LONDON (AP) -- The World Health Organization said Friday that Tamiflu should only be given to particularly vulnerable people - a warning to countries like Britain where the swine flu drug is being handed out freely.

WHO previously said it was up to doctors to decide who should get Tamiflu. On Friday, the U.N. agency said healthy people who catch mild to moderate cases of swine flu don't need the drug, but the young, old, pregnant, and those with underlying health problems surely do.

If countries use Tamiflu too liberally, that could lead to resistant viruses, leaving the world with few resources to fight swine flu.

WHO said people thought to be at risk for complications from swine flu - children less than 5 years old, pregnant women, people over age 65 and those with other health problems like heart disease, HIV or diabetes - should definitely get the drug.

The agency also recommended all patients, including children, who have severe cases of swine flu, with breathing difficulties, chest pain or severe weakness, should get Tamiflu immediately, perhaps in higher doses than now used.

WHO said most patients infected with swine flu worldwide recover within a week without any medical treatment. Still, about 40 percent of the severe swine flu cases are occurring in previously healthy children and adults, usually under 50 years of age.

WHO has estimated that as many as 2 billion people could become infected over the next two years with swine flu - nearly one-third of the world's population. At a swine flu conference in Beijing, a top WHO official predicted there would be "an explosion" of cases in the future.

By MARIA CHENG
AP Medical Writer