BOSTON -- Massachusetts health officials say they have tested 24 cases of possible swine flu infections and all have been negative, although they are awaiting the results of tests on two Lowell middle school students who got sick after returning from a spring break trip to Mexico.

The students' pediatricians took ear and nose swab samples on Friday that were sent to the U.S. Centers for Disease Control and Prevention for testing on Monday morning, Lowell Health Director Frank Singleton said.

"Lab sample was taken. There were two suspected swine flu cases in Lowell and the confirmation will be sought at CDC's laboratory and will be reported back to us as soon as it's known," Singleton said.

The Lowell Health Department expects results Tuesday and, if confirmed, the students would be the first two cases in the Bay State.

The siblings attend a private middle school in Lowell and their mother is a teacher's aide in the public school system.

"They haven't been hospitalized and there's no information we have that there's anything going on, other than that they just have the flu," Singleton said.

By the middle of this week, the state health lab in Boston's Jamaica Plain neighborhood should be able to test locally for swine flu cases. Singleton said if the Lowell cases are concerned, the children's playmates will be notified.

"Kids who played together closely, kids who were in intimate contact while the children were symptomatic, before they were isolated back being sick in their homes and see whether or not those kids need to be observed, to see if they're coming down with the flu," Singleton said.

If they have symptoms, others will be treated with antiviral medications that have been made available to the state lab. More are expected to arrive from the national stockpile later in the week.

Meanwhile, U.S. Airways Flight 804, returning to Boston from Cancun, Mexico made a scheduled stop in Philadelphia. Passengers were kept on board the plane when one woman reported flu-like symptoms.

"Paramedics came on board. They were wearing masks and trying not to get contaminated," one male passenger said when the flight arrived in Boston.

The 24-year-old female passenger from Ontario was kept in Philadelphia to see doctors. Fellow passengers were asked to fill out forms so they could be contacted later if the woman was confirmed as having swine flu.

The Department of Homeland Security is now distributing flyers at Logan International Airport, informing passengers from Mexico and other areas of the swine flu outbreak.