzenegger has advised Californians to take common-sense precautions.