Flu concerns are apparently keeping U.S. tourists out of Mexico.

A check by The Associated Press finds that some planes were nearly empty today on U.S.-to-Mexico routes.

Continental Airlines says fewer than half the available seats were booked on six flights tonight. One Continental flight from Houston to Cancun had reservations for just five of the 157 available seats. Ninety-three of the 124 seats on a flight from Newark, N.J., to Mexico City were unfilled just minutes before its scheduled departure.

Continental spokesman Dave Messing says flights heading back north, on the other hand, have been full.

American Airlines reports some of its passengers have been changing travel plans or requesting refunds but describes it as nothing unexpected. And the airline says, for now, it's flying its normal schedule.

Empty planes flying to Mexico would not be a surprise; the U.S. has warned Americans to avoid nonessential travel to Mexico.