The World Health Organisation has cautioned people to continue to protect themselves against swine flu and vaccination may greatly help in this exercise.
"At the WHO we remain quite concerned about the pattern we are seeing, particularly because a sizeable number of people do develop serious complications and death," said Keiji Fukuda, special adviser on pandemic.
Advertisement"Again, we are seeing most of this develop in people who are younger than 65 years, a picture which is different from seasonal influenza," he explained.
Fukuda reiterated that the cases of serious complications were largely found among the chronically ill, pregnant women as well as some otherwise healthy younger people.
He also countered signs of public scepticism about the vaccine revealed by surveys in several countries, despite the resurgence of swine flu in the northern hemisphere with the onset of the colder season.
"WHO believes that these vaccines are very useful against the pandemic virus," Fukuda said in a telephone news conference.
Vaccination against A(H1N1) influenza has now started in more than 20 countries and were demonstrably safe, according to the WHO.
"What we are seeing is that these vaccines are highly safe and have been received by a significant number of people without any problems," Fukuda said.
Swine flu has differed from seasonal flu in persisting in the northern hemisphere during the summer period and in a pattern of occasional serious illness affecting under 65-year-olds.
However, Fukuda acknowledged that it was as transmissible as seasonal flu, showed the same pattern of spreading more widely in winter and was "self limiting" in that the large majority of people who caught it got better without the need for special treatment.
More than 5,700 people have died worldwide since the pandemic virus was first discovered in April, with most deaths 4,175 reported in the Americas region, according to WHO data.