NEW YORK (Reuters) - U.S. President Barack Obama on Saturday commended New Yorkers for their calm reaction to the city's first case of Ebola and said that medical authorities were responding effectively to the threat from the deadly virus.

He used his weekly address for the second time in a row to speak directly to Americans about the response to Ebola, that has killed thousands in Africa and has become a political issue in the United States ahead of Nov. 4 congressional elections.

"It's important to remember that of the seven Americans treated so far for Ebola - the five who contracted it in West Africa, plus the two nurses from Dallas - all seven have survived," Obama said.

He did not refer to new 21-day quarantines implemented late on Friday by New York and New Jersey for medical workers returning from Ebola hotspots. His administration is discussing similar measures.

"We have been examining the protocols for protecting our brave healthcare workers, and, guided by the science, we'll continue to work with state and local officials to take the necessary steps to ensure the safety and health of the American people," Obama said.

The first person to be quarantined under the states' new measures was a medical worker who arrived at Newark Liberty International Airport on Friday after treating Ebola victims in West Africa.

The worker, who has not been publicly identified, has tested negative for the virus, New Jersey's health department said on Saturday, but remains in isolation at University Hospital in Newark.

Under the new policy, which exceeds federal guidelines, anyone arriving at airports in the two states after having contact with Ebola patients in Liberia, Sierra Leone or Guinea must submit to a mandatory 21-day quarantine.

The worst Ebola outbreak since the disease was identified in 1976 has killed almost half of more than 10,000 people diagnosed with the disease -- predominantly in Liberia, Sierra Leone and Guinea -- although the true toll is far higher, according to the World Health Organization.

New York Governor Andrew Cuomo and New Jersey Governor Chris Christie put the screening measures in place after Dr. Craig Spencer, a 33-year-old New Yorker who had been treating Ebola patients in Guinea, tested positive for Ebola on Thursday.

Spencer, who spent a month in the West African nation working with the humanitarian group Doctors Without Borders, was the fourth person diagnosed with the virus in the United States and the first in its largest city.

The unidentified healthcare worker in New Jersey did not have any symptoms when she arrived at Newark Liberty International Airport on Friday, officials said, but developed a fever while quarantined at the airport before being taken to the Newark hospital.

Despite the negative test result, she will remain under mandatory quarantine for the full 21 days, the virus's maximum incubation period, the New Jersey health department said.

The federal government is considering similar quarantine rules, according to a spokesman for the U.S. Centers for Disease Control and Prevention.

The virus is not airborne but is spread through direct contact with bodily fluids from an infected person who is showing symptoms.

(By Jonathan Allen; Editing by Frank McGurty and Louise Ireland)