Complacency looms as a big risk in the fight against the Ebola virus, those leading the battle say.

Although West Africa has about 50 new cases (confirmed, probable and suspected) every day, mostly in Sierra Leone, the rate of new cases is on a downward trend. But that has happened before during this epidemic.

In May, for example, the World Health Organization projected that in a matter of days the Ebola outbreak in Liberia "could be declared over."

However, in July, the number of cases in Liberia began to rise rapidly, peaking in August and September, followed by an equally rapid decline.

Three months ago, health workers were identifying about 240 new cases a day in West Africa. Now the WHO talks about new case numbers halving, because in Guinea new case numbers go down by half every 10 days, in Liberia every 14 days and in Sierra Leone every 19 days.

As it released those numbers, the WHO warned, "Complacency is the biggest risk to not getting to zero cases. Continued vigilance is essential."

Doctors Without Borders, which says it has cared for about a quarter of all declared Ebola cases in West Africa, now has just over 50 patients in its Ebola care centres.

The group, known by its French initials MSF, is also worried that "loss of vigilance now would jeopardize the progress made in stemming the epidemic."

Dr. Allison McGeer, who helped lead efforts to halt the SARS outbreak in Toronto in 2003, also worries that the world could become complacent as case numbers drop. McGeer, based at Mount Sinai Hospital in Toronto, began working with the WHO on Ebola last fall, making several trips to West Africa.

How the outbreak was slowed

Several factors were critical in slowing the epidemic.

Ebola Liberia James Flomo

Three days after his wife died of Ebola in Monrovia, Liberia, James Flomo is in isolation with his children, Jan. 21, because he may have the virus. (James Giahyue/Reuters)

This was the first Ebola outbreak to hit West Africa, so health workers there were slow to recognize it and respond. With weak public health systems, countries lacked sufficient resources to respond, and the rest of the world was also slow to help out at required levels.

McGeer says there has been a huge community education campaign in the three countries about what Ebola does, how people can protect themselves, the importance of changing funeral practices such as washing the body, and the need to report and identify illness.

Quickly identifying ill people and treating them in isolation were also key, she says.

Brice de le Vingne, MSF director of operations, says improved efforts and resources for contact tracing — identifying and tracking everyone who was recently in contact with a new Ebola patient — were also key to slowing the epidemic.

De le Vingne tells CBC News that people not on any Ebola contact list are still getting the disease, indicating the surveillance system is still not good enough.

All those measures sounds simple, McGeer says, but it is an enormous amount of work. Because of the three countries' "very fragile health and public health systems, there has been a need for funding both within those systems and for people to help them."

Virus speculation

There has been speculation that something about the virus has changed, but no one CBC News interviewed for this story says there is evidence of that.

Tom Geisbert

Tom Geisbert, at his biosafety Level 4 lab at the University of Texas Medical Branch in Galveston, works on treatments and and vaccines for Ebola. Time magazine included him among Ebola fighters it named 2014 Person of the Year. (UTMB-Galveston)

Microbiologist Tom Geisbert says research is underway in biosafety Level 4 labs, including his own at the University of Texas Medical Branch in Galveston, on samples of the virus from West Africa.

So far researchers have confirmed the virus is very similar to the Zaire strain of Ebola, which has been responsible for most of the outbreaks in Central Africa. Geisbert says it's too early to tell whether the small genetic differences would have any impact on the effectiveness of a vaccine.

Time magazine included Geisbert as one of the Ebola fighters it named as Person of the Year for 2014, for his work on vaccines and treatments.

McGeer, who is also a microbiologist, says it's not impossible that the virus is losing strength, "just unlikely." If that was happening, she says it's odd that it would be "losing strength differentially in different countries."

While corona viruses like SARS and MERS have changed virulence, she says there is no evidence that Ebola can do this.

Lessons learned

McGeer, Geisbert and de le Vingne stressed vigilance for what the Liberians call "getting to zero" in the Ebola outbreak.

Dr. Allison McGeer

Since last fall, Dr. Allison McGeer, a microbiologist and infectious disease consultant at Mount Sinai Hospital in Toronto, has been working with the World Health Organization. (CBC)

De le Vingne says MSF will stay in an emergency mindset because of the need to track down every case and monitor every contact. Cross-border spread requires a regional response.

He notes the delay in the global response allowed the virus to spread out of control. If there's another Ebola outbreak tomorrow, "we still don't have a functioning organizational system to respond to that kind of epidemic."

But, he adds, "You cannot have a one-size-fits-all system, you need to know very well the people you are working with and interacting with and you need to have a very well qualified staff to do that."

McGeer says it's critical to continue investment and hard work, although it is difficult to keep up the pace. "As soon as you let up, you are likely to have relapses."

All the people she has worked with at the Liberian health ministry have been at it "24/7 for nine months now — no holidays, no days off and one in 20 of their colleagues have died."

Geisbert would like to have stocks of treatments such as ZMapp and TKM-Ebola readily available near potential outbreak zones. He says that although the vaccines have the potential to make a big difference, they are "no substitute, especially in Africa where you have poor public health infrastructure."

Nevertheless, he'd like to see "everything ready for that vaccine, so that when you do have an outbreak, you can move in quickly."

Geisbert, along with Canadian scientists, came up with VSV-EBOV, an experimental vaccine now in trials. Because it is a single-injection, fast-acting vaccine, it holds promise for use as an emergency response.