Only two Ebola cases reported in past week, but risks remain - WHO

* Guinea and Sierra Leone each report a case in past week

* WHO cites "real progress", warns on "unrealistic expectations"

* Contact tracing key; 600 quarantined in Sierra Leone village (Adds details, quotes)

By Stephanie Nebehay

GENEVA, Aug 4 (Reuters) - Guinea and Sierra Leone each recorded a single cases of Ebola in the past week, putting a year-end goal of ending the deadly epidemic within reach, although risks remain, the World Health Organization (WHO) said on Tuesday.

Tight surveillance and tracing contacts of infected people remain crucial, WHO Assistant-Director Bruce Aylward said. They are especially challenging during the heavy rains in August.

In the previous week to July 26, the two countries had seven confirmed cases, which was the lowest in the past year up until then, according to the WHO.

"We have gone over the last four weeks from 30 cases (per week) to 25 to seven and, in the last week, two cases. That progress is real," Aylward told a news briefing.

Ebola re-emerged last month in Liberia, after the country was declared officially free of the virus in May, but "appears now to be stopped, although it is early days", Aylward said.

He stressed the need to maintain vigilance in all three countries, while dampening hopes of quickly stamping out the haemorrhagic disease, which has killed more than 11,200 people in West Africa since December 2013.

"There is a huge risk of unrealistic expectations that this will go from here to zero. It won't. We will have additional flares. This could still go on for additional months before it gets stopped," he said.

Already this week, two new Ebola cases have been confirmed in Tonkolili, Sierra Leone, where several other suspect cases are being investigated, he said.

"In Tonkolil,i where this person went to two health facilities and then eventually died, they have actually quarantined a whole village plus a whole hospital. So they have got nearly 600 people quarantined around a single case," Aylward said, pointing to "massive operations" to stop the spread.

It was a "realistic goal to have transmission of this epidemic stopped this year," Aylward said.

"There is a lot of challenges between here and there, one very, very difficult rainy season in August. Remember, last year it took off in August."

The WHO said last Friday that a trial in Guinea found an Ebola vaccine to have been 100 percent effective.

Initial results from the trial, which tested Merck (Jakarta: 28586808.JK - news) and NewLink Genetics VSV-ZEBOV vaccine on some 4,000 people who had been in close contact with a confirmed Ebola case, showed complete protection after 10 days.

"It's not a game changer as much as it is an enhancer," Aylward said. "The game is still about case-finding, contact-tracing, rapid isolation, safe burials. That's how you stop Ebola, certainly in the current period."

(Reporting by Stephanie Nebehay; Editing by Larry King)