All passengers and crew have been screened after a flu-like illness hit passengers on two American Airlines flights.
Medical screening has been carried out on 250 passengers and crew on two flights from Munich and Paris to Philadelphia, after 12 people aboard suffered flu-like symptoms.
A similar outbreak occurred on a flight from Dubai to New York earlier this week.
The American Airlines passengers and crew were "held for a medical review" as a precaution, and the US Centers for Disease Control and Prevention (CDC) was notified, Philadelphia International Airport spokeswoman Diane Gerace said.
Flight 717 from Munich Airport and Flight 755 from Charles de Gaulle Airport in Paris both arrived in Philadelphia on Thursday afternoon, she said.
The CDC worked with Philadelphia health officers, emergency responders and Customs and Border Patrol agents to evaluate the sick passengers for influenza and other respiratory illnesses, CDC spokesman Benjamin Haynes said.
"Twelve passengers from the two flights reported sore throat and cough, none were identified with fever. None of the passengers are severely ill, and they will be released and informed of test results in 24 hours," Mr Haynes said.
The rest of the passengers from the two flights were allowed to continue on to their destination, he said, adding that the CDC would provide further information when tests results were confirmed.
No one on either plane was quarantined, American Airlines spokeswoman Leslie Scott said.
Members of the crews had not contacted medical personnel before landing to alert them of sick passengers on board, she said.
On Wednesday, at least 19 people aboard an Emirates airlines double-deck Airbus 380 flight from Dubai were confirmed ill when the aircraft landed at John F. Kennedy International Airport.
Ten of them were treated at a local hospital.
A press secretary for New York Mayor Bill de Blasio said test results had confirmed influenza.
"Some tests came back inconclusive on other viruses, which is common," de Blasio spokesman Eric Phillips said on Twitter, adding that the tests were being re-administered.
"All 10 patients will be kept in the hospital as a precaution until we know those final results," Mr Phillips said.
There was no indication the incidents, which occurred over two days at separate US airports, were related.