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With three cases of Ebola diagnosed in the United States but more than 100 people being monitored in case they contract the disease, U.S. President Barack Obama urged Americans on Saturday not to give in to "hysteria" about the spread of the virus.

As though to illustrate his point, a Dallas bus and train station was closed on Saturday afternoon over concern about a woman who fell ill. The woman was first reported to be on the checklist for possible Ebola exposure, but then turned out not to be on any such list.

Obama made plain he is not currently planning to give in to demands from some lawmakers for a ban on travellers from the worst-hit countries.

"We can't just cut ourselves off from West Africa," Obama said in his weekly radio address. "Trying to seal off an entire region of the world - if that were even possible - could actually make the situation worse," he said.

The worst Ebola outbreak on record has killed more than 4,500 people, most of them in the West African countries of Liberia, Sierra Leone and Guinea.

We can't just cut ourselves off from West Africa. - U.S. President Barack Obama

Obama, whose approval rating is already low, has been criticized over his administration's handling of Ebola. He held a flurry of meetings on the issue in the past week and on Friday appointed Ron Klain, a lawyer with long Washington experience, to oversee the effort to contain the disease.

Republicans questioned why he did not pick a medical expert.

"I hope he (Klain) is successful in this. I think it's a step in the right direction, but I just question picking someone without any background in public health," Republican Representative Ed Royce, chairman of the House Foreign Affairs Committee, told CNN on Saturday.

The Obama administration is not alone in facing criticism. The World Health Organization has been faulted for failing to do enough to halt the spread since the outbreak was detected in March.

The agency promised it would publish a full review of its handling of the crisis once the outbreak was under control, in response to a leaked document that appeared to acknowledge that it had failed to do enough.

Experimental vaccines tested

There is no cure or approved vaccine yet for Ebola but pharmaceutical companies have been working on experimental drugs. The virus is transmitted through an infected person's bodily fluids and it is not airborne.

Canada said on Saturday it would ship 800 vials of its experimental Ebola vaccine, undergoing clinical trials, to the WHO in Geneva, starting on Monday. Iowa-based NewLink Genetics Corp holds the commercial licence for the Canadian vaccine.

U.S. President Barack Obama speaks to Dr. Thomas Frieden, Director of the Centers for Disease Control and Prevention, about the government’s Ebola response. (Jacquelyn Martin/Associated Press) Britain's biggest drugmaker GlaxoSmithKline said on Saturday that work to develop a vaccine was moving at an "unprecedented rate" and the next phase - if successful - involving vaccinating frontline health-care workers, would begin early 2015.

Obama sought to put the extent of the disease in the United States in perspective. "What we're seeing now is not an 'outbreak' or an 'epidemic' of Ebola in America," he said. "This is a serious disease, but we can't give in to hysteria or fear."

A rash of Ebola scares has hit the country since late September when a Liberian visitor to Dallas, Texas, became the first person to be diagnosed in the United States. Americans' faith in the medical system and in authorities' ability to prevent the disease from spreading was jolted by a series of mis-steps when the Liberian was initially not diagnosed by a Dallas hospital.

The man, Thomas Eric Duncan, was admitted to Texas Health Presbyterian Hospital days later and diagnosed with Ebola. Two nurses who were part of the team caring for Duncan, who died on Oct. 8, contracted Ebola. Amber Vinson is being cared for at Atlanta's Emory University Hospital, while Nina Pham is being treated at the National Institutes of Health (NIH) just outside Washington.

A chain of people who had contact with either Duncan or the sick nurses are being monitored in case they develop the disease, which has an incubation period of up to 21 days.

Chain of people monitored

The Texas Department of State Health Services said in a statement on its website dated Saturday that it was working to monitor people who had contact with the Ebola patients, specimens, or potentially contaminated surfaces. A total of 145 "contacts and possible contacts" were being monitored for symptoms, while 14 people had completed surveillance, it said.

Some 800 passengers who took the same planes as Vinson on a trip she made to Ohio before being diagnosed, and passengers on subsequent flights using the same planes, have been contacted by the airline, Frontier Airlines, the carrier said on Saturday.

The airline said in a letter to employees that it had been informed by the U.S. Centers for Disease Control and Prevention (CDC) that the Dallas nurse may have been in a more advanced stage of the illness than previously thought, when she traveled back to Dallas from Cleveland on Oct. 13.

The White House said late on Friday it would send senior personnel to Dallas to help federal, state and local officials there trying to identify and monitor people who came in contact with the three people who fell sick with Ebola.

Those being monitored include a lab worker at the Dallas hospital, who is not ill but is in isolation at sea: in her cabin on the Carnival Magic cruise ship. The lab worker did not have contact with Duncan, but may have come in contact with test samples.

Her presence on the ship caused Mexican authorities to deny docking at the Mexican port of Cozumel on Friday. The Carnival Magic, owned by Carnival Corp unit Carnival Cruise Lines, was en route back to Galveston on Sunday.

At Dallas' White Rock station, a woman was taken to the hospital for evaluation on Saturday, the city's transit authority said. Local media reported the station was closed as she was being monitored for possible Ebola exposure, but the Dallas Area Rapid Transit system said she was not on a watchlist.

Obama has stressed that containing Ebola should include help for the worst-hit countries and Washington plans to deploy up to 4,000 military personnel to the region by late October. Obama is preparing to ask for additional funds from Congress to beat Ebola, and could make the request next week, according to a Bloomberg report.

British Prime Minister David Cameron said on Saturday European Union leaders should raise the amount of money pledged to fight Ebola to US $1.3 billion and mobilize at least 2,000 workers to head to West Africa.

He made the appeal in a letter to the President of the European Council Herman Van Rompuy. A spokeswoman at Cameron's office said the EU commission and 28 member states had pledged a total of 500 million euros so far to fight Ebola.

Combatting the disease was also among the subjects of talks being held on Friday and Saturday between American and Chinese top diplomats.