The World Health Organization tried this week to dampen fears about mutations seen in the swine flu virus in several countries, noting that both mutations had been found in very few people.

A change that created Tamiflu resistance has been found in about 75 people around the world, said Dr. Keiji Fukuda, chief flu adviser to the W.H.O.’s director general. Two clusters, in cancer units at Duke University Medical Center in North Carolina and a hospital in Wales, were both among patients whose immune systems had been severely suppressed by cancer treatment ; some had had their bone marrow, which produces infection-fighting white blood cells, wiped out so that replacement blood stem cells could be injected.

Such patients are more likely to develop resistant viruses when on Tamiflu because they can not clear a virus on their own. But the mutant strain appears not to spread easily in people with normal immunity, like hospital workers.

“We don’t know the full answer, but it is more likely that we are not seeing a major shift,” Dr. Fukuda said.

Widespread Tamiflu resistance is a serious problem in the seasonal H1N1 virus, but it has not crossed over into the swine H1N1.

Dr. Fukuda also said W.H.O. scientists were “not sure” of the level of threat posed by a separate mutation that helps the virus reach the lungs. It has been found in Norway, Ukraine, Brazil, China, Japan, Mexico and the United States, in both serious and mild cases.

Experts still need to see whether the mutation — whose shorthand name in virology is D222G or D225G — is becoming more common, and how often it leads to severe disease, he said.

One isolate from Ukraine with the mutation had changed so that swine flu vaccine probably would not protect against it well, Britain’s national medical laboratory reported Friday.

Flus mutate so fast, Dr. Fukuda cautioned, that announcing each change is “like reporting changes in the weather.”