WASHINGTON (AP) — Health officials on Thursday stressed that people with flu-like symptoms should avoid public transportation but said everyone else needs only to follow commonsense precautions, as the nation’s swine flu cases passed 100, reaching 16 states.

The Obama administration stood solidly against closing the U.S.-Mexico border, with Vice President Joe Biden calling it “a monumental undertaking” that would do little good.

Authorities noted the virus already has hit several states — including South Carolina, with 10 confirmed cases, and four cases confirmed so far among University of Delaware students. Closing the border now would be, as President Barack Obama put it Wednesday night, “akin to closing the barn door after the horses are out.”

There have been schools closed in roughly 100 school systems, and Seattle and Huntsville, Ala., joined the list Thursday as officials awaited word on whether some sick children had the infection. Texas authorities suspended high school sports.

Biden reiterated on Thursday advice the administration has been eagerly dispensing: “A parent whose child’s school is closed out of a precaution or because there’s been a confirmed case of flu should not take the child then to a day care center. They’re going to have to take them home.”

“And the hope is that the employers will be generous in terms of how they treat that employee’s necessary action of taking that child home and not being at work,” he said.

At a congressional hearing, Dr. Anne Schuchat of the Centers for Disease Control and Prevention sought to strike a balance: No one knows what the never-before-seen virus ultimately will do, but so far in most U.S. cases people are recovering without even needing a doctor’s care. The big message is to try not to spread infection.

“This is a time when we don’t want the worried well flooding the emergency rooms,” she said. “At no time in our nation’s history have we been more prepared to face this kind of challenge.”

The CDC and officials in several states have confirmed at least 120 cases. They are in New York, Texas, California, South Carolina, Delaware and scattered cases in Arizona, Indiana, Kansas, Massachusetts, Michigan, Nevada, Ohio, Minnesota, Colorado, Georgia and Maine.

The vice president also was asked whether he would advise his own family against flying to Mexico and gave a surprise answer: “I would tell members of my family — and I have — I wouldn’t go anywhere in confined places now,” Biden said. “It’s not that it’s going to Mexico. It’s you’re in a confined aircraft. When one person sneezes, it goes all the way through the aircraft.” Biden went on to say that he wouldn’t suggest that they ride the subway either.

Biden’s office quickly issued a statement backing off the remarks and suggesting he was talking only about travel to Mexico, where the new virus has hit hardest, and urging people not to use public transportation while sick.

Lawmakers asked Schuchat about the risk of confined spaces, and she said there may have been a misstatement.

“If you’re ill, you shouldn’t get on an airplane or any public transport to travel,” Schuchat said. “If you’re sick, stay home. I can’t tell you how many times I’ve said that this week.”

She added, “I’m looking forward to getting on an airplane later today,” to return to Atlanta, where the CDC is based.

Schools aren’t the only focus. In California, dozens of Marines were under quarantine to see if they’ll develop illness after contact with a comrade confirmed to have the new flu.

U.S. scientists are racing to develop the key vaccine ingredient — a strain of the virus engineered to trigger the immune system. But they cautioned again Thursday that it would take several months before enough doses could roll off assembly lines for the necessary testing in human volunteers.

The U.S. has reported the only death outside Mexico — a Mexican toddler who visited Texas with his family.

Biden was interviewed on ABC’s “Good Morning America,” CBS’s “The Early Show” and NBC’s “Today” and Besser appeared on ABC and CBS. Fauci was on the “Today” show.