Millions of doses of H1N1 vaccine have been given so far and no serious side-effects have been seen, the World Health Organization said Thursday.
Twenty countries have started to vaccinate against the pandemic virus and the initial picture suggests the vaccine is very safe, Dr. Keiji Fukuda, the global health agency's top flu expert, said in a news conference in Geneva.
"We now have good evidence, based on many people receiving the vaccines, that we have no picture of unusual side-effects emerging," Dr. Fukuda said.
"And the side-effects which are expected - such as a painful injection or perhaps some swelling at the injection site - these are occurring at rates which are expected and usually seen with seasonal influenza vaccine. So the picture right now looks quite good in terms of the safety."
Dr. Fukuda said rates of local reactions may actually be lower than what is seen with seasonal flu shots.
In a wide-ranging update on the H1N1 situation, Dr. Fukuda said the viruses still aren't changing much - something that bodes well for the effectiveness of the H1N1 vaccine. But he said as immunity to this H1N1 virus starts to rise in the global population, the WHO expects the virus will begin to mutate.
He warned that the public may be concluding that this virus is insignificant or mild, but said it's too soon to write off H1N1.
"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. And again, we are seeing most of these occur in people who are younger than 65 years, a picture which is different from seasonal influenza."
The pandemic viruses remain susceptible to the antiviral drugs Tamiflu and Relenza, with only sporadic and isolated cases of resistance to Tamiflu, Dr. Fukuda said. To date none of the viruses have been found to be resistant to the less commonly used Relenza.
He said the WHO does not object to a decision by Norway to make flu antivirals temporarily available this winter on an over-the-counter basis, saying it was an "innovative and prudent" way to relieve strain on an overburdened health-care system.
Global flu surveillance shows pandemic H1N1 viruses are the dominant flu viruses around the world, said Fukuda, who serves as special adviser on pandemic influenza to WHO Director General Margaret Chan.
That's been the case in North America for a while; the pandemic virus makes up about 99 per cent of flu viruses that have been found and typed.
But there have been pockets of the world where seasonal flu viruses - especially the H3N2 virus family - have continued to circulate. East Asia has continued to see a fair proportion of these viruses, making experts skeptical the pandemic H1N1 will extinguish or replace this subtype.
It is thought the new virus is crowding out or replacing the family of seasonal H1N1 viruses, which are being dropped from future flu shot formulations. This phenomenon of viral replacement has been seen in previous pandemics.
Dr. Fukuda said recently it's been seen that even in East Asia the pandemic virus is taking over.
"As we head into the winter, even in that part of the world the pandemic virus is clearly becoming dominant, and so we are seeing it crowd out the H3N2 viruses, and these are really falling in terms of the overall proportion."
He did not venture a prediction, though, as to whether the H3N2 viruses would disappear entirely.
This family of viruses has evolved significantly since decisions were made last February on what should be included in the Northern Hemisphere flu shot for this winter. As a consequence, it is feared that if there is H3N2 activity later in the winter, the seasonal shot may not offer good protection against the H3 viruses.
But Dr. Fukuda said it's impossible to know that from looking at the genetic blueprint of a virus. It has to be tested - and those tests have not yet been done. He pointed out, however, that it remains unclear how much seasonal flu activity there will be this winter.