WASHINGTON, Oct. 22 -- U.S. President Barack Obama said Wednesday he is "cautiously optimistic" about the Ebola situation in the United States, as his government announced a new program that will actively monitor for 21 days every person coming from West African nations hard hit by the deadly virus.

"The public health infrastructure and systems that we are now putting in place across the board around the country should give the American people confidence that we're going to be in a position to deal with any additional cases of Ebola," Obama said after meeting his new Ebola response coordinator Ron Klain and other officials in the Oval Office.

Obama mentioned "a number of things" that made him "cautiously optimistic," including the clearance of dozens of persons who had initial interaction with the now deceased Thomas Eric Duncan, the first person diagnosed with Ebola on U.S. soil.

He reported that two nurses from the Texas Presbyterian Hospital who were infected while treating Duncan "seem to be doing better" and that the spirits of coworkers of the two infected nurses whom he spoke to earlier in the day "were good."

Obama also said he is "very happy" about the news that two American patients, who got Ebola in West Africa but were brought back for treatment, have been released from the hospital this week.

Internationally, he spoke of a declaration by the World Health Organization that Nigeria and Senegal are now Ebola-free, saying that "it gives you some sense that when it's caught early and where the public health infrastructure operates effectively, this outbreak can be stopped."

He reassured Americans that the prospect of an outbreak in the U.S. is "extremely low."

Earlier, the U.S. government announced all travelers from West African Ebola-affected countries will be actively monitored for 21 days, the incubation period for the deadly virus.

Under the measures starting Monday, travelers from Liberia, Sierra Leone and Guinea will be required to give their contact information including email, phone numbers and a U.S. address, and state or local public health officials will follow up with them on a daily basis.

Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, told a telebriefing that these new measures are expected to "give additional levels of safety" to make sure those who develop symptoms of Ebola are isolated quickly.

About 150 people from the affected countries in West Africa enter the United States each day.

Frieden said the new program will start in six states of New York, Pennsylvania, Maryland, Virginia, New Jersey and Georgia, where about 70 percent of incoming travelers are located.

The announcement came one day after the Department of Homeland Security required all travelers originating from West African Ebola-affected countries to fly into one of the five airports that are conducting enhanced screening for Ebola symptoms.

Also on Wednesday, the U.S. National Institutes of Health (NIH) announced the start of human clinical trials of a second Ebola vaccine candidate called VSV-ZEBOV at its clinical center.

The vaccine was developed by researchers at the Public Health Agency of Canada's National Microbiology Laboratory and has been licensed to NewLink Genetics Corp. in the U.S. state of Iowa.

Early human testing of another Ebola vaccine candidate, co- developed by the NIH and GlaxoSmithKline, began in early September at the NIH clinical center. Initial data on that vaccine's safety and ability to generate an immune response are expected by the end of 2014.