Millions of doses of H1N1 vaccine have already been administered, and so far, no unusual side-effects have been seen, reports The World Health Organization.

Dr. Keiji Fukuda, the agency's top flu expert, told a press briefing Thursday that 20 countries have started to administer the pandemic vaccine. He says the rates of reactions at the injection site, such as sore arms, are similar and maybe even lower than seen with seasonal flu shots.

"We now have good evidence, based on many people receiving the vaccines, that we have no picture of unusual side-effects emerging," Fukuda said.

"So the picture right now looks quite good in terms of the safety."

He said the agency was convinced the vaccine was "highly safe."

Fukuda reiterated that WHO experts have found that the vaccine appears to provide good protection after just one dose. Even in children under 10, WHO recommends that one dose appears effective, Fukuda said.

Some countries, including the U.S., are recommending that children get two doses of the swine flu vaccine. But the U.S. is using vaccine without an adjuvant, which boosts immune response. Canada, as well as many countries in Europe, is using a vaccine with adjuvant for most of the population.

While the Public Health Agency of Canada still advises children under the age of 10 to get two half-doses of vaccine, given 21 days apart, it remains unclear whether that advice will change based on the WHO recommendations.

Fukuda said the important thing is to ensure that as many children as possible get at least one dose.

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

The flu chief also noted that swine flu has now become the predominant flu strain worldwide and even appears to be starting to "crowd out" H3N2 viruses in East Asia.

And he noted that H1N1 still hasn't mutated much, an observation that has surprised many flu experts who had expected to see some change with the virus by now, given how far the strain has travelled in the last seven months.

Fukuda said H1N1 samples from around the world remain very similar to those seen when the virus was first identified in April. WHO experts do expect they will start to see some change in the strain soon, though, as immunity in the human population rises and the virus adapts.

In many countries, swine flu now accounts for up to 70 per cent of the flu viruses being sampled, Fukuda said.

Reports from across North America have suggested the virus makes up more than 90 per cent of flu viruses that have been typed.

While most people recover from the illness without needing medical treatment, Fukuda warned that the public may be concluding that this virus is insignificant. But he noted that officials are continuing to see severe cases in all age groups.

"At WHO, we remain quite concerned about the patterns we are seeing, particularly, again, because a sizable number of people do develop serious complications and death," he said. 

He added that cases of patients showing resistance to the antiviral drug Tamiflu remain isolated and sporadic.

"We see no evidence at all that there is widespread occurrence of antiviral resistance," Fukuda told the news conference.