The Asian countries that suffered the worst avian flu outbreaks are now largely free of the disease, international health officials said this week. Vietnam, which had about half the worlds human cases of bird flu, hasnt seen an outbreakeven among poultrythis year. Thailand hasnt had one in six months. Vietnam logged its success by vaccinating every single chicken. Thailand, which exports poultry, killed all the birds anywhere near infected flocks and compensated farmers for their losses. Officials said the countries should remain vigilant. Tomorrow, the whole thing could change again, said U.N. flu expert Dr. David Nabarro.