Officials from Doctors Without Borders say the strategy has not worked in Congo, in part because it has not been possible to track down every person who has come into contact with someone infected with Ebola, and because some contacts have refused to cooperate. The group has urged more widespread vaccination in regions where the disease is spreading, whether people are known contacts or not.

But it says that instead the W.H.O. has doled out limited amounts of vaccine. About 225,000 people have been vaccinated, but Doctors Without Borders says 450,000 to 600,000 should have received the vaccine by now.

The vaccine that has been used so far, made by Merck, takes effect after one dose.

On Monday, the W.H.O. announced that the health authorities in Congo had decided to allow use of a second vaccine, made by Johnson & Johnson. That vaccine requires two doses, about two months apart, and will be used only in areas that are at risk but where the need is not urgent because the disease is not being actively transmitted.

People at high risk, including contacts of cases, and their contacts, will still receive the Merck vaccine.

Congo’s previous minister of health, Dr. Oly Ilunga, vehemently opposed using any vaccine but the Merck one, saying people had come to trust it and would be unlikely to accept a new one. He resigned in July after Congo’s president took control of the Ebola response away from him. Dr. Ilunga was arrested on Sept. 14, accused of mismanaging some $4 million meant for the Ebola response, a claim that his lawyers deny.

Concerns about possible spread of the disease to Tanzania arose on Sept. 10, when the W.H.O. obtained “unofficial reports” of the death in Dar es Salaam, the largest city in Tanzania, of someone suspected to have had Ebola. Further unofficial reports said the person had tested positive for Ebola. Then the W.H.O. heard of two more possible cases.

The W.H.O. said it requested information from Tanzania, but received none, even though International Health Regulations, agreed on by all W.H.O. member countries, require that suspected cases of Ebola be reported.