Passengers and crew on two flights arriving in Philadelphia from Europe on Thursday were screened by medical teams after 12 people aboard became ill with flu-like symptoms, a day after a similar outbreak on a flight from Dubai to New York.
All 250 people on separate American Airlines flights from Munich and Paris were "held for a medical review" as a precaution, and the US Centres 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. She said members of the crews had not contacted medical personnel ahead of landing to alert them of sick passengers on board.
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 on Thursday that 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," Phillips said.
There was no indication the incidents, which occurred over two days at separate U.S. airports, were related.
In an another apparently unrelated health scare, passengers on four Southwest Airlines flights in Texas may have been exposed to measles in the last 16 days.
Southwest Airlines confirmed passengers on two flights in and out of Dallas Love Field may have been exposed and they were working with the Centres for Disease Control and Prevention to inform those affected, NBC 5 reported.