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or the first time in more than two years, all three West African countries at the heart of the worst Ebola outbreak ever seen were declared free of the disease on Thursday, marking a landmark moment in a hard-fought victory.

The World Health Organization announced that Liberia has gone 42 days without a new Ebola case — the period used as the yardstick to measure whether transmission of the disease has stopped. Liberia joined the ranks of Sierra Leone, where Ebola transmission was declared over in early November, and Guinea, where the spread of the disease was declared to have been stopped in late December.

“Detecting and breaking every chain of transmission has been a monumental achievement,” WHO’s director general, Dr. Margaret Chan, said in a statement. “So much was needed and so much was accomplished by national authorities, heroic health workers, civil society, local and international organizations and generous partners. But our work is not done and vigilance is necessary to prevent new outbreaks.”

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The news will bring sighs of relief. But that relief will be tempered with wariness. While the worst appears over, public health experts say, it will take more time before people truly believe the last embers of this conflagration have gone cold.

“I don’t think we’ve seen the very last case,’’ said Dr. Jeremy Farrar, a tropical diseases expert and director of Britain’s Wellcome Trust.

Whereas past Ebola outbreaks have been declared over when two incubation periods have elapsed without new cases, this time the bar has been set higher, acknowledged Dr. Bruce Aylward, the senior WHO official who has led the UN agency’s Ebola response since the fall of 2014.

That means an additional 90 days of enhanced surveillance to make sure that what appears to be over is really over. “This is the time when you don’t want to blink. Because this is the time when real mistakes could be made,” said Aylward, the WHO’s assistant director general for outbreaks and health emergencies.

Aylward said he is “fairly confident” all the chains of transmission in the outbreak have been broken. That means he believes there is no ongoing spread from cases that can be traced back to the first in the outbreak, believed to have started in Guinea in December 2013. In the intervening time, at least 28,637 people in 10 countries contracted the disease, and, of them, 11,315 died.

From March 2014 to November 2015, the suspected, probable, and confirmed cases of the Ebola virus in Guinea, Liberia, and Sierra Leone topped 28,000. Alex Hogan/STAT

What remains to be seen, though, is whether the region experiences any more “flares” — a term the WHO is using to describe small clusters of cases that ignite from a survivor-related phenomenon that has come into sharper focus in the waning months of this epidemic.

Survivors can harbor Ebola viruses in their bodies after their recoveries in what are known as immune privileged sites — places such as testicles or eyes from which the immune system has a harder time routing them. Males survivors, for instance, can discharge Ebola viruses in their semen for months after they recover.

Aylward said there are at least 10 small groups of cases in this outbreak where it’s become apparent that virus from a survivor reignited the spread of the disease. The genetic fingerprints of viruses in these cases more closely resemble those that circulated months earlier than more contemporary viruses, he said.

In some cases, it’s clear these flares were set off when a male survivor infected a sexual partner. But it’s not apparent that all 10 of these instances were the result of sexual transmission, Aylward said.

However transmission occurs in these cases, there may be more to come. Hundreds of male survivors in West Africa are probably still emitting virus in semen — though with each passing week that number drops. A study conducted among survivors suggested about 20 percent still have viruses in their semen nine months after their illness. After nine months, viruses in semen appears to be a rare occurrence, Aylward said.

The three West African countries will need to be ready to spot and contain any new flares, he said.

Responsibility for surveillance and containment will be shifting to new parties going forward, Aylward said. While the WHO will maintain a strong presence in the region for the rest of this year, most of the international responders have left. In Sierra Leone, the National Ebola Response Team is closing down. There and elsewhere, ministries of health are taking over Ebola responsibilities.

While West Africa had not experienced an Ebola outbreak in the past, countries there must now be viewed as among those — like the Democratic Republic of Congo and Uganda — at risk of having more outbreaks in the future.

Farrar, of the Wellcome Trust, thinks one of the questions that needs to be answered is whether Ebola now circulates, undetected, in animal species in the region. “We have so little knowledge. We would be wise to keep an open mind,” he suggested.

The Wellcome Trust is funding some research aimed at trying to answer that question. It was also one of the funders of the report released Wednesday by the Commission on a Global Health Risk Framework for the Future, set up by the National Academy of Medicine. Other sponsors include the Bill and Melinda Gates Foundation and the Ford, Rockefeller, and Paul G. Allen Family foundations.

Like other Ebola reports that have come before it, this one attempts to cast forward more than to cast blame. And it supports the emerging consensus that the WHO needs to have built into it an arms-length center responsible for identifying and responding to disease threats. There is no “realistic alternative” to having this responsibility rest with the WHO, the report stated.

The WHO needs more funding as well as a $100 million emergency response fund, the report said.

It also called for the WHO to establish a committee responsible for pushing development of drugs and vaccines needed to combat disease threats, to ensure the world does not again face a deadly disease without any vaccines or drugs to curb its spread.

The report said that work will need about $1 billion of investment a year, and, in total, the reforms it proposes would cost about $4.5 billion a year, saying that is a fraction of what the global community spends defending against other treats to humankind.

Farrar suggested the recommendations are ones the world could and should agree to — and soon. “That window of opportunity coming out of the horror of Ebola will not last long,” he said. “2016 is the year to make the changes.”

This story has been updated with news of the World Heath Organization announcement. An earlier version incorrectly stated that the WHO would announce Friday that Liberia has gone 42 days without a new Ebola case. 

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