UNITED NATIONS — A shortage of international health workers and delays in building Ebola treatment clinics in West Africa are forcing the United Nations to change course in fighting the virus, and to call for smaller and more mobile treatment units that make greater use of local staff — and in turn require more money.

The shift comes eight months after the Ebola outbreak in West Africa was first identified. The virus is waning in some places and growing stronger elsewhere, and the international response so far has been unable to get the outbreak under control.

The most prominent international efforts have so far been focused on building large treatment centers. But by the time they are completed, they may not be where they are most needed.

For example, four 100-bed centers are to open soon in the Liberian capital, Monrovia, where reports of new cases have slowed significantly, while Sierra Leone, where new cases are escalating, is seriously short of treatment centers.