LONDON -- The World Health Organization's flu chief said the swine flu virus has now become the predominant flu strain worldwide.

In some countries, swine flu accounts for up to 70 percent of the flu viruses being sampled, said Dr. Keiji Fukuda, WHO's top flu official.

While most people recover from the illness without needing medical treatment, officials are also continuing to see severe cases in people under 65 -- people who are not usually at risk during regular flu seasons.

"We remain quite concerned about the patterns that we're seeing," Fukuda said during a press briefing Thursday.

He said the swine flu virus appeared to be fairly stable, and that samples from around the world remained very similar to when the virus was first identified in April.

Regarding the recent surge of cases in the Ukraine -- which has reported more than 250,000 cases and 70 deaths of people with flu-like illness in recent weeks -- Fukuda said the virus appeared no different there than anywhere else.

"We just simply have to understand that influenza can cause outbreaks in very large numbers of people," he said. "Patterns can be quite different from country to country."

Fukuda said the agency was also monitoring the impact of the virus on particularly susceptible populations.

On Wednesday, Venezuelan officials reported that swine flu had hit the Yanomami Indians, killing seven people in a population of 28,000.

Fukuda said WHO had observed that aboriginals in Australia were disproportionately hit by swine flu, but could not say whether they were more genetically vulnerable to the virus or if that was due to underlying health problems.

With swine flu vaccination programs now under way in more than 20 countries, Fukuda said no rare or dangerous side effects had been reported, and the agency was convinced the vaccine was "highly safe."

Fukuda said WHO had been surprised that the vaccine appeared to work after just one dose, but was happy about it because that mean the world's vaccine supplies could be stretched.

Even in children under 10, Fukuda said WHO recommended that one dose could be effective. Some countries, including the U.S., are recommending that children get two doses of the swine flu shot.

"It is better to provide one dose to as many children as possible rather than two doses to fewer children," Fukuda said.