London ? Poor countries likely to suffer most in a swine flu outbreak have the smallest stockpiles of antiviral medicines to fight it.

Affluent countries like Japan, Britain and the United States have enough Tamiflu and similar medicines to reach about a quarter to half their populations, while developing countries like Guatemala, Indonesia and India have enough for only 2 percent of their people or less.

The disparities are not surprising given the high cost of stockpiling and maintaining expensive medicines for future epidemics that may never develop, especially when many are already coping with high rates of malaria and HIV. But experts fear the countries with low supplies may suffer higher death rates if swine flu becomes more lethal outside Mexico, where it has already killed more than 150 people.

Elspeth Garman, an Oxford University professor of molecular biophysics who co-authored an influential 2001 report urging Britain to stockpile antivirals, said having low supplies creates severe political problems because the government has to determine who gets access to scarce tablets.

“With 50 percent there is a good chance all the people in the UK would get it if they need it, but with 1 percent it’s very problematic,” she said. “They won’t be able to treat as many people, that’s obvious, but the much harder issue is who decides who gets it.”

The problem for poorer countries is compounded, she said, because Tamiflu is only effective if used within 48 hours of the time when symptoms develop, and there is no quick, easy way to diagnose and confirm the presence of swine flu.

The flu medicines Tamiflu and Relenza are not cures for any type of flu, but lab tests suggest they might be effective against the new virus. Little is known about whether they lower the chances for serious flu complications, like pneumonia, and few studies have tested them in children. But they have been found to cut the duration of ordinary seasonal flu by about one day.