As violence makes it harder to reach stricken villages in Congo, experts plan to stretch supplies and to give the vaccine to everyone, not just contacts of victims.
Confronting an Ebola outbreak spiraling out of control in the Democratic Republic of Congo, the World Health Organization announced plans on Tuesday to change its vaccination strategy, offering smaller doses and eventually introducing a second vaccine.
The outbreak, which has lasted nine months, has now claimed more than 1,000 lives. Rebel groups have attacked health care centers or medical teams on dozens of occasions, leading many relief organizations to withdraw from the area.
According to a W.H.O. official quoted by The Associated Press, 85 health workers have been killed or wounded since January.
Because driving to villages with Ebola victims is so risky, the agency intends to launch brief and unexpected “pop-up” vaccination campaigns in communities considered safe.
The W.H.O. also wants to shift to immunizing as many people in each target area as possible, instead of relying solely on its current tactic: “ring vaccinating” the immediate contacts of each known case, along with health care workers.
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Before such a change is made, the Congolese government must accept the recommendations made Tuesday by a W.H.O. panel of experts. Dr. Jean-Jacques Muyembe, who heads Congo’s ethical review board, welcomed the recommendations and said he would work to see them implemented quickly.
Demand for the vaccine is increasing in the affected districts of eastern Congo, Dr. Muyembe said.
There have been contradictory reports from the area about acceptance of the vaccine. Some observers said villagers and even health workers were rejecting the vaccine.
Others said villagers were angry that the vaccine was given only to contacts of victims when everyone was afraid of the infection.
According to the W.H.O., more than 111,000 people have been vaccinated against Ebola since the outbreak began in August. To stretch supplies of the vaccine, made by Merck and known as rVSV-ZEBOV, the agency recommended switching to smaller doses.
Those at highest risk — contacts of known Ebola cases, and their contacts — should get 0.5 milliliter of vaccine, which is half the dose currently used, the W.H.O. said. The dose was used in a 2015 trial in Guinea and found to be protective.
Everyone else at lower risk but willing to be vaccinated should get 0.2 milliliter, or one-fifth of the current dose, the W.H.O. said. It will take longer for them to build immunity against the virus, but they should be at least partially protected.
Even at the full 1.0 milliliter dose, it is unclear how long the Merck vaccine’s protection lasts. In October, the W.H.O. said some studies indicated the vaccine protected against Ebola for about one year.
In addition, the W.H.O. recommended rolling out a second vaccine made by Johnson and Johnson, currently known as Ad26.ZEBOV/MVA-BN.
(Preliminary vaccines are named with initials describing their ingredients. The Merck vaccine uses a recombinant vesicular stomatitis virus to deliver the vaccine to cells, while the Johnson and Johnson vaccine used an adenovirus. Both viruses can infect humans but are rendered harmless in the vaccines.)
A consortium, including the Coalition for Epidemic Preparedness, which develops vaccines against pandemic diseases, and the London School of Hygiene and Tropical Medicine, is planning how to safely roll out the new vaccine.
Donald G. McNeil Jr. is a science reporter covering epidemics and diseases of the world’s poor. He joined The Times in 1976, and has reported from 60 countries.