U.S. freelancer now free of Ebola; Texas nurse Nina Pham upgraded to good condition

(CNN) -- Freelance cameraman Ashoka Mukpo no longer has the Ebola virus in his bloodstream and will be allowed to leave Nebraska Medical Center, the hospital said Tuesday.

"Just got my results," Mukpo tweeted. "3 consecutive days negative. Ebola free and feeling so blessed. I fought and won, with lots of help. Amazing feeling."

The 33-year-old was working for NBC News when he tested positive for Ebola in Liberia. Mukpo was among a team working with Dr. Nancy Snyderman, the network's chief medical correspondent.

Mukpo spent about two weeks at the hospital in Omaha, Neb. The hospital said he can head back home to Rhode Island on Wednesday.

"Recovering from Ebola is a truly humbling feeling," the hospital quoted Mukpo as saying. "Too many are not as fortunate and lucky as I've been. I'm very happy to be alive."

Two nurses undergoing treatment for the virus also got good news on Tuesday.

The National Institutes of Health said the condition of Nina Pham, a Texas nurse who contracted Ebola while caring for a patient, was upgraded from fair to good.

Pham is at the NIH Hospital in Bethesda, Maryland. She cared for Thomas Eric Duncan, the first person to die of Ebola in the United States, at a Texas hospital.

And in Spain, nurse's aide Teresa Romero Ramos, who contracted Ebola after treating virus-stricken patients in Madrid, is now free of the virus, her doctors announced.

U.S. limits airports for passengers from Ebola-stricken region

The United States is doing more to help prevent the spread of the virus. The Department of Homeland Security said Tuesday that all arriving passengers from West African countries that Ebola has hit hardest -- Liberia, Sierra Leone and Guinea -- must land in one of the five U.S. airports that have enhanced Ebola screening.

Those airports are New York's John F. Kennedy International, D.C.'s Washington Dulles, New Jersey's Newark Liberty International, Chicago's O'Hare International and Hartsfield-Jackson International in Atlanta.

On its website, the Department of Homeland Security shows how many people have been screened and then taken to health care facilities for further checks. JFK appears to be outpacing the other airports in screenings.

Vaccine testing

Also Tuesday, the World Health Organization announced that testing was underway at the NIH for an Ebola vaccine. A trial for a second vaccine, developed in Canada, has started at the Walter Reed Army Institute of Research in Silver Spring, Maryland.

The goal is to launch vaccine trials in West Africa by January, said Dr. Marie Paule Kieny, the WHO's assistant director general for health systems and innovation.

The initial vaccine tests are being given to volunteers in countries such as Mali, the United States and England. It is impossible to get Ebola from the vaccines, Kieny said, because they do not contain enough of the virus' genetic material. But "there is no vaccine that has no side effects at all," she added.

It's not clear when vaccines could be distributed to the masses. That won't be determined until after test results come in. When the testing reaches West Africa, candidates could include relatives of infected Ebola patients, Kieny said.

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Debate over experimental drugs

The WHO said it is also visiting sites in the three countries most devastated by Ebola -- Sierra Leone, Guinea and Liberia -- to see which treatment centers could participate in the testing of experimental Ebola drugs.

But there is debate among medical ethicists about the drug trials -- namely, whether to use placebos in testing.

While some say placebos are necessary to gauge the effectiveness of drugs, others say it's unethical to withhold treatment for a disease with a mortality rate of about 50%.

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CDC gives new guidelines

The news from the WHO comes a day after the Centers for Disease Control and Prevention issued updated Ebola guidelines, focusing on better protecting health care workers.

Dr. Tom Frieden, the director of the CDC, stressed the importance of more training and supervision, and he said no skin should be exposed when workers are wearing personal protective equipment, or PPE.

"We're increasing the margin of safety with a real consensus guideline that has three key changes. One, training, practicing -- demonstrated hands-on experience so that the health care workers are comfortable donning and doffing PPE. Two, no skin exposure. Three, observation of every single step, putting on and taking off the PPE," Frieden said.